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Originally Posted by kh2gr Hey all, I am getting ready to go out to Illinois to get my high altitude endorsement in a cessna 340 Simulator at recurrent training center. Any opinions on good reading material, or things I should study up on to better prepare me for the endorsement? I do not have any experience in the 340 but will be flying for a local company in one upon completion of the endorsement. Ive been given the cessna 340 systems book from a local pilot which he got from SIMCOM so I feel pretty good I am up to par there, but just looking for advise on exactly what I should study up on. do you think they expect me to know much about the 340? or should I devote my attention all to the things specified in the FARs on high altitude training: rapid decompression, Hypoxia, oxygen systems etc....?
thanks for the help. |
Hiya Kh2gr,
If you are asking for what I think you may be asking, then It may be what I have in my ATP library. AC 61-107
OPERATIONS OF AIRCRAFT AT ALTITUDES ABOVE 25,000 FEET MSL AND/OR MACH NUMBERS (Mmo) GREATER THAN .75
Here is the AC in its entirety ( apologies in advance to those for a long thread ) Quote:
U.S. Department
of Transportation
Federal Aviation
Administration
Subject: OPERATIONS OF AIRCRAFT AT ALTITUDES ABOVE Date 01/23/91 AC 61-107
25,000 FEET MSL AND/OR MACH NUMBERS (Mmo) Initiated by: AFS-840
GREATER THAN .75
1. PURPOSE. This advisory circular (AC) is issued to alert pilots
transitioning to complex, high-performance aircraft which are capable of
operating at high altitudes and high airspeeds of the need to be knowledgeable
of the special physiological and aerodynamic considerations involved within
this realm of operation.
2. CANCELLATION. AC 91-8B, Use of Oxygen by Aviation Pilots/Passengers,
dated April 7, 1982, is canceled.
3. RELATED READING MATERIAL. Additional information can be found in the
latest edition of AC 67-2, Medical Handbook for Pilots.
4. BACKGROUND. On September 17, 1982, the National Transportation Safety
Board (NTSB) issued a series of safety recommendations which included, among
other things, that a minimum training curriculum be established for use at
pilot schools covering pilots' initial transition into general aviation
turbojet airplanes. Aerodynamics and physiological aspects of high-performance
aircraft operating at high altitudes were among the subjects recommended for
inclusion in this training curriculum. These recommendations were the result
of an NTSB review of a series of fatal accidents which were believed to involve
a lack of flightcrew knowledge and proficiency in general aviation turbojet
airplanes capable of operating in a high-altitude environment. Although the
near total destruction of physical evidence and the absence of installed flight
recorders have inhibited investigators' abilities to pinpoint the circumstances
which led to these accidents, the NTSB is concerned that a lack of flightcrew
knowledge and proficiency in the subject matter of this AC were involved in
either the initial loss of control or the inability to regain control, or both,
of the aircraft. A requirement has been added to the Federal Aviation
Regulations (FAR) Part 61 for high-altitude training of pilots who transition
to any pressurized airplane that has a service ceiling or maximum operating
altitude, whichever is lower, above 25,000 feet mean sea level (MSL).
Recommended training in high altitude operations that would meet the
requirements of this regulation can be found in Chapter 1 of this AC.
5. DEFINITIONS.
a. Aspect Ratio is the relationship between the wing chord and the wingspan. A
short wingspan and wide wing chord equal a low aspect ratio.
b. Drag Divergence is a phenomenon that occurs when an airfoil's drag increases
sharply and requires substantial increases in power (thrust) to produce further
increases in speed. This is not to be confused with MACH
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AC 61-107 01/23/91
crit The drag increase is due to the unstable formation of shock waves that
transform a large amount of energy into heat and into pressure pulses that act
to consume a major portion of the available propulsive energy (thrust).
Turbulent air may produce a resultant increase in the coefficient of drag.
c. Force is generally defined as the cause for motion or of change or stoppage
of motion. The ocean of air through which an aircraft must fly has both mass
and inertia and, thus, is capable of exerting tremendous forces on an aircraft
moving through the atmosphere. When all of the above forces are equal, the
aircraft is said to be in a state of equilibrium. For instance, when an
aircraft is in level, unaccelerated 1 G flight, thrust and drag are equal, and
lift and gravity (or weight plus aerodynamic downloads on the aircraft) are
equal. Forces that act on any aircraft as the result of air resistance,
friction, and other factors are:
(1) Thrust. The force required to counteract the forces of drag in order to
move an aircraft in forward flight
(2) Drag. The force which acts in opposition to thrust
(3) Lift. The force which sustains the air-craft during flight.
(4) Gravity. The force which acts in opposition to lift
d. MACH, named after Ernst Mach, a 19th century Austrian physicist, is the
ratio of an aircraft's true speed as compared to the local speed of sound at a
given time or place.
e. MACH Buffet is the airflow separation behind a shock-wave pressure barrier
caused by airflow over flight surfaces exceeding the speed of sound.
f. MACH (or Aileron) Buzz is a term used to describe a shock-induced flow
separation of the boundary layer air before reaching the ailerons.
g. MACH Meter is an instrument designed to indicate MACH number. MACH
indicating capability is incorporated into the airspeed indicator(s) of current
generation turbine-powered aircraft capable of MACH range speeds.
h. MACH number is a decimal number (M) representing the true airspeed (TAS)
relationship to the local speed of sound (e.g., TAS 75 percent (.75M) of the
speed of sound where 100 percent of the speed of sound is represented as MACH 1
(1.OM)). The local speed of sound varies with changes in temperature.
i. MACH number (Critical) is the free stream MACH number at which local
sonic flow such as buffet, airflow separation, and shock waves becomes evident.
These phenomena occur above the critical MACH number, often referred to as
MACH crit. These phenomena are listed as follows:
SUBSONIC MACH Numbers below .75
TRANSONIC MACH Numbers from .75 to 1.20
SUPERSONIC MACH Numbers from 1.20 to 5.0
HYPERSONIC MACH Numbers above 5.0
ii
01/23/91 AC 61-107
j. MACH Speed is the ratio or percentage of the TAS to the speed of sound
(e.g., 1,120 feet per second (660 Knots (K)) at MSL). This may be represented
by MACH number.
k. MACH Tuck is the result of an aftward shift in the center of lift causing a
nose down pitching moment.
l. Mmo (MACH, maximum operation) is an airplane's maximum certificated MACH
number. Any excursion past Mmo, whether intentional or accidental, may cause
induced flow separation of boundary layer air over the ailerons and elevators
of an airplane and result in a loss of control surface authority and/or control
surface buzz or snatch.
m. Q-Corner or Coffin Corner is a term used to describe operations at high
altitudes where low indicated airspeeds yield high true airspeeds (MACH number)
at high angles of attack. The high angle of attack results in flow separation
which causes buffet. Turning maneuvers at these altitudes increase the angle
of attack and result in stability deterioration with a decrease in control
effectiveness. The relationship of stall speed to MACH crit narrows to a
point where sudden increases in angle of attack, roll rates, and/or
disturbances; e.g., clear air turbulence, cause the limits of the airspeed
envelope to be exceeded. Coffin comer exists in the upper portion of the
maneuvering envelope for a given gross weight and G-force.
n. Vmo (Velocity maximum operation) is an airplane's indicated airspeed limit.
Exceeding Vmo may cause aerodynamic flutter and G-load limitations to become
critical during the dive recovery.
6. DISCUSSION.
a. FAR Part 61 prescribes the knowledge and skill requirements for the various
airman certificates and ratings, including category, class, and type ratings
authorized to be placed thereon. The civil aircraft fleet consists of numerous
aircraft capable of flight in the high-altitude environment. Certain
knowledge elements pertaining to high-altitude flight are essential for the
pilots of these aircraft. Pilots who fly in this realm of flight must receive
training in the critical factors relating to safe flight operations in the
high-altitude environment. These critical factors include knowledge of the
special physiological and/or aerodynamic considerations which should be given
to high-performance aircraft operating in the high-altitude environment. The
high-altitude environment has different effects on the human body than those
experienced at the lower altitudes. ne aerodynamic characteristics of an
aircraft in high-altitude flight may differ significantly from those of
aircraft operated at the lower altitudes.
b. Pilots who are not familiar with operations in the high-speed environment
are encouraged to obtain thorough and comprehensive training and a checkout in
complex high-performance aircraft before engaging in extensive high-speed
flight in such aircraft, particularly at high altitudes. The training should
enable the pilot to become thoroughly familiar with aircraft performance
charts and aircraft systems and procedures. The more critical elements of
high-altitude flight planning and operations should also be reviewed. The
aircraft checkout should enable the pilot to demonstrate a comprehensive
knowledge of the aircraft performance charts, systems, emergency procedures,
and operating limitations, along with a high degree of proficiency in
performing all flight maneuvers and in-flight emergency procedures. The
attainment of such knowledge and skill requirements by a pilot of high-
performance aircraft should enhance the pilot's preparedness to transition to
the operation of a high-speed aircraft in the high-altitude environment
safely and efficiently.
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AC 61-107 01/23/91
7. SUMMARY. It is beyond the scope of this AC to provide a more definitive
treatment of the subject matter discussed herein. Rather, this AC will have
served its purpose if it aids pilots in becoming familiar with the basic
phenomena associated with high-altitude and high-speed flight. Pilots should
recognize that greater knowledge and skills are needed for the safe and
efficient operation of state-of-the-art turbine-powered aircraft at high
altitude. Pilots are strongly urged to pursue further study from the many
excellent textbooks, charts, and other technical reference material available
through industry sources, and to obtain a detailed understanding of both
physiological and aerodynamic factors which relate to the safe and efficient
operation of the broad variety of high-altitude aircraft available today and
envisioned for the future.
Thomas C. Accardi
Acting Director, Flight Standards Service
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01/23/91 AC 61-107
TABLE OF CONTENTS
CHAPTER 1. RECOMMENDATIONS HIGH-ALTITUDE TRAINING
1. PURPOSE........................................... ............
2. OUTLINE........................................... ............
3. GROUND TRAINING.......................................... .....
5. WEATHER........................................... ............
6. FLIGHT PLANNING AND NAVIGATION................................
7. PHYSIOLOGICAL TRAINING........................................
8. ADDITIONAL PHYSIOLOGICAL TRAINING.............................
9. HIGH-ALTITUDE SYSTEMS AND EQUIPMENT...........................
10. AERODYNAMICS AND PERFORMANCE FACTORS..........................
11. EMERGENCIES AND IRREGULARITIES AT HIGH ALTITUDES..............
12. FLIGHT TRAINING.......................................... .....
CHAPTER 2. MACH FLIGHT AT HIGH ALTITUDES................................
13. PURPOSE........................................... ............
14. CRITICAL ASPECTS OF MACH FLIGHT...............................
15. AIRCRAFT AERODYNAMICS AND PERFORMANCE.........................
v(and vi)
01/23/91 AC 61-107
CHAPTER 1. RECOMMENDATIONS HIGH-ALTITUDE TRAINING
1. PURPOSE. This chapter presents an outline for recommended high-altitude
training that meets the requirements of FAR SS 61.31(f). The actual training,
which may be derived from this outline, should include both ground and flight
training in high-altitude operations. Upon completion of the ground and flight
training, the flight instructor who conducted the training should provide an
endorsement in the pilot's logbook or training record, certifying that
training in high-altitude operations was given. A sample high-altitude
endorsement is available in the most recent version of AC 61-65, Certification:
Pilots and Flight Instructors.
a. Although FAR SS 61.31(f) applies only to pilots who fly pressurized
airplanes with a service ceiling or maximum operating altitude, whichever is
lower, above 25,000 feet MSL, this training is recommended for all pilots who
fly at altitudes above 10,000 feet MSL.
(1) A service ceiling is the maximum height above MSL at which an airplane can
maintain a rate of climb of 100 feet per minute under normal conditions.
(2) All pressurized airplanes have a specified maximum operating altitude above
which operation is not permitted. This maximum operating altitude is determined
by flight, structural, powerplant, functional, or equipment characteristics.
An airplane's maximum operating altitude is limited to 25,000 feet or lower
unless certain airworthiness standards are met.
(3) Maximum operating altitudes and service ceilings are specified in the
Airplane Flight Manual.
b. The training outlined in this chapter is designed primarily for light
twin-engine airplanes that fly at high altitudes but do not require type
ratings. The training should, however, be incorporated into type rating
courses for aircraft that fly above 25,000 feet MSL if the pilot has not
already received training in high-altitude flight. The training in this
chapter does not encompass high-speed flight factors such as acceleration,
G-forces, MACH, and turbine systems that do not apply to reciprocating engine
and turboprop aircraft. Information on high-speed flight can be found in
Chapter 2 of this AC.
2. OUTLINE. Additional information should be used to complement the training
provided herein. The training outlined below, and explained in further detail
in the remainder of this chapter, covers the minimum information needed by
pilots to operate safely at high altitudes.
a. Ground Training.
(1) The High-Altitude Flight Environment.
(i) Airspace.
(ii) FAR.
(2) Weather.
(i) The atmosphere.
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AC 61-107 01/23/91
(ii) Winds and clear air turbulence.
(iii) Clouds and thunderstorms.
(iv) Icing.
(3) Flight Planning and Navigation.
(i) Flight planning.
(ii) Weather charts.
(iii) Navigation.
(iv) Navaids.
(4) Physiological Training.
(i) Respiration.
(ii) Hypoxia.
(iii) Effects of prolonged oxygen use.
(iv) Decompression sickness.
(v) Vision
(vi) Altitude chamber (optional).
(5) High-Altitude Systems and Components.
(i) Turbochargers.
(ii) Oxygen and oxygen equipment.
(iii) Pressurization systems.
(iv) High-altitude components.
(6) Aerodynamics and Performance Factors.
(7) Emergencies.
(i) Decompressions.
(ii) Turbocharger malfunction.
(iii) In-flight fire.
(iv) Flight into severe turbulence or thunderstorms.
b. Flight Training.
(1) Preflight Briefing.
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AC 61-107 01/23/91
(2) Preflight Planning.
(i) Weather briefing and considerations.
(ii) Course plotting.
(iii) Airplane Flight Manual review.
(iv) Flight plan.
(3) Preflight Inspection.
(4) Runup, Takeoff, and Initial Climb.
(5) Climb to High Altitude and Normal Cruise Operations While
Operating Above 25,000 Feet MSL.
(6) Emergencies.
(i) Simulated rapid decompression.
(ii) Emergency descent.
(7) Planned Descents.
(8) Shutdown Procedures.
(9) Postflight Discussion.
3. GROUND TRAINING. Thorough ground training should cover all aspects of
high-altitude flight, including the flight environment, weather, flight
planning and navigation, physiological aspects of high-altitude flight, systems
and equipment, aerodynamics and performance, and high-altitude emergencies.
The ground training should include the history and causes of some past
accidents and incidents involving the topics included in paragraph 2.
Accident reports are available from the NTSB and some aviation organizations.
4. THE HIGH-ALTITUDE FLIGHT ENVIRONMENT. For the purposes of FAR 61.31(f),
flight operations conducted above 25,000 feet are considered to be high
altitude. However, the high-altitude environment itself begins below 25,000
feet. For example, flight levels (FL) are used at and above 18,000 feet
(e.g., FL 180) to indicate levels of constant atmospheric pressure in relation
to a reference datum of 29.92" Hg. Certain airspace designations and Federal
Aviation Administration (FAA) requirements become effective at different
altitudes. Pilots must be familiar with these elements before operating in
each realm of flight.
a. Airspace. Pilots of high-altitude aircraft are subject to three principle
types of airspace at altitudes above 10,000 feet MSL. These are the Positive
Control Area (PCA), which extends from FL 180 to FL 600; the Continental Control
Area, which covers the continental United States above 14,500 feet MSL; and
control zones that do not underlie the Continental Control Area, which extend
upward from the surface and have no upper limit. (Other control zones
terminate at the base of the Continental Control Area.)
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AC 61-107 01/23/91
b. Federal Aviation Regulations. In addition to the training required by FAR
61.31(f), pilots of high-altitude aircraft should be familiar with FAR Part 91
regulations that apply specifically to flight at high altitudes.
(1) FAR 91.215 requires that all aircraft operating within the continental
United States at and above 10,000 feet MSL be equipped with an operable
transponder with Mode C capability (unless operating at or below 2,500 feet
above ground level (AGL), below the PCA).
(2) FAR 91.211(a) requires that the minimum flightcrew on civil aircraft of
U.S. registry be provided with and use supplemental oxygen at cabin pressure
altitudes above 12,500 feet MSL up to and including 14,000 feet MSL for that
portion of the Right that is at those altitudes for more than 30 minutes. The
required minimum flightcrew must be provided with and use supplemental oxygen
at all times when operating an aircraft above 14,000 feet MSL. At cabin
pressure altitudes above 15,000 feet MSL, all occupants of the aircraft must be
provided with supplemental oxygen.
(3) FAR 91.21 1 (b) requires pressurized aircraft to have at least a 10-minute
additional supply of supplemental oxygen for each occupant at flight altitudes
above FL 250 in the event of a decompression. At flight altitudes above FL
350, one pilot at the controls of the airplane must wear and use an oxygen mask
that is secured and sealed. The oxygen mask must supply oxygen at all times or
must automatically supply oxygen when the cabin pressure altitude of the
airplane exceeds 14,000 feet MSL. An exception to this regulation exists for
two-pilot crews that operate at or below FL 410. One pilot does not need to
wear and use an oxygen mask if both pilots are at the controls and each
pilot has a quick donning type of oxygen mask that can be placed on the face
with one hand from the ready position and be properly secured, sealed, and
operational within 5 seconds. If one pilot of a two-pilot crew is away from the
controls, then the pilot that is at the controls must wear and use an oxygen
mask that is secured and sealed.
(4) FAR 91.121 requires that aircraft use an altimeter setting of 29.92 at all
times when operating at or above FL 180.
(5) FAR 91.135 requires that all flights within the PCA be conducted under
instrument flight rules (IFR) in an aircraft equipped for IFR and flown by a
pilot who is rated for instrument flight.
(6) FAR 91.159 and 91.179 specify cruising altitudes and flight levels for
visual flight rules (VFR) and IFR flights, respectively. For VFR flights
between FL 180 to FL 290 (except within the PCA where VFR flight is
prohibited), odd flight levels plus 500 feet should be flown if the magnetic
course is 0 to 179 , and even flight levels plus 500 feet should be flown if
the magnetic course is 180 to 359. VFR flights above FL 290 should be flown at
4,000 foot intervals beginning at FL 300 if the magnetic course is 0 to 179 and
FL 320 if the magnetic course is 180 to 359. For IFR flights in uncontrolled
airspace between FL 180 and FL 290, odd flight levels should be flown if the
magnetic course is 0 to 179, and even flight levels should be flown if the
magnetic course is 180 to 359. IFR flights in uncontrolled airspace at or
above FL 290 should be flown at 4,000 foot intervals beginning at FL 290 if the
magnetic course is 0 to 179 and FL 310 if the magnetic course is 180 to 359.
When flying in the PCA, flight levels assigned by air traffic control (ATC)
should be maintained.
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AC 61-107 01/23/91
5. WEATHER. Pilots should be aware of and recognize the meteorological
phenomena associated with high altitudes and the effects of these phenomena on
flight.
a. The Atmosphere. The atmosphere is a mixture of gases in constant motion.
It is composed of approximately 78 percent nitrogen, 21 percent oxygen, and 1
percent other gases. Water vapor is constantly being absorbed and released in
the atmosphere which causes changes in weather. The three levels of the
atmosphere where high-altitude flight may occur are the troposphere, which can
extend from sea level to approximately FL 350 around the poles and up to FL 650
around the equator; the tropopause, a thin layer at the top of the troposphere
that traps water vapor in the lower level; and the stratosphere, which extends
from the tropopause to approximately 22 miles. The stratosphere is
characterized by lack of moisture and a constant temperature of -55 deg C,
while the temperature in the troposphere decreases at a rate of 2 deg C per
1,000 feet. Condensation trails, or contrails, are common in the upper levels
of the troposphere and in the stratosphere. These cloud-like streamers that
are generated in the wake of aircraft flying in clear, cold, humid air, form by
water vapor from aircraft exhaust gases being added to the atmosphere causing
saturation or supersaturation of the air. Contrails can also form
aerodynamically by the pressure reduction around airfoils, engine nacelles, and
propellers cooling the air to saturation.
b. Atmospheric density in the troposphere decreases 50 percent at 18,000 feet.
This means that at FL 180, the air contains only one-half the oxygen molecules
as at sea level. Because the human body requires a certain amount of oxygen
for survival, aircraft that fly at high altitudes must be equipped with some
means of creating an artificial atmosphere, such as cabin pressurization.
c. Winds.
(1) The jet stream is a narrow band of high-altitude winds, near or in the
tropopause, that results from large temperature contrasts over a short distance
(typically along fronts) creating large pressure gradients aloft. The jet
stream usually travels in an easterly direction between 50 and 200 K. The speed
of the jet stream is greater in the winter than in the summer months because of
greater temperature differences. It generally drops more rapidly on the polar
side than on the equatorial side. In the mid-latitudes, the polar front jet
stream is found in association with the polar front. This jet stream has a
variable path, sometimes flowing almost due north and south.
(2) Because of its meandering path, the polar front jet strewn is not found on
most circulation charts. One almost permanent jet is a westerly jet found over
the subtropics at 25 deg latitude about 8 miles above the surface. Low
pressure systems usually form to the south of the jet stream and move northward
until they become occluded lows which move north of the jet stream. Horizontal
windshear and turbulence are frequently found on the northern side of the jet
stream.
5
AC 61-107 01/23/91
d. Clear Air Turbulence (CAT). CAT is a meteorological phenomenon associated
with high-altitude winds. This high-level turbulence occurs where no clouds
are present and can take place at any altitude (normally above 15,000 feet
AGL), although it usually develops in or near the jet stream where there is a
rapid change in temperature. CAT is generally stronger on the polar side of
the jet and is greatest during the winter months. CAT can be caused by
windshear, convective currents, mountain waves, strong low pressures aloft, or
other obstructions to normal wind flow. CAT is difficult to forecast because
it gives no visual warning of its presence and winds can carry it far from its
point of origin.
e. Clouds and Thunderstorms.
(1) Cirrus and cirriform clouds are high-altitude clouds that are composed of
ice crystals. Cirrus clouds are found in stable air above 30,000 feet in
patches or narrow bands. Cirriform clouds, such as the white clouds in long
bands against a blue background known as cirrostratus clouds, generally
indicate some type of system below. Cirrostratus clouds form in stable air as
a result of shallow convective currents and also may produce light turbulence.
Clouds with extensive vertical development (e.g., towering cumulus and
cumulonimbus clouds) indicate a deep layer of unstable air and contain moderate
to heavy turbulence with icing. The bases of these clouds are found at
altitudes associated with low to middle clouds but their tops can extend up to
60,000 feet or more.
(2) Cumulonimbus clouds are thunderstorm clouds that present a particularly
severe hazard to pilots and should be circumnavigated if possible. Hazards
associated with cumulonimbus clouds include embedded thunderstorms, severe or
extreme turbulence, lightning, icing, and dangerously strong winds and updrafts.
f. Icing. Icing at high altitudes is not as common or extreme as it can be at
low altitudes. When it does occur, the rate of accumulation at high altitudes
is generally slower than at. low altitudes. Rime ice is generally more common
at high altitudes than clear ice, although clear ice is possible. Despite the
composition of cirrus clouds, severe icing is generally not a problem
although it can occur in some detached cirrus. It is more common in tops of
tall cumulus buildups, anvils, and over mountainous regions. Many airplanes
that operate above 25,000 feet are equipped with deice or anti-ice systems,
reducing even further the dangers of icing.
6. FLIGHT PLANNING AND NAVIGATION.
a. Flight Planning.
(1) Careful flight planning is critical to safe high-altitude flight.
Consideration must be given to power settings, particularly on takeoff, climb,
and descent to assure operation in accordance with the manufacturer's
recommendations. Fuel management, reporting points, weather briefings (not
only thunderstorms, the freezing level, and icing at altitude but at all
levels and destinations, including alternates, that may affect the flight),
direction of flight, airplane performance charts, high speed winds aloft, and
oxygen duration charts must also be considered. When possible, additional
oxygen should be provided to allow for emergency situations. Breathing rates
increase under stress and extra oxygen could be necessary.
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AC 61-107 01/23/91
(2) Flight planning should take into consideration factors associated with
altitudes that will be transited while climbing to or descending from the high
altitudes (e.g., airspeed limitations below 10,000 feet MSL, airspace, and
minimum altitudes). Westward flights should generally be made away from the
jet stream to avoid the strong headwind, and eastward flights should be made in
the jet stream when possible to increase groundspeed. Groundspeed checks
are particularly important in high-altitude flight. If fuel runs low because
of headwinds or poor flight planning, a decision to fly to an alternate
airport should be made as early as possible to allow time to replan descents
and advise ATC.
b. Knowledge of Aircraft. Complete familiarity with the aircraft systems and
limitations is extremely important. For example, many high-altitude airplanes
feed from only one fuel tank at a time. If this is the case, it is important
to know the fuel consumption rate to know when to change tanks. This knowledge
should be made part of the preflight planning and its accuracy confirmed
regularly during the flight.
c. Gradual Descents. Gradual descents from high altitudes should be planned in
advance to prevent excessive engine cooling and provide passenger comfort. The
manufacturer's recommendations found in the Airplane Flight Manual should be
complied with, especially regarding descent power settings to avoid stress on
the engines. Although most jets can descend rapidly at idle power, many
turboprop and light twin airplanes require some power to avoid excessive engine
cooling, cold shock, and metal fatigue. ATC does not always take aircraft type
into consideration when issuing descent instructions. It is the pilot's
responsibility to fly the airplane in the safest manner possible. Cabin rates
of descent are particularly important and should generally not exceed 500 or 600
feet per minute. Before landing, cabin pressure should be equal to ambient
pressure or inner ear injury can result. If delays occur en route, descents
should be adjusted accordingly.
d. Weather Charts. Before beginning a high-altitude flight, all weather charts
should be consulted, including those designed for low levels. Although
high-altitude flight may allow a pilot to overfly adverse weather, low altitudes
must be transited on arrival, departure, and in an emergency situation that may
require landing at any point en route.
e. Types of Weather Charts. Weather charts that provide information on
high-altitude weather include Constant Pressure Charts, which provide
information on pressure systems, temperature, winds, and temperature/dewpoint
spread at the 850 millibar (mb), 700 mb, 500 mb, 300 mb, and 200 mb levels (5
charts are issued every 12 hours). Prognostic Charts forecast winds,
temperature, and expected movement of weather over the 6-hour valid time of the
chart. Observed Tropopause Charts provide jet stream, turbulence, and
temperature-wind-pressure reportings at the tropopause over each station.
Tropopause Wind Prognostic Charts and Tropopause Height Vertical Windshear
Charts are helpful in determining jet stream patterns and the presence of CAT
and windshear.
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AC 61-107 01/23/91
f. Windshear. Windshear is indicated by dashed lines on Tropopause Height
Vertical Windshear Charts. Horizontal wind changes of 40 K within 150 NM, or
vertical windshear of 6 K or greater per 1,000 feet usually indicate moderate to
severe turbulence and should be avoided. Pilot reports (PIREPS) are one of the
best methods of receiving timely and accurate reports on icing and turbulence
at high altitudes.
g. Navigation. Specific charts have been designed for flight at FL 180 and
above. Enroute high-altitude charts delineate the jet route system, which
consists of routes established from FL 180 up to and including FL 450. The
VOR airways established below FL 180 found on low-altitude charts must not be
used at FL 180 and above. High-altitude jet routes are an independent matrix of
airways, and pilots must have the appropriate enroute high-altitude charts
before transitioning to the flight levels.
h. Jet Routes. Jet routes in the U.S. are predicated solely on VOR or VORTAC
navigation facilities, except in Alaska where some are based on L/MF navigation
aids. AH jet routes are identified by the letter "J" and followed by the
airway number.
i. Reporting Points. Reporting points are designated for jet route systems and
must be used by flights using the jet route unless otherwise advised by ATC.
Flights above FL 450 may be conducted on a point-to-point basis, using the
facilities depicted on the enroute high-altitude chart as navigational guidance.
Random and fixed Area Navigation (RNAV) Routes are also used for direct
navigation at high altitudes and are based on area navigation capability between
waypoints defined in terms of latitude/longitude coordinates, degree-distance
fixes, or offsets from established routes or airways at a specified distance and
direction. Radar monitoring by ATC is required on all random RNAV routes.
j. Point-to-Point Navigation. In addition to RNAV, many high-altitude airplanes
are equipped with point-to-point navigation systems for high-altitude en route
flight. These include LORAN-C, OMEGA, Inertial Navigation System, and Doppler
Radar. Further information about these and additional navigation systems are
available in the Airman's Information Manual.
k. Navaids. VOR, DME, and TACAN depicted on high-altitude charts are designated
as class H navaids, signifying that their standard service volume is from 1,000
feet AGL up to and including 14,500 AGL at radial distances out to 40 NM; from
14,500 feet AGL up to and including 60,000 feet AGL at radial distances out to
100 NM; and from 18,000 feet AGL up to and including 45,000 feet AGL at radial
distances out to 130 NM. Ranges of NDB service volumes are the same at all
altitudes.
7. PHYSIOLOGICAL TRAINING. To ensure safe flights at high altitudes, pilots of
high-altitude aircraft must understand the physiological effects of
high-altitude flight.
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Additional physiological training information, including locations and
application procedures for attending an altitude chamber, can be found in
paragraph 8 of this chapter. Although not required, altitude chamber training
is highly recommended for all pilots.
a. Respiration is the exchange of gases between the organism and its environment
In humans, external respiration is the intake of oxygen from the atmosphere by
the lungs and the elimination of some carbon dioxide from the body into the
surrounding atmosphere. Each breath intake is comprised of approximately 21
percent oxygen, which is absorbed into the bloodstream and carried by the blood
throughout the body to bum food material and to produce heat and kinetic energy.
The partial pressure of oxygen forces oxygen through air sacs (alveoli),
located at the end of each of the smaller tubes that branches out from the
bronchial tubes and lungs, into the bloodstream. Other gases contained in the
lungs reduce the partial pressure of oxygen entering the air sacs to about
102 mm Hg at ground level, which is approximately 21 percent of the total
atmospheric pressure.
b. The human body functions normally in the atmospheric area extending from sea
level to 12,000 feet MSL. In this range, brain oxygen saturation is at a level
that allows for normal functioning. (Optimal functioning is 96 percent
saturation. At 12,000 feet, brain oxygen saturation is approximately 87 percent
which begins to approach a level that could affect human performance. Although
oxygen is not required below 12,500 feet MSL, its use is recommended when flying
above 10,000 feet MSL during the day and above 5,000 feet MSL at night when the
eyes become more sensitive to oxygen deprivation.)
c. Although minor physiological problems, such as middle ear and sinus trapped
gas difficulties, can occur when flying below 12,000 feet, shortness of breath,
dizziness, and headaches will result when an individual ascends to an altitude
higher than that to which his or her body is acclimated. From 12,000 to 50,000
feet MSL, atmospheric pressure drops by 396 mm Hg. This area contains less
partial pressure of oxygen which can result in problems such as trapped or
evolved gases within the body. Flight at and above 50,000 feet MSL requires
sealed cabins or pressure suits.
d. Hypoxia is a lack of sufficient oxygen in the body cells or tissues caused by
an inadequate supply of oxygen, inadequate, transportation of oxygen, or
inability of the body tissues to use oxygen. A common misconception among many
pilots who are inexperienced in high-altitude flight operations and who have not
been exposed to physiological training is that it is possible to recognize the
symptoms of hypoxia and to take corrective action before becoming seriously
impaired. While this concept may be appealing in theory, it is both misleading
and dangerous for an untrained crewmember. Symptoms of hypoxia vary from pilot
to pilot, but one of the earliest effects of hypoxia is impairment of judgment.
Other symptoms can include one or more of the following:
(1) Behavioral changes (e.g., a sense of euphoria).
(2) Poor coordination.
(3) Discoloration at the fingernail beds (cyanosis).
(4) Sweating.
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(5) Increased breathing rate, headache, sleepiness, or fatigue.
(6) Loss or deterioration of vision.
(7) Light-headedness or dizzy sensations and listlessness.
(8) Tingling or warm sensations.
e. While other significant effects of hypoxia usually do not occur in a healthy
pilot in an unpressurized aircraft below 12,000 feet, there is no assurance that
this will always be the case. The onset of hypoxic symptoms may seriously
affect the safety of flight and may well occur even in short periods of
exposure to altitudes from 12,000 to 15,000 feet. The ability to take
corrective measures may be totally lost in 5 minutes at 22,000 feet. However,
that time would be reduced to only 18 seconds at 40,000 feet and the crewmember
may suffer total loss of consciousness soon thereafter. A description of the
four major hypoxia groups and the recommended methods to combat each follows.
(1) Hypoxic (Altitude) Hypoxia. Altitude hypoxia poses the greatest potential
physiological hazard to a flight crewmember while flying in the high-altitude
environment. This type of hypoxia is caused by an insufficient partial
pressure of oxygen in the inhaled air resulting from reduced oxygen pressure in
the atmosphere at altitude. If a person is able to recognize the onset of
hypoxic symptoms, immediate use of supplemental oxygen will combat hypoxic
hypoxia within seconds. Oxygen systems should be checked periodically to
ensure that there is an adequate supply of oxygen and that the system is
functioning properly. This check should be performed frequently with increasing
altitude. If supplemental oxygen is not available, an emergency descent to
an altitude below 10,000 feet should be initiated.
(2) Histotoxic Hypoxia. This is the inability of the body cells to use oxygen
because of impaired cellular respiration. This type of hypoxia, caused by
alcohol or drug use, cannot be corrected by using supplemental oxygen because
the uptake of oxygen is impaired at the tissue level. The only method of
avoiding this type of hypoxia is to abstain, before flight, from alcohol or
drugs that are not approved by a flight surgeon or an aviation medical examiner.
(3) Hypemic (Anemic) Hypoxia. This type of hypoxia is defined as a reduction
in the oxygen-carrying capacity of the blood. Hypemic hypoxia is caused by
a reduction in circulating red blood cells (hemoglobin) or contamination of
blood with gases other than oxygen as a result of anemia, carbon monoxide
poisoning, or excessive smoking. Pilots should take into consideration the
effect of smoking on altitude tolerance when determining appropriate cabin
pressures. If heavy smokers are among the crew or passengers, a lower cabin
altitude should be set because apparent altitudes for smokers are generally much
higher than actual altitudes. For example, a smoker's apparent altitude at sea
level is approximately 7,000 feet. Twenty thousand feet actual altitude for a
nonsmoker would be equivalent to an apparent altitude of 22,000 feet for a
smoker. The smoker is thus more susceptible to hypoxia at lower altitudes
than the nonsmoker. Hypemic hypoxia is corrected by locating and eliminating
the source of the contaminating gases. A careful preflight of heating systems
and exhaust manifold equipment is mandatory. Also, cutting down on smoking
would minimize the onset of this type of hypoxia. If symptoms are recognized,
initiate use of supplemental oxygen and/or descend to an altitude below 10,000
feet. If symptoms persist, ventilate the cabin and land as soon as possible
because the symptoms may be indicative of carbon monoxide poisoning and medical
attention should be sought.
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(4) Stagnant Hypoxia. This is an oxygen deficiency in the body resulting from
poor circulation of the blood because of a failure of the circulatory system to
pump blood (and oxygen) to the issues. Evidence of coronary artery disease is
grounds for immediate denial or revocation of a medical certificate. In flight,
this type of hypoxia can sometimes be caused by positive pressure breathing for
long periods of time or excessive G-forces.
f. Effective Performance Time (EPT) or Time of Useful Consciousness (TUC) is the
amount of time in which a person is able to effectively or adequately perform
flight duties with an insufficient supply of oxygen. EPT decreases with
altitude, until eventually coinciding with the time it takes for blood to
circulate from the lungs to the head usually at an altitude above 35,000 feet.
Table 1 shows the TUC (shown as average TUC) at various altitudes.
Table 1. Times Of Useful Consciousness At Various Altitudes
Altitude Sitting Moderate
(Feet) Quietly Activity
22,000 10 minutes 5 minutes
25,000 5 minutes 3 minutes
30,000 1 minute 45 seconds
35,000 45 seconds 30 seconds
40,000 25 seconds 18 seconds
g. Other factors that determine EPT are the rate of ascent (faster rates of
ascent result in shorter EPT's), physical activities (exercise decreases EPT's),
and day-today factors such as physical fitness, diet, rest, prescription drugs,
smoking, and illness. Altitude chamber experiments found a significantly longer
TUC for nonsmoker pilots who exercise and watch their diet than for pilots who
smoke and are not physically fit.
h. Prolonged oxygen use can also be harmful to human health. One hundred
percent aviation oxygen can produce toxic symptoms if used for extended periods
of time. The symptoms can consist of bronchial cough, fever, vomiting,
nervousness, irregular heart beat, and lowered energy. These symptoms appeared
on the second day of breathing 90 percent oxygen during controlled experiments.
It is unlikely that oxygen would be used long enough to produce the most severe
of these symptoms in any aviation incidence. However, prolonged flights at high
altitudes using a high concentration of oxygen can produce some symptoms of
oxygen poisoning such as infection or bronchial irritation. The sudden supply
of pure oxygen following a decompression can often aggravate the symptoms of
hypoxia. Therefore, oxygen should be taken gradually, particularly when the
body is already suffering from lack of oxygen, to build up the supply in small
doses. If symptoms of oxygen poisoning develop, high concentrations of oxygen
should be avoided until the symptoms completely disappear.
i. When nitrogen is inhaled, it dilutes the air we breathe. While most
nitrogen is exhaled from the lungs along with carbon dioxide, some nitrogen is
absorbed by the body. The nitrogen absorbed into the body tissues does not
nominally present any problem because it is carried in a liquid state. If the
ambient surrounding atmospheric pressure lowers drastically, this nitrogen could
change from a liquid and return to its gaseous state in the form of bubbles.
These evolving and expanding gases in the body are known as decompression
sickness and are divided into two groups.
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(1) Trapped Gas. Expanding or contracting gas in certain body cavities during
altitude changes can result in abdominal pain, toothache, or pain in ears and
sinuses if the person is unable to equalize the pressure changes. Above 25,000
feet, distention can produce particularly severe gastrointestinal pain.
(2) Evolved Gas. When the pressure on the body drops sufficiently, nitrogen
comes out of solution and forms bubbles which can have adverse effects on some
body tissues. Fatty tissue contains more nitrogen than other tissue; thus
making overweight people more susceptible to evolved gas decompression
sicknesses.
(i) SCUBA diving will compound this problem because of the compressed air used
in the breathing tanks. After SCUBA diving, a person who flies in an aircraft
to an altitude of 8,000 feet would experience the same effects as a nondiver
flying at 40,000 feet unpressurized. The recommended waiting period before
going to flight altitudes of 8,000 feet is at least 12 hours after
non-decompression stop diving (diving which does not require a controlled
ascent), and 24 hours after decompression stop diving (diving which requires a
controlled ascent). For flight altitudes above 8,000 feet, the recommended
waiting time is at least 24 hours after any SCUBA diving.
(ii) The bends, also known as caisson disease, is one type of evolved gas
decompression sickness and is characterized by pain in and around the joints.
The term bends is used because the resultant pain is eased by bending the
joints. The pain gradually becomes more severe, can eventually become
temporarily incapacitating, and can result in collapse. The chokes refers to
a decompression sickness that manifests itself through chest pains and burning
sensations, a desire to cough, possible cyanosis, a sensation of suffocation,
progressively shallower breathing and, if a descent is not made immediately,
collapse and unconsciousness. Paresthesia is a third type of decompression
sickness, characterized by tingling, itching, a red rash, and cold and warm
sensations, probably resulting from bubbles in the central nervous system (CNS).
CNS disturbances can result in visual deficiencies such as illusionary lines
or spots, or a blurred field of vision. Some other effects of CNS disturbances
are temporary partial paralysis, sensory disorders, slurred speech, and seizures.
j. Shock can often result from decompression sicknesses as a form of body
protest to disrupted circulation. Shock can cause nausea, fainting, dizziness,
sweating, and/or loss of consciousness. The best treatment for decompression
sickness is descent to a lower altitude and landing. If conditions persist
after landing, recompression chambers can be located through an aviation medical
examiner.
k. Vision has a tendency to deteriorate with altitude. A reversal of light
distribution at high altitudes (bright clouds below the airplane and darker,
blue sky above) can cause a glare inside the cockpit. Glare effects and
deteriorated vision are enhanced at night when the body becomes more susceptible
to hypoxia. Night vision can begin to deteriorate at cabin pressure altitudes
as low as 5,000 feet. In addition, the empty visual field caused by cloudless,
blue skies during the day can cause inaccuracies when judging the speed, size,
and distance of other aircraft. Sunglasses are recommended to minimize the
intensity of the sun's ultraviolet rays at high altitudes.
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AC 61-107 01/23/91
8. ADDITIONAL PHYSIOLOGICAL TRAINING. There are no specific requirements
in FAR Part 91 or Part 125 for physiological training. However, in addition to
the high altitude training required by FAR SS 61.31(f), which should include the
physiological training outlined in this chapter, FAR Parts 121 and 135 require
flight crewmembers that serve in operations above 25,000 feet to receive
training in specified subjects of aviation physiology. None of the requirements
includes altitude chamber training. The U.S. military services require its
flight crewmembers to complete both initial and refresher physiological
training, including instruction in basic aviation physiology and altitude
chamber training. Other U.S. Government agencies, such as the National Aviation
and Space Administration and FAA, also require their flight personnel who
operate pressurized aircraft in the high-altitude flight environment to complete
similar training. Although most of the subject material normally covered in
physiological training concerns problems associated with reduced atmospheric
pressure at high-flight altitudes, other equally important subjects are covered
as well. Such subjects of aviation physiology as vision, disorientation,
physical fitness, stress, and survival affect flight safety and are normally
presented in a good training program.
a. Physiological training programs are offered at locations across the United
States (Table 2) for pilots who are interested in learning to recognize and
overcome vertigo, hypoxia, hyperventilation, etc., during flight. Trainees
who attend these programs will be given classroom lectures, a high-altitude
"flight" in an altitude chamber, and time in a jet aircraft cockpit spatial
disorientation training device at some of the military bases that offer the
course.
b. Persons who wish to take this training must be at least 18 years of age,
hold a current FAA Airman Medical Certificate, and must not have a cold or any
other significant health problem when enrolling for the course.
c. Applications for physiological training may be obtained at any FAA Flight
Standards District Office. Persons who wish to enroll should send a completed
application and payment (minimal fee for the course is $20) to the Mike
Monroney Aeronautical Center, General Accounting Branch, AAC-23B, Box 25082,
Oklahoma City, Oklahoma 73125.
d. Within 30 to 60 days, the applicant will be notified of the time and place
of training.
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Table 2. List of Training Locations
Aeronautical Center, OK Andrews AFB, MD Barbers Point NAS, HI
Beale AFB, TX Brooks AFB, TX Brunswick NAS, ME
Cherry Point MCAS, NC Columbus AFB, MS Edwards AFB, CA
Ellsworth AFB, CA El Toro MCAS, CA Fairchild AFB, WA
Jacksonville NAS, FL Laughlin AFB, TX Lemoore NAS, CA
Little Rock AFB, AR MacDill AFB, FL Mather AFB, CA
NASA Johnson Space Ctr, TX Norfolk NAS, VA Patuxent River NAS, MD
Pease AFB, NH Peterson AFB, CO Point Mugu NMC, CA
Reese AFB, TX San Diego NAS, CA Sheppard AFB, TX
Vance AFB, OK Whidbey Island NAS, WA Williams AFB, AZ
Wright AFB, AZ Wright-Patterson AFB, OH
9. HIGH-ALTITUDE SYSTEMS AND EQUIPMENT. Several systems and equipment are
unique to aircraft that fly at high altitudes, and pilots should be familiar
with their operation before using them. Before any flight, a pilot should be
familiar with all the systems on the aircraft to be flown.
a. Turbochargers. Most light piston engine airplanes that fly above 25,000 feet
MSL are turbocharged. Turbochargers compress air in the carburetor or cylinder
intake by using exhaust gases from an engine-driven turbine wheel. The
increased air density provides greater power and improved performance. Light
aircraft use one of two types of turbocharging systems. The first is the
nominalizer system, which allows the engine to develop sea level pressure from
approximately 29 inches of manifold pressure up to a critical altitude
(generally between 14,000-16,000 feet MSL). The supercharger system is a more
powerful system which allows the engine to develop higher than sea level
pressure (up to 60 inches of manifold pressure) up to a critical altitude. To
prevent overboosting at altitudes below the critical altitude, a waste gate is
installed in the turbocompressor system to release unnecessary gases. The
waste gate is a damper-like device that controls the amount of exhaust that
strikes the turbine rotor. As the waste gate closes with altitude, it sends
more gases through the turbine compressor, causing the rotor to spin faster.
This allows the engine to function as if it were maintaining sea level or, in
the case of a supercharger, above sea level manifold pressure. The time
principle types of waste gate operations are manual, fixed, and automatic.
(1) Manual Waste Gate. Manual waste gate systems are common in older aircraft
but have been discontinued due to the additional burden on the pilot. Waste
gates were often left closed on takeoff or open on landing, resulting in an
overboost that could harm the engine.
(2) Fixed Waste Gate. Fixed waste gates pose less of a burden on the pilot,
but the pilot must still be careful not to overboost the engine, especially on
takeoff, initial climb, and on cold days when the air is especially dense.
This type of waste gate remains in the same position during all engine
operations, but it splits the exhaust flow allowing only partial exhaust
access to the turbine. The pilot simply controls manifold pressure with
smooth, slow application of the throttle to control against overboost. If
overboost does occur, a relief valve on the intake manifold protects the engine
from damage. This is not a favorable system due to fluctuations in manifold
pressure and limited additional power from the restricted control over the
exhaust flow. In addition, the compressor can produce excessive pressure and
cause overheating.
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(3) Automatic Waste Gate. Automatic waste gates operate on internal pressure.
When internal pressure builds towards an overboost, the waste gate opens to
relieve pressure, keeping the engine within normal operating limits regardless
of the air density.
(i) The pressure-reference automatic waste gate system maintains the manifold
pressure set by the throttle. Engine oil pressure moves the waste gate to
maintain the appropriate manifold pressure, thus reducing the pilot's workload
and eliminating the possibility of overboost. If the airplane engine is started
up and followed by an immediate takeoff, cold oil may cause a higher than
intended manifold pressure. Allow the oil to warm up and circulate throughout
the system before takeoff.
(ii) The density-reference waste gate system is controlled by compressor
discharge air. A density controller holds a given density of air by
automatically adjusting manifold pressure as airspeed, ambient pressure,
temperature, altitude, and other variables change.
b. Turbocharged engines are particularly temperature sensitive. Manufacturers
often recommend increasing the fuel flow during climbs to prevent overheating.
It is also important to cool the engine after landing. Allowing the engine to
idle for approximately 1 minute before shutting it down permits engine oil to
flow through the system, cooling the engine while simultaneously cooling and
lubricating the turbocharger.
c. Most high-altitude airplanes come equipped with some type of fixed oxygen
installation. If the airplane does not have a fixed installation, portable
oxygen equipment must be readily accessible during flight. The portable
equipment usually consists of a container, regulator, mask outlet, and pressure
gauge. A typical 22 cubic-foot portable container will allow four people
enough oxygen to last approximately 1.5 hours at 18,000 feet MSL. Aircraft
oxygen is usually stored in high pressure system containers of 1,800-2,200
pounds per square inch (PSI). The container should be fastened securely in the
aircraft before flight. When the ambient temperature surrounding an oxygen
cylinder decreases, pressure within that cylinder will decrease because
pressure varies directly with temperature if the volume of a gas remains
constant. Therefore, if a drop in indicated pressure on a supplemental oxygen
cylinder is noted, there is no reason to suspect depletion of the oxygen supply,
which has simply been compacted due to storage of the containers in an unheated
area of the aircraft. High pressure oxygen containers should be marked with the
PSI tolerance (i.e., 1,800 PSI) before filling the container to that pressure.
The containers should be supplied with aviation oxygen only, which is 100
percent pure oxygen. Industrial oxygen is not intended for breathing and may
contain impurities, and medical oxygen contains water vapor that can freeze in
the regulator when exposed to cold temperatures. To assure safety, oxygen
system periodic inspection and servicing should be done at FAA certificated
stations found at some fixed base operations and terminal complexes.
d. Regulators and masks work on continuous flow, diluter demand, or on pressure
demand systems. The continuous flow system supplies oxygen at a rate that may
either be controlled by the user or controlled automatically on some regulators.
The mask is designed so the oxygen can be diluted with ambient air by allowing
the user to exhale around the face piece, and comes with a rebreather bag which
allows the individual to reuse pan of the exhaled oxygen. The pilots' masks
sometimes allow greater oxygen flow than passengers' masks, so it is important
that the pilots use the masks that are indicated for them. Although
certificated up to 41,000 feet, very careful attention to system capabilities
is required when using continuous flow oxygen systems above 25,000 feet.
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e. Diluter demand and pressure demand systems supply oxygen only when the
user inhales through the mask. An automix lever allows the regulators to
automatically mix cabin air and oxygen or supply 100 percent oxygen, depending
on the altitude. The demand mask provides a tight seal over the face to
prevent dilution with outside air and can be used safely up to 40,000 feet.
Pilots who fly at those altitudes should not have beards and moustaches
because air can easily seep in through the border of the mask. Pressure demand
regulators also create airtight and oxygen-tight seals but they also provide a
positive pressure application of oxygen to the mask face piece which allows the
user's lungs to be pressurized with oxygen. This feature makes pressure
demand regulators safe at altitudes above 40,000 feet.
f. Pilots should be aware of the danger of fire when using oxygen. Materials
that are nearly fireproof in ordinary air may be susceptible to burning in
oxygen. Oils and greases may catch fire if exposed to oxygen and, therefore,
cannot be used for sealing the valves and fittings of oxygen equipment. Smoking
during any kind of oxygen equipment use must also be strictly forbidden.
g. Surplus oxygen equipment must be inspected and approved by a certified FAA
inspection station before being used. Before each flight, the pilot should
thoroughly inspect and test all oxygen equipment. The inspection should be
accomplished with clean hands and should include a visual inspection of the
mask and tubing for tears, cracks, or deterioration; the regulator for valve and
lever condition and positions; oxygen quantity; and the location and functioning
of oxygen pressure gauges, flow indicators and connections. The mask should be
donned and the system should be tested. After any oxygen use, verify that all
components and valves are shut off.
h. Cabin pressurization is the compression of air in the aircraft cabin to
maintain a cabin altitude lower than the actual flight altitude. Because of the
ever-present possibility of decompression, supplemental oxygen is still
required. Pressurized aircraft meeting specific requirements of FAR Part 23 or
Pan 25 have cabin altitude warning systems which are activated at 10,000 feet.
Pressurized aircraft meeting the still more stringent requirements of FAR Part
25 have automatic passenger oxygen mask dispensing devices which activate
before exceeding 15,000 feet cabin altitude.
i. Pressurization in most light aircraft is sent to the cabin from the
turbocharger's compressor or from an engine-driven pneumatic pump. The flow of
compressed air into the cabin is regulated by an outflow valve which keeps the
pressure constant by releasing excess pressure into the atmosphere. The cabin
altitude can be manually selected and is monitored by a gauge which indicates
the pressure difference between the cabin and ambient altitudes. The rate of
change between these two pressures is automatically controlled with a manual
backup control.
j. Each pressurized aircraft has a determined maximum pressure differential,
which is the maximum differential between cabin and ambient altitudes that the
pressurized section of the aircraft can support. The pilot must be familiar
with these limitations, as well as the manifold pressure settings recommended
for various pressure differentials. Some aircraft have a negative pressure
relief valve to equalize pressure in the event of a sudden decompression
or rapid descent to prevent the cabin pressure from becoming higher than the
ambient pressure.
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k. Reducing exposure to low barometric pressure lowers the occurrence of
decompression sickness and the need for an oxygen mask is eliminated as a full
time oxygen source above certain altitudes. Many airplanes are equipped with
automatic visual and aural warning systems that indicate an unintentional
loss of pressure.
l. Technology is continuously improving flight at high altitudes through the
development of new devices and the improvement of existing systems. One such
example is the pressurized magneto. Thin air at high altitudes makes the
unpressurized magneto susceptible to crossfiring. The high tension pressurized
system is composed of sealed caps and plugs that keep the electrodes contained
within the body. A pressure line extends directly from the turbodischarger to
the magneto. Pressurized magnetos perform better at high altitudes where low
pressure and cold atmosphere have a detrimental effect on electrical
conductivity. Flight above 14,000 feet with an unpressurized magneto should be
avoided because of its higher susceptibility to arcing.
m. Another airplane component recommended for flight at high altitudes is the
dry vacuum pump. Engine-driven wet vacuum pumps cannot create sufficient
vacuum to drive the gyros in the low density found at high altitudes.
Furthermore, gyros and rubber deicing boots can be ruined by oil contamination
from the wet pump system, which uses engine oil for lubrication and cooling.
Dry vacuum pumps are lightweight, self-lubricating systems that eliminate oil
contamination and cooling problems. These pumps can power either a vacuum or
pressure pneumatic system, allowing them to drive the gyros, deice boots, and
pressurize the door seals.
10. AERODYNAMICS AND PERFORMANCE FACTORS. Thinner air at high altitudes has a
significant impact on an airplane's flying characteristics because surface
control effects, lift, drag, and horsepower are all functions of air density.
a. The reduced weight of air moving over control surfaces at high altitudes
decreases their effectiveness. As the airplane approaches its absolute
altitude, the controls become sluggish, making altitude and heading difficult
to maintain. For this reason, most airplanes that fly at above 25,000 feet are
equipped with an autopilot.
b. A determined weight of air is used by the engine for producing an identified
amount of horsepower through internal combustion. For a given decrease of air
density, horsepower decreases at a higher rate which is approximately 1.3 times
that of the corresponding decrease in air density.
c. For an airplane to maintain level flight, drag and thrust must be equal.
Because density is always greatest at sea level, the velocity at altitude given
the same angle of attack will be greater than at sea level, although the
indicated air speed (IAS) win not change. Therefore, an airplane's TAS
increases with altitude while its IAS remains constant. In addition, an
airplane's rate of climb will decrease with altitude.
11. EMERGENCIES AND IRREGULARITIES AT HIGH ALTITUDES. All emergency procedures
in the Airplane Flight Manual should be reviewed before flying any airplane, and
that manual should be readily accessible during every flight. A description of
some of the most significant high-altitude emergencies and remedial action for
each follows.
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a. Decompression is defined as the inability of the aircraft's pressurization
system to maintain its designed pressure schedule. Decompression can be caused
by a malfunction of the system itself or by structural damage to the aircraft.
A decompression will often result in cabin fog because of the rapid drop in
temperature, and the change in relative humidity. A decompression will also
affect the human body. Air will escape from the lungs through the nose and
mouth because of a sudden lower pressure outside of the lungs. Differential
air pressure on either side of the eardrum should clear automatically.
Exposure to windblast and extremely cold temperatures are other hazards the
human body may face with a decompression.
b. Decompression of a small cabin volume pressurized aircraft is more critical
than a large one, given the same size hole or conditions, primarily because of
the difference in cabin volumes. Table 3 is a comparison of cabin volume
ratios between several large transport airplanes and some of the more popular
general aviation turbojet airplanes in current use. Table 3 shows that, under
the same conditions, a typical small pressurized aircraft can be expected to
decompress on the order of 10 to 200 times as fast as a large aircraft.
The B-747/Learjet comparison is an extreme example in that the human response,
TUC, and the protective equipment necessary are the same. Actual decompression
times are difficult to calculate due to many variables involved (e.g., the type
of failure, differential pressure, cabin volume, etc.). However, it is more
probable that the crew of the small aircraft will have less time in which to
take lifesaving actions.
(1) An explosive decompression is a change in cabin pressure faster than
the lungs can decompress. Most authorities consider any decompression which
occurs in less than 0.5 seconds as explosive and potentially dangerous. This
type of decompression is more likely to occur in small volume pressurized
aircraft than in large pressurized aircraft and often results in lung damage.
To avoid potentially dangerous flying debris in the event of an explosive
decompression, all loose items such as baggage and oxygen cylinders should be
properly secured.
Table 3. Aircraft Cabin Volume Ratios
Aircraft Type Cabin Volumes in Cubic Feet Ratio
DC-9 vs CE-650 5,840 vs 576 10:1
B-737 vs LR-55 8,010 vs 502 16:1
B-727 vs NA-265 9,045 vs 430 21:1
L-1011 vs G-1159 35,000 vs 1,850 19:1
B-747 vs Learjet 59,000 vs 265 223:1
Data Source: Physiological Considerations and Limitations in the High-altitude
Operation of Small-Volume Pressurized Aircraft. E. B. McFadden and D. de
Steigner, Federal Aviation Administration (FAA) Civil Aeromedical Institute
(CAMI).
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(2) A rapid decompression is a change in cabin pressure where the lungs can
decompress faster than the cabin. The risk of lung damage is significantly
reduced in this decompression as compared with an explosive decompression.
(3) Gradual or slow decompression is dangerous because it may not be detected.
Automatic visual and aural warning systems generally provide an indication of
a slow decompression.
(4) Recovery from all types of decompression is similar. Oxygen masks should
be donned, and a rapid descent initiated as soon as possible to avoid the onset
of hypoxia. Although top priority in such a situation is reaching a safe
altitude, pilots should be aware that cold-shock in piston engines can result
from a high-altitude rapid descent, causing cracked cylinders or other engine
damage. The time allowed to make a recovery to a safe altitude before loss of
useful consciousness is, of course, much 'less with an explosive than with a
gradual decompression.
c. Increased oil temperature, decreased oil pressure, and a drop in
manifold pressure could indicate a turbocharger malfunction or a partial or
complete turbocharger failure. The consequences of such a malfunction or
failure are twofold. The airplane would not be capable of sustaining altitude
without the additional power supplied by the turbocharging system. The loss in
altitude in itself would not create a significant problem, weather and terrain
permitting, but ATC must be notified of the descent. A more serious problem
associated with a failed turbocharger would be loss of cabin pressurization if
the pressurization system is dependent on the turbocharger compressor.
Careful monitoring of pressurization levels is essential during the descent to
avoid the onset of hypoxia from a slow decompression.
d. Another potential problem associated with turbochargers is fuel
vaporization. Engine driven pumps that pull fuel into the intake manifold are
susceptible to vapor lock at high altitudes. Most high-altitude aircraft are
equipped with tank-mounted boost pumps to feed fuel to the engine-driven pump
under positive pressure. These pumps should be turned on if fuel starvation
occurs as a result of vapor lock.
e. Because of the highly combustible composition of oxygen, an immediate
descent to an altitude where oxygen is not required should be initiated if a
fire breaks out during a flight at high altitude. The procedures in the
Airplane Flight Manual should be closely adhered to.
f. Flight through thunderstorm activity or known severe turbulence should be
avoided, if possible. When flight through severe turbulence is anticipated
and/or unavoidable, the following procedures are highly recommended:
(1) Airspeed is critical for any type of turbulent air penetration. Use the
Airplane Flight Manual recommended turbulence penetration target speed or, if
unknown, an airspeed below maneuvering speed. Use of high airspeeds can result
in structural damage and injury to passengers and crewmembers. Severe gusts
may cause large and rapid variations in indicated airspeed. Do not chase
airspeed.
(2) Penetration should be at an altitude that provides adequate maneuvering
margins in case severe turbulence is encountered to avoid the potential for
catastrophic upset.
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AC 61-107 01/23/92
(3) If severe turbulence is penetrated with the autopilot on, the altitude
hold mode should be off. If the autopilot has an attitude hold mode, it should
be engaged. The autopilot attitude hold mode can usually maintain attitude
more successfully than a pilot under stress. With the autopilot off, the yaw
damper should be engaged. Controllability of the aircraft in turbulence
becomes more difficult with the yaw damper off. Rudder controls should be
centered before engaging the yaw damper.
(4) When flight through a thunderstorm cannot be avoided, turn up the intensity
of panel and cabin lights so lightening does not cause temporary blindness.
White lighting in the cockpit is better than red lighting during thunderstorms.
(5) Keep wings level and maintain the desired pitch attitude and approximate
heading. Do not attempt to turn around and fly out of the storm because the
speed associated with thunderstorms usually makes such attempts unsuccessful.
Use smooth, moderate control movements to resist changes in attitude. If large
attitude changes occur, avoid abrupt or large control inputs. Avoid, as much
as possible, use of the stabilizer trim in controlling pitch attitudes. Do not
chase altitude.
12. FLIGHT TRAINING. Flight training required to comply with FAR 61.31(f) may
be conducted in a high-altitude airplane or a simulator that meets the
requirements of FAR SS 121.407. The simulator should be representative of an
airplane that has a service ceiling or maximum operating altitude, whichever is
lower, above 25,000 feet MSL. The training should consist of as many flights
as necessary to cover the following procedures and maneuvers. Each flight
should consist of a preflight briefing, flight planning, a preflight inspection
(if an airplane is being used), demonstrations by the instructor of certain
maneuvers or procedures when necessary, and a postflight briefing and
discussion.
a. Preflight Briefing. The instructor should verbally cover the material that
will be introduced during the flight. If more than one flight is required,
previous flights should be reviewed at this time. The preflight briefing is
a good time to go over any questions the trainee may have regarding operations
at high altitudes or about the aircraft itself. Questions by the trainee
should be encouraged during all portions of the flight training.
b. Preflight Planning. A thorough flight plan should be completed for a
predetermined route. The flight plan should include a complete weather
briefing. If possible, a trip to a Flight Service Station (FSS) is encouraged
rather than a telephone briefing so the trainee can use actual weather charts.
Winds, pilot reports, the freezing level and other meteorological information
obtained from the briefing should be used to determine the best altitude for
the flight. The information should be retained for future calculations.
(1) The course should be plotted on a high-altitude navigation chart noting
the appropriate jet routes and required reporting points on a navigation log.
Low-altitude charts should be available for planning departures and arrivals to
comply with airspace and airspeed requirements. Alternate airports should also
be identified and noted.
(2) The Airplane Flight Manual should be reviewed with particular attention to
weight and balance, performance charts, and emergency procedures. Oxygen
requirements, airspeeds, groundspeeds, time en route, and fuel burn should be
calculated using the Airplane Flight Manual and weather data, when applicable.
Fuel management and descents should also be planned at this time. The Airplane
Flight Manual should be readily accessible in the cabin in the event of an
emergency.
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AC 61-107 01/23/91
(3) A flight plan should be completed using appropriate jet routes from the
enroute high-altitude chart. The flight plan should be filed with the local
FSS.
c. Preflight Inspection. The aircraft checklist should be followed carefully.
Particular attention should be given to the aircraft's fuselage, windshields,
window panels, and canopies to identify any cracks or damage that could rupture
under the stress of cabin pressurization. The inspection should include a
thorough examination of the aircraft oxygen equipment, including available
supply, an operational check of the system, and assurance that the supplemental
oxygen is in a readily accessible location.
d. Runup, Takeoff and Initial Climb. Procedures in the Airplane Flight Manual
should be followed, particularly the manufacturer's recommended power settings
and airspeeds to avoid overboosting the engine. Standard call-out procedures
are highly recommended and should be used for each phase of flight where the
airplane crew consists of more than one crewmember.
e. Climb to high altitude and normal cruise operations while operating above
25,000 feet MSL. The transition from low to high altitude should be performed
repeatedly to assure familiarity with appropriate procedures. Specific
oxygen requirements should be met when climbing above 12,500 feet and
pressurization should be adjusted with altitude. When passing through FL 180,
the altimeter should be set to 29.92 and left untouched until descending below
that altitude. Reporting points should be complied with, as should appropriate
altitude selection for direction of flight. Throughout the entire climb and
cruise above 25,000 feet, emphasis should be given to monitoring cabin
pressurization.
f. Simulated Emergencies. Training should include at least one simulated rapid
decompression and emergency descent. Do not actually depressurize the airplane
for this or any other training. Actual decompression of an airplane can be
extremely dangerous and should never be done intentionally for training
purposes. The decompression should be simulated by donning the oxygen masks,
turning on the supplemental oxygen controls, configuring the airplane for an
emergency descent, and performing the emergency descent as soon as possible.
This maneuver can be practiced at any altitude.
g. Descents. Gradual descents from altitude should be practiced to provide
passenger comfort and compliance with procedures for transitioning out of the
high-altitude realm of flight. The airplane manufacturer's recommendations
should be followed with regard to descent power settings to avoid stress on the
engine and excessive cooling. Particular emphasis should be given to cabin
pressurization and procedures for equalizing cabin and ambient pressures before
landing. Emphasis should also be given to changing to low-altitude charts when
transitioning through FL 180, obtaining altimeter settings below FL 180, and
complying with airspace and airspeed restrictions at appropriate altitudes.
h. Engine Shutdown. Allow the turbocharged engine to cool for at least 1
minute and assure that all shutdown procedures in the Airplane Flight Manual
are followed. Before exiting the airplane, always check that all oxygen
equipment has been turned off and that the valves on that equipment are closed.
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AC 61-107 01/23/91
i. Postflight Discussion. The instructor should review the flight and answer
any questions the trainee may have. If additional nights are necessary to
ensure thorough understanding of high-altitude operations, the material for the
next flight should be previewed during the postflight discussion.
22(and 23)
AC 61-107 01/23/91
CHAPTER 2. MACH FLIGHT AT HIGH ALTITUDES
13. PURPOSE. To present certain factors involved in the high-speed flight
environment at high altitudes. It is the lack of understanding of many of
these factors involving the laws of aerodynamics, performance, and MACH speeds
that has produced a somewhat higher accident rate in some types of turbojet
aircraft.
14. CRITICAL ASPECTS OF MACH FLIGHT. In recent years, a number of corporate
jet airplanes have been involved in catastrophic loss of control during
high-altitude/high-speed flight. A significant causal factor in these
accidents may well have been a lack of knowledge by the pilot regarding
critical aspects of high-altitude/MACH flight.
a. Maximum operating altitudes of general aviation turbojet airplanes have now
reached 51,000 feet. It is, therefore, logical to expect these types of
accidents to continue unless pilots learn to respect the more critical aspects
of high-altitude/high-speed flight and gain as much knowledge as possible about
the specific make and model of aircraft to be flown and its unique limitations.
b. From the pilot's viewpoint, MACH is the ratio of the aircraft's true
airspeed to the local speed of sound. At sea level, on a standard day (59 deg
F/15 deg C) the speed of sound equals approximately 660 K or 1,120 feet per
second. MACH 0.75 at sea level is equivalent to a TAS of approximately 498 K
(0.75 x 660 K) or 840 feet per second. The temperature of the atmosphere
normally decreases with an increase in altitude. The speed of sound is
directly related only to temperature. The result is a decrease in the speed of
sound up to about 36,000 feet.
c. The sleek design of some turbojet airplanes has caused some operators to
ignore critical airspeed and MACH limitations. There are known cases in which
corporate turbojet airplanes have been modified by disabling the airspeed and
MACH warning systems to permit intentional excursions beyond the FAA
certificated Vmo/Mmo limit for the specific airplane. Such action may
critically jeopardize the safety of the airplane by setting the stage for
potentially hazardous occurrences.
d. The compulsion to go faster may result in the onset of aerodynamic flutter,
which in itself can be disastrous, excessive G-loading in maneuvering, and
induced flow separation over the ailerons and elevators. This may be closely
followed by a loss of control surface authority and aileron buzz or snatch,
coupled with yet another dangerous phenomenon called MACH-tuck, leading to
catastrophic loss of the airplane and the persons onboard.
e. MACH-tuck is caused principally by two basic factors:
(1) Shock wave-induced flow separation, which normally begins near the wing
root, causes a decrease in the downwash velocity over the elevator and produces
a tendency for the aircraft to nose down.
(2) Aftward movement of the center of pressure, which tends to unbalance the
equilibrium of the aircraft in relation to its center of gravity (CG) in
subsonic flight.
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AC 61-107 01/23/91
f. The airplane's CG is now farther ahead of the aircraft's aerodynamic center
than it was in slower flight. Ibis dramatically increases the tendency of the
airplane to pitch more nosedown.
g. Pressure disturbances in the air, caused by an airfoil in high-altitude/
high-speed flight, result from molecular collisions. These molecular
collisions are the result of air that moves over an airfoil faster than the air
it is overtaking can dissipate. When the disturbance reaches a point at which
its propagation achieves the local speed of sound, MACH 1 is attained. One
hundred percent (100%) of the speed of sound at MSL with a temperature of
15 deg C is 760 statute or 660 nautical miles per hour. This speed is affected
by temperature of the atmosphere at altitude. Thus, optimum thrust, fuel, and
range considerations are significant factors in the design of most general
aviation turbine-powered airplanes which cruise at some percentage of MACH 1.
h. Because of the critical aspects of high-altitude/high-MACH flight, most
turbojet airplanes capable of operating in the MACH speed ranges are designed
with some form of trim and autopilot MACH compensating device (stick puller) to
alert the pilot to inadvertent excursions beyond its certificated Mmo. This
stick puller should never be disabled during normal flight operations in the
aircraft.
i. If for any reason there is a malfunction that requires disabling the stick
puller, the aircraft must be operated at speeds well below Mmo as prescribed in
the applicable Airplane Flight Manual procedures for the aircraft.
J. An airplane's IAS decreases in relation to TAS as altitude increases.
As the IAS decreases with altitude, it progressively merges with the low-speed
buffet boundary where prestall buffet occurs for the airplane at a load factor
of 1.0 G. The point where high speed MACH, IAS, and low-speed buffet boundary
IAS merge is the airplane's absolute or aerodynamic ceiling. Once an aircraft
has reached its aerodynamic ceiling, which is higher than the altitude limit
stipulated in the Airplane Flight Manual, the aircraft can neither be made
to go faster without activating the design stick puller at MACH limit nor can
it be made to go slower without activating the stick shaker or pusher. This
critical area of the aircraft's flight envelope is known as coffin corner.
k. MACH buffet occurs as a result of supersonic airflow on the wing. Stall
buffet occurs at angles of attack that produce airflow disturbances (burbling)
over the upper surface of the wing which decreases lift. As density altitude
increases, the angle of attack that is required to produce an airflow
disturbance over the top of the wing is reduced until a density altitude is
reached where MACH buffet and stall buffet converge (described in introductory
paragraph 5m as coffin comer). When this phenomenon is encountered, serious
consequences may result causing loss of control of the aircraft.
25
AC 61-107 01/23/91
l. Increasing either gross weight or load-factor (G-factor) will increase the
low-speed buffet and decrease MACH buffet speeds. A typical turbojet airplane
flying at 51,000 feet altitude at 1.0 G may encounter MACH buffet slightly
above the airplane's Mmo (0.82 MACH) and low speed buffet at 0.60 MACH.
However, only 1.4 G (an increase of only 0.4 G) may bring on buffet at the
optimum speed of 0.73 MACH and any change in airspeed, bank angle, or gust
loading may reduce this straight and level flight 1.4 G protection to no
protection. Consequently, a maximum cruising flight altitude must be
selected which will allow sufficient buffet margin for the maneuvering
necessary and for gust conditions likely to be encountered. Therefore, it is
important for pilots to be familiar with the use of charts showing cruise
maneuvering and buffet limits. Flight crews operating airplanes at high speeds
must be adequately trained to operate them safely. This training cannot be
complete until pilots are thoroughly educated in the critical aspect of
aerodynamic factors described herein pertinent to MACH flight at high
altitudes.
15. AIRCRAFT AERODYNAMICS AND PERFORMANCE. Pilots who operate aircraft at high
speeds and high altitudes are concerned with the forces affecting aircraft
performance caused by the interaction of air on the aircraft. With an
understanding of these forces, the pilot will have a sound basis for predicting
how the aircraft will respond to control inputs. The importance of these
aerodynamic forces and their direct application to performance and the
execution of aircraft maneuvers and procedures at altitude will be evident.
The basic aerodynamics definitions that apply to high-altitude flight are
contained in paragraph 5 of the introduction to this AC.
a. Wing Design.
(1) The wing of an airplane is an airfoil or aircraft surface designed to
obtain the desired reaction from the air through which it moves. The profile
of an aircraft wing is an excellent example of an efficient airfoil. The
difference in curvature between the upper and lower surfaces of the wing
generates a lifting force. Air passing over the upper wing surface moves at a
higher velocity than the air passing beneath the wing because of the greater
distance it must travel over the upper surface. This increased velocity
results in a decrease in pressure on the upper surface. The pressure
differential created between the upper and lower surfaces of the wing lifts
the wing upward in the direction of the lowered pressure. This lifting
force is known as induced lift. Induced lift may be increased, within limits,
by:
(i) Increasing the angle of attack of the wing or changing the shape of the
airfoil, changing the geometry, e.g., aspect ratio.
(ii) Increasing the wing area.
(iii) Increasing the free-stream velocity.
(iv) A change in air density.
(2) The pilot may have only varying degrees of control over these factors.
Thus, the pilot must keep firmly in mind that an aircraft will obey the laws of
physics just as precisely at its high-speed limits as it does during a slower
routine flight, and that regardless of wing shape or design, MACH range flight
requires precise control of a high volume of potential energy without exceeding
the critical MACH number or MACH crit.
26
AC 61-107 01/23/91
(3) MACH crit is important to high speed aerodynamics because it is the speed
at which the flow of air over a specific airfoil design reaches MACH 1, but the
most important effect is formation of a shock wave and drag divergence.
(4) Sweeping the wings of an airplane is one method used by aircraft designers
to delay the adverse effects of high MACH flight and bring about economical
cruise with an increase in the critical MACH number. Sweep allows a faster
airfoil speed before critical MACH is reached when compared to an equal
straight wing. This occurs because the airflow now travels over a different
cross section (camber) of the airfoil. This new cross section has less
effective camber which results in a reduced acceleration of airflow over the
wing, thus allowing a higher speed before critical MACH is reached. Sweep may
be designed either forward or rearward; the overall effect is the same.
However, rearward sweep appears to be somewhat more desirable, since it has
presented fewer problems to manufacturers of models of general aviation
aircraft in terms of unwanted design side effects. In effect, the wing is
flying slower than the airspeed indicator indicates and, similarly, it is
developing less drag than the airspeed indicator would suggest. Since less
drag is being developed for a given indicated airspeed, less thrust is required
to sustain the air-craft at cruise flight.
(5) There is a penalty, however, on the low-speed end of the spectrum.
Sweeping the wings of an aircraft increases the landing/stall speed which, in
turn, means higher touchdown speed, with proportionally longer runway
requirements and more tire and brake wear as opposed to a straight-wing design.
A well-stabilized approach with precise control of critical "V" speeds is
necessary. In other words, to achieve a safe margin airspeed on the wing
that will not result in a stalled condition with the wingtips stalling prior to
the rest of the wing and possibly rolling uncontrollably to the right or left,
the swept-wing aircraft must be flown at a higher actual airspeed than a
straight-wing aircraft.
(6) Drag curves are approximately the reverse of the lift curves, in that a
rapid increase in drag component may be expected with an increase of angle of
attack with the swept wing; the amount being directly related to the degree of
sweep or reduction of aspect ratio.
(7) The extension of trailing edge flaps and leading edge devices may, in
effect, further reduce the aspect ratio of the swept wing by increasing the
wing chord. This interplay of forces should be well understood by the pilot of
the swept-wing aircraft, since raising the nose of the aircraft to compensate
for a mild undershoot during a landing approach at normal approach speeds will
produce little lift, but may instead lead to a rapid decay in airspeed, thus
rapidly and critically compromising the margin of safety.
(8) Another method of increasing the critical MACH number of an aircraft wing
is through the use of a high-speed laminar airflow airfoil in which a small
leading edge radius is combined with a reduced thickness ratio. This type of
wing design is more tapered with its maximum thickness further aft, thus
distributing pressures and boundary layer air more evenly along the chord of
the wing. This tends to reduce the local flow velocities at high MACH numbers
and improve aircraft control qualities.
27
AC 61-107 01/23/91
(9) Several modem straight-wing, turbojet aircraft make use of the design
method described in paragraph 14h. To delay the onset of MACH buzz and obtain
a higher Mmo, these aircraft designs may incorporate the use of both vortex
generators and small triangular upper wing strips as boundary layer energizers.
Both systems seem to work equally well, although the boundary layer energizers
generally produce less drag. Vortex generators are small vanes affixed to the
upper wing surface, extending approximately 1 to 2 inches in height. This
arrangement permits these vanes to protrude through the boundary layer air.
The vortex generators deflect the higher energy airstream downward over the
trailing edge of the wing and accelerate the boundary layer aft of the shock
wave. This tends to delay shock-induced flow separation of the boundary layer
air which causes aileron buzz, and thus permits a higher Mmo. The lift
characteristics of straight-wing and swept-wing airplanes related to changes
in angle of attack are more favorable for swept-wing airplanes. An increase in
the angle of attack of the straight wing airplane produces a substantial and
constantly increasing lift vector up to its maximum coefficient of lift and,
soon thereafter, flow separation (stall) occurs with a rapid deterioration of
lift.
(10) By contrast, the swept wing produces a much more gradual buildup of lift
with no well-defined maximum coefficient, the ability to fly well beyond this
point, and no pronounced stall break. The lift curve of the short, low-aspect
ratio (short span, long chord) wing used on present-day military fighter
aircraft compares favorably with that of the swept wing, and that of other wing
designs which may be even more shallow and gentle in profile.
(11) Regardless of the method used to increase the critical MACH number,
airflow over the wing is normally smooth. However, as airspeed increases, the
smooth flow becomes disturbed. The speed at which this disturbance is usually
encountered is determined by the shape of the wing and the degree of sweep.
(12) When the aircraft accelerates, the airflow over the surface of the wing
also accelerates until, at some point on the wing, it becomes sonic. The
indicated airspeed at which this occurs is the critical MACH number (MACH crit)
for that wing.
b. Jet Engine Efficiency.
(1) The efficiency of the jet engine at high altitudes is the primary reason
for operating in the high-altitude environment. The specific fuel consumption
of jet engines decreases as the outside air temperature decreases for constant
revolutions per minute (RPM) and TAS. Thus, by flying at a high altitude, the
pilot is able to operate at flight levels where fuel economy is best and with
the most advantageous cruise speed. For efficiency, jet aircraft are typically
operated at high altitudes where cruise is usually very close to RPM or exhaust
gas temperature limits. At high altitudes, little excess thrust may be
available for maneuvering. Therefore, it is often impossible for the jet
aircraft to climb and turn simultaneously, and all maneuvering must be
accomplished within the limits of available thrust and without sacrificing
stability and controllability.
(2) Compressibility also is a significant factor in high-altitude flight. The
low temperatures that make jet engines more efficient at high altitudes also
decrease the speed of sound. Thus, for a given TAS, the MACH number will be
significantly higher at high altitude than at sea level. This compressibility
effect due to supersonic airflow win be encountered at slower speeds at high
altitude than when at low altitude.
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AC 61-107 01/23/91
c. Controllability Factors.
(1) Static stability is the inherent flight characteristic of an aircraft to
return to equilibrium after being disturbed by an unbalanced force or movement.
(2) Controllability is the ability of an aircraft to respond positively to
control surface displacement, and to achieve the desired condition of flight.
(3) At high-flight altitudes, aircraft stability and control may be greatly
reduced. Thus, while high-altitude flight may result in high TAS and high MACH
numbers, calibrated airspeed is much slower because of reduced air density.
This reduction in density means that the angle of attack must be increased to
maintain the same coefficient of lift with increased altitude. Consequently,
jet aircraft operating at high altitudes and high MACH numbers may
simultaneously experience problems associated with slow-speed flight such
as Dutch roll, adverse yaw, and stall. In addition, the reduced air density
reduces aerodynamic damping, overall stability, and control of the aircraft in
flight.
(i) Dutch roll is a coupled oscillation in roll and yaw that becomes
objectionable when roll, or lateral stability is reduced in comparison with
yaw or directional stability. A stability augmentation system is required to
be installed on the aircraft to dampen the Dutch roll tendency when it is
determined to be obje | |
January 14th, 2006, 15:14
