Vy or best glide

Ophir

Well-Known Member
I don't know what it would be called in a big jet, but what is your Vy?

What was specifically interested in is what would be your airspeed and decent rate in fpm in a large aircraft with no power? Do those things glide much? How does operating in a landing configuration alter the decent rate?

Can a big jet or fighter recover from a power off stall?
 

jetman

New Member
Years ago an Air Canada B767 run out of fuel.At 28000 ft both engines had stoped,with only backup instruments Captain Bob Pearson elected to glide the 767 at 220 KIAS,since they did not have a working VSI the FO was calculating the rate of descent by distance travel/altitude lost ,they were able to glide from 22000 ft to an abandoned airstrip about 40 NM away,average rate of descent was 2500 fpm,the landing was safe without injuries,aicraft sustained some damage [colapsed nose gear]
 

Ophir

Well-Known Member
Oh, yeah, I think I read about that incident. Wasn't he flying from the West coast up to Alaska? That is an amazing story.
 

C650CPT

Well-Known Member
Ophir
Vy is predicated on weight, and the bigger the airplane with a larger weight envelope it can change significantly. For Max performance when I fly my 22,000# Jet I target .4 on my angle of attack indicator, this giving me my best lift over drag. I don't need to know the weight unless I want to know the ballpark airspeed prior to my finall reference to angle of attack.

We often climb out on an airspeed schedule ie:250KIAS untill reaching .67 mach, and sometimes ATC will assign an airspeed to maintain in the climb, accepting a flater climb profile. Some Class B's want you out of the area ASAP. I was departing Houston a few months back and climbing out 4000 the controller asked me to speed up, I told him I was doing 250KIAS and he advised me the Houston Class B was a "test" for higher speeds below 10,000, so I flew 320KIAS.

The Jet glides very well, very clean wing and if you don't plan your descents carefully you have your hands full trying to slow down and get down, that's why they put speed brakes and spoilers on high performance wings. Any time you add drag ie: landing flaps and / or gear you can either increase your descent rate or slow your airspeed.

There is no one answer for your last question, Assuming proper recognition and recovery techniques there is no reason no to be able to recover a jet from a power off stall. That being said ...who knows what could happen if one gets really behind an airplane.
 

jetman

New Member
The flight was from Ottawa [Ontario] to Edmonton [Alberta]
A couple of years ago an Airbus in the middle of the ATLANTIC suffered a total power loss ,also they made it safely to the Azores
 

Ophir

Well-Known Member
[ QUOTE ]
For Max performance when I fly my 22,000# Jet I target .4 on my angle of attack indicator,

[/ QUOTE ]

Do most commercial jets have AOA indicators?

I was wondering about the power off stall when thinking of what a swept wing aircraft must act like in that situation. I would think the more swept the wings, the more difficult it would be recover. In the little GA planes with the engine in the front it naturally results in a nose down attitude and therefore increased airspeed. What is the CG like on a fighter?
 

pilot602

If specified, this will replace the title that
[ QUOTE ]
I told him I was doing 250KIAS and he advised me the Houston Class B was a "test" for higher speeds below 10,000, so I flew 320KIAS.

[/ QUOTE ]

I thought ATC could simply override the restriction when they felt like it (i.e. determined there was a need/reason too). That something I just made up?
 

xdashdriver

Well-Known Member
Theoretically ATC can allow you to break the 250 below 10k speed limit. It is their discretion but I believe that in the U.S. it is a universal policy not to do it routinely (if ever!) In the UK, airplanes climbing out of LHR or LGW are often told "no speed restriction" which means they can fly as fast as they want.

Ray
 

E_Dawg

Moderator
It can be waived by the administrator, but ATC doesn't qualify as the 'administrator' as I remember... though I wish I had a reference.

The other exception is when the minimum safe speed is faster than 250.
 

Derg

Cap, Roci
Staff member
If you're doing less than 330 knots on the arrival below 10,000 to FLL, ATC will usually remind you that you're more than 12 miles out.
 

Derg

Cap, Roci
Staff member
[ QUOTE ]
I don't know what it would be called in a big jet, but what is your Vy?

[/ QUOTE ]

Best angle, I think, but the standard profile is 250 until 10k, then about 300 to 330 until reaching your cruise mach, and then maintaining that until cruise altitude.

[ QUOTE ]
What was specifically interested in is what would be your airspeed and decent rate in fpm in a large aircraft with no power? Do those things glide much?

[/ QUOTE ]

Probably depends on your airspeed when you start and the type aircraft. Like an MD-88 has a good descent rate, whereas the 737 is like a Grob motorglider.

But yes, they'll descend pretty well power off, but as well as a 150,000 chunk of metal will.

[ QUOTE ]
Can a big jet or fighter recover from a power off stall?

[/ QUOTE ]

The trick is not to stall the aircraft!
You figure after the "STALL!" aural warning, then the stick shaker, and then the stick pusher, you'd have gotten the hint. But it's usually not a "good" idea to fully stall a swept wing jet.
 

Nick

Well-Known Member
[ QUOTE ]
[ QUOTE ]
I don't know what it would be called in a big jet, but what is your Vy?

[/ QUOTE ]

Best angle, I think, but the standard profile is 250 until 10k, then about 300 to 330 until reaching your cruise mach, and then maintaining that until cruise altitude.

[ QUOTE ]
What was specifically interested in is what would be your airspeed and decent rate in fpm in a large aircraft with no power? Do those things glide much?

[/ QUOTE ]

Probably depends on your airspeed when you start and the type aircraft. Like an MD-88 has a good descent rate, whereas the 737 is like a Grob motorglider.

But yes, they'll descend pretty well power off, but as well as a 150,000 chunk of metal will.

[ QUOTE ]
Can a big jet or fighter recover from a power off stall?

[/ QUOTE ]

The trick is not to stall the aircraft!
You figure after the "STALL!" aural warning, then the stick shaker, and then the stick pusher, you'd have gotten the hint. But it's usually not a "good" idea to fully stall a swept wing jet.

[/ QUOTE ]


Actually, the Grob motorgliders I've flown seem to have a glide ratio more like the MD-88!

Nick
 

Ophir

Well-Known Member
[ QUOTE ]
Best angle, I think, but the standard profile is 250 until 10k, then about 300 to 330 until reaching your cruise mach, and then maintaining that until cruise altitude.

[/ QUOTE ]

So what are the throttles set to for take off? I am sure there are settings for all those speeds dependant on weight and atmospheric conditions, but when you say you go from 250 to 300-330 is there a dramatic increase in thrust or mostly AOA?
 

seagull

Well-Known Member
Jumped on this thread a bit late. A few points:

1. Vy does corrospond to best glide in a jet just as in any other aircraft, L/Dmax as it were. The speed varies
in each aircraft type, but usually you can sort it out by the recommended speeds with more than
one engine out in an aircraft with more than 2 engines, for twins, I think it is published somewhere.
For example, in the MD-11, it works out to be 1.3 stall +30 kts, approximately.

Glide ratio is quite good, as that is dependent on the forward speed and cleanliness of the design,
so glide ratios tend to be signifcantly better than light aircraft. The larger aircraft actually has
less surface area and frontal area for the volume of the aircraft, and that also reduces the drag a bit.

The climb speed profile is not really indicative of the Vy, as it is a tradeoff between the forward speed
and climb rate. A higher forward speed in climb can result in less overall fuel burn for the flight,
as well as time savings. Depends a lot on the cost index the individual carrier wants to use.

Yes, you can do full stalls in all transport jets out there, and they are done routinely both during
initial flight test and any time the aircraft comes out of a heavy maintenance check, as the
amount of warning the stick shaker is giving must be recorded and stall characteristics must be
good. There are configurations where the stall characteristics are bad, and there are normally
operating limitations placed on the aircraft such that line pilots will not encounter those regimes.

Descent speeds are often closer to the L/Dmax, but vary for the carriers, and due to traffic flow
are generally a compromise to allow for consistent speeds into the airport. The farther you are
from L/Dmax the greater the angle of descent. More operators descend at speeds well above
L/Dmax. The heavier you are, the higher the L/Dmax IAS will be, consequently, the aircraft will
be closer to L/Dmax at higher weights, and so a higher rate results in a better glide at the
same IAS than a lower weight will.

Finally, yes, power on takeoff varies. Max power is a function of OAT, pressure, etc., but normally
we use a reduced power that reduces the thrust by assuming a higher temperature that still allows
us to meet Part 25 performance requirements.
 

seagull

Well-Known Member
In answer to two other questions posted,

1. No, most transports do not directly read AoA, but they do use AoA for various items, such as
stall warning. We can access AoA through our CFDS system in the MD-11/MD-10, but that is
not something we train pilots to do. We also can see AoA with the flight path vector display, and just
look at the delta between that and the pitch attitude. Finally, we have a Pitch Limit Indicator,
which does give us some idea of AoA, although that has an airspeed algorithm incorporated as well.

2. No, thrust is not increased when accelerating in climb. Climb power is constant, and AoA is
reduced to allow for acceleration.
 

Derg

Cap, Roci
Staff member
Man, those are pretty words man. I remember when I was in college I used to know some of that stuff.

Now I just feel like a dog watchin' golf on TV!
j/k!
 
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