Rudder usage

flying_ME

New Member
I'll start by saying that I am a mechanical engineer with hopes of someday getting a PPL (still looking for that couch cushion with 5 large underneath it). I came across the investigation of an Airbus crash which investigators believe may have been caused by large rudder movements (http://abcnews.go.com/sections/travel/DailyNews/Flt.587_probe021029.html). There are several factors involved (was the rudder movement caused by the pilots or some malfunction, etc) but I was suprised to find out that the vertical stabilizer can be overstressed from moving the rudder. For the pilots: does this concern you at all or is rudder usage rare enough that it is acceptable?
 

Zone5AB

New Member
I wonder the same thing. Seems as though you should be able to throw the rudder around without snapping of the vert. stab. I am concerned that we don't have enough "history" with composite materials.
 

pscraig

Well-Known Member
Yes, it does bother me. All I have flown is small sigle engine aircraft so far, but have used full rudder deflection in many cases, including spin recovery.

And no, rudder usage isn't rare.
 

Derg

Cap, Roci
Staff member
Well, the captain and I were discussing this tonight and came to the same conclusion.

Since flying large multi-engine jets, we rarely, if ever, besides takeoff roll or landing in a crosswind ever use the rudder. The only time I EVER aggresively used the rudder is in ad hoc training in the simulator when we'd replicate the B-737 rudder "hard over" situation like what happened with USAir and UAL.

Beyond that, we arrived at the conclusion that it's easier to blame dead pilots because they're not able to defend themselves.

We had an FAA jumpseater about six months ago and his quote was "...well, if we grounded the A-300 to investigate, we'd run American and Airbus out of business..."

So basically, it's easier and more financially sound to blame dead pilots.

Thinking about flying the MD-88/90, I only use rudder for alignment during takeoff until we clear the runway, and only in the last 50 feet prior to flare during landing. Other than that, the yaw damper takes care of about 99.999% of the rudder activity.
 

Pilot21

Well-Known Member
I agree entirely with Doug on this one. The only time we use the rudder is on the takeoff roll and just prior to landing to align yourself and even then we dont have to use much rudder.
 

IrishSheepdog

Sitting in the median
Many aircraft have rudder limiting units to keep rudder movement to a small amount at higher airspeeds. For instance the ERJ-135/140/145, ATR-72, Saab 340B. This is because at higher airspeeds, a full rudder movement could cause structural damage. I'm sure that many other aircraft have the same feature.

The above examples work in one of two ways. In the ERJ, there are two rudders, both combined. It is basically taking the whole rudder, and cutting it in two longways. At higher speeds, one of the rudders is locked, restricting rudder deflection angle.

In the ATR and Saab, the rudder has a limiting device, that limits deflection angle of the whole rudder (not split). This is also activated at higher speeds.

When I was working at Trans States, we had an ATR-72 that had a malfunction of the rudder limiting unit. It had to hold until the problem was fixed, because as the captain explained "if we get above a certain speed and kick that rudder in the event of an engine failure, we could bend the fuselage. It's just too long."

I have to question whether the rudder problem on the A300 was truly rudder movements as stated. From the procedures I have had experience with (B737, B757/B767), yaw damper is engaged before start/before takeoff. This limits rudder movement, and has "automatic" (in a sense) movement for rudder changes. If the crew had this engaged, and were below 250 knots (maybe lower), how could they have overstressed the vertical stabilizer if it was in good condition? Even without yaw damper, it would be some nasty inputs to do that.

The other day I was flying a single engine ILS with a nasty crosswind and windshear, and was fighting the plane the whole time (B737). At times I was putting in a lot of rudder when the shear hit, and we didn't have problems. Of course it is a simulator. But what I'm getting at is directional control was maintained, and the inputs were not so drastic as to cause structural failure.

IMHO, I don't think that the American A300 crash in JFK was caused by pilot inputs alone.

Am I worried? Not really. I figure if it is going to happen such as it did in JFK, well there might not be much I can do about it. I will fight the plane to the end trying to save myself, my crew, and my passengers, but if it is structural, what can you really do?
 

seagull

Well-Known Member
Regardless of the source of the rudder movement, if you move the rudders first one way and then the other as has been reported in the AA accident, it will exceed the design overload limits on ANY transport jet out there, that includes the entire Airbus, Boeing, Douglas and Lockheed lines. This has been well known in the transport and certification community since the 1960s if not before. Not sure how it came to be that line pilots were not aware of it, but that's the facts.
 

Derg

Cap, Roci
Staff member
The only question I have is that your first instinctive control input in a wake turbulence even is to use the ailerons. I generally only use the rudder, other than takeoff roll (because I let the aircraft establish a crab during the initial climb) is during a crosswind landing . And even liberal usage of the rudder during a crosswind landing is highly uncomfortable because it's somehting that just isn't used often in a swept-wing jet aircraft.

The tail of the Airbus came off fairly cleanly, I'm sure if it was a failure due to reaching the maximum load of the surface, you'd have some 'tell tell' signs of torsion, tension or some other type of pressure dynamic on the surface.

Personally, in my cynical opinion, I really think it's easier to blame dead pilots due to pilot error, while quietly working out the problem with the FAA and Airbus to avoid grounding the entire US-operated fleet -- similar to what happened with the ATR accident.

"Oh, the plane is safe, fly it at will and don't use the autopilot during icing conditions"

[work work work work]

"Ok, we fixed the problem, use the autopilot in icing... all is well, carry on..."
 

seagull

Well-Known Member
As for rudder useage, it depends on what you were taught, the push on rudder useage happened before your time, I think. All of this will come out in the wash, the WSJ recently had an article that got into it in detail.

On the clean failure, it did show signs of overload at the points one would expect, based on the scant evidence released to the public.
 

IrishSheepdog

Sitting in the median
Wouldn't operation with the yaw damper have kept this from occuring? I'm not sure how low they were when this happened, but in most SOPs I've seen the YD should be on out of 400' AGL.
 

seagull

Well-Known Member
Yaw dampers are never shut off on most larger aircraft, and the yaw damper has no effect in this case.
 

averyrm

Well-Known Member
[ QUOTE ]
How do yaw dampers work?

[/ QUOTE ]

They dampen the yaw


Seriously though, they basically are a gyro and servo that work together to cancel out yaw oscillations through the manipulation of the rudder. They also keep turns coordinated and they cut down on turbulence "feel" in the cabin.
 

E_Dawg

Moderator
I think it was AOPA Pilot that had the article about the crash and its implications on our flying. It pretty much said that planes are tested to withstand a single control deflection below Va. If you do one max deflection followed by another in the opposite direction, you will really load up the part regardless of airspeed.
 

FDXA300

New Member
14 CFR part 25, subpart C, are the regulations which pertain to strength and load capabilities of the airplane structure. although many pilots believe that flyingat or below Va will allow for full deflections of the controls without concern for strutural damage, this is not srrictly true for a transport category airplane. FAR 25.331 and FAR 25.349relate the full deflection design loadsto Va for the aileron and elevator, but not the rudder. FAR25.351specifies the requirement for the tail loading for yawing conditions. at any speed within the range of Vmc to Vd-design dive speed- with the airplane in unaccelerated fligt at zero yaw, it is assumed that the rudder control is suddenly displaced to the maximum deflection, as limited by the control surface stops, or by a rudder pedal design forcelimit, whichever is less. with the rudder deflected as specified, it is assumed that the airplane yaws and stabilizes at the resulting sideslip angle. it is then assumed the rudder is returned to the neutral, faired position. there is NO, repeat, NO requirement for the tail to withstand the load from asudden and full rudder reversal the intent for this requirement is that the tail is able to withstand the induced load from a sudden and full rudder deflection within the normal operating speed range to counter the loss of engine thrust. far 25.351 also defines the load requirements in response to a lateral gust.
Summary: the tail fin combination is designed to a loading criteria that allows for deflection, within prescribed limits, through the speed range of Vmc to Vd, but pilots must be aware that rudder reversals from a large side slip condition could expose the airplane to loads beyond limit design loads, even at speeds below Va.
FDR data forthe AA airbus indicates several full rudder reversals in rapid succession......in 20+ years fo training and checking jet transport crews, i have NEVER seen those pilot inputs.......i can't speak for my airbus computer controlled rudder actuator/computer interface..............hope this wasn't too long winded.
regards to all
 

EricT

New Member
Speaking of rudder usage...

For folks in the IFR environment. When on an ILS and you have let's say a 1 dot deflection from center on the CDI do you correct with aileron or rudder? I have heard pros and cons for each. Thanks.
 

MikeD

Administrator
Staff member
Correcting with aileron works well. If using an HSI, make corrections no more than the width of the heading set marker (or captain's bars), or if using a DG, no more than 5 degrees either way. Anymore is normally too much, especially inside glideslope intercept distance, or the OM.

MD
 
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