RNOcommctr
Topic Author
Posts: 774
Joined: Tue Jan 16, 2001 12:26 pm

Landing With Assymetrical Flaps

Sun Jul 19, 2009 1:32 am

We had an Alert II here at RNO with an AA 757 landing with assymetrical flaps. The aircraft landed safely although it looked slightly crabbed on touch-down. I would guess that with one flap extended more than the other, you would have a different lift force on each wing, resulting in one wing being higher than the other. Would additional thrust on the opposing engine be used? Or is assymetrical thrust not a serious matter? What causes this condition-- hydraulic problem?

Thanks for any information.
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rendezvous
Posts: 531
Joined: Sun May 20, 2001 9:14 pm

RE: Landing With Assymetrical Flaps

Sun Jul 19, 2009 1:48 am

Having one side with more flap means more lift on that side, and also more drag. The asymmetric lift would have to be countered with aileron. The asymmetric drag would cause the aircraft to yaw to the left or the right, and this is countered with the rudder.

Depending on the degree of asymmetry, it can be a pretty serious problem. You have to be able to apply enough control deflection to counter the asymmetry.
 
Tristarsteve
Posts: 3400
Joined: Tue Nov 22, 2005 11:04 pm

RE: Landing With Assymetrical Flaps

Sun Jul 19, 2009 9:01 am

Asymmetric flaps is very serious. Just compare the area of the flaps to the area of the ailerons and you can see that it would not take much asymmetric flap for the ailerons to be overpowered. So airliners have devices to stop it.
The usual flap motor drives a single torque tube that runs across the aircraft driving all the flaps. At the wingtips are rotary transducers that measure the movement. Any discrepancy, and the flap motor stops. Sometimes there are also brakes at the wingtips that lock on. This cannot be reset in flight. The crew will get an Asymmetric flap, or flap locked message. There are quite often also no back brakes on the individual flap drives.
All this is done to avoid asymmetry.

Now a story. We had just completed a B737-200 D check, and the aircraft went out on a manual reversion test flight. The pilot could not stall the aircraft as the left wing was dropping before he could slow to the stall. We adjusted one side of the flaps IIRC it was one turn of the flap drive tube (that turns about 250 turns flaps up to down). Problem solved.
I have seen many reports of Flap asymmetry recordrd in the log book, but never seen a real one. The flaps had always locked up.
 
boeing767mech
Posts: 806
Joined: Fri Dec 15, 2000 5:03 pm

RE: Landing With Assymetrical Flaps

Sun Jul 19, 2009 10:42 am

Quoting RNOcommctr (Thread starter):
We had an Alert II here at RNO with an AA 757 landing with assymetrical flaps. The aircraft landed safely although it looked slightly crabbed on touch-down.

N622AA on flight 1933, that was a indication problem. The airplane was turned with only a 29 minute delay.

David

[Edited 2009-07-19 03:43:53]
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Dufo
Posts: 799
Joined: Tue May 18, 1999 2:41 am

RE: Landing With Assymetrical Flaps

Sun Jul 19, 2009 8:00 pm

On embraer 120, flap assymetry is one of two things (the other is propeller overspeed) that can kill you if you don't react immediately. Luckily, some types have left and right sides mehanically interconnected to prevent such problems.
I seriously think I just creamed my pants without any influence from any outside variables.

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