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A340 question for those who fly them

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rumpletumbler

Well-known member
Joined
Jul 3, 2002
Posts
1,209
I was on an A340 and during taxi, etc, the ailerons were fully deflected. During the takeoff roll ditto. I'm watching and wondering if we are going to do an unannounced roll on departure. I dismiss this idea as we are loaded for about an 8 hour flight and say goodbye to my wife, etc. The instant the aircraft started to rotate however they "snapped" into a neutral position. While being gratefull that they did I was still curious as to the why of it all and if it was computer or pilot controlled. By the way and the speed at which they moved I would guess computer.

RT
 
I've flown 3-4 times on a Virgin Atlantic A340 in the past 2 yrs and I've never seen that.

Very strange.

Maybe it was for a strong crosswind? Thats how we deal with it in the bugsmashers I fly anyway. Not sure about heavys.
 
Like most large aircraft, it probably has servo tabs on the ailerons. The aileron itself is actually unpowered, and floats freely up and down. The tab on the trailing edge of the aileron is moved hydraulically when the pilot makes a roll input, which "flies" the aileron up or down and rolls the airplane. The plane was probably parked with a quartering tailwind, which forced the aileron up or down. As it started the takeoff roll, the slipstream caused the aileron to neutralize and fair with the wing. Some large aircraft use a combination of inboard and outboard ailerons, and spoilers for roll control, and the surfaces that are active may change depending on if the plane is in low speed or high speed flight.
 
Not sure of the A-340 systems, perhaps it has deflected ailerons for additional lift on takeoff. That would be my guess, as some other types do have that feature.

As to what Eagle said, no, most large aircraft do NOT have servo tabs. Some of the smaller transport jets, B-727, etc, have that as a backup if hydraulics fail, and some others in that class have servo tabs with no hydraulics at all, but the larger widebodies are totally hydraulic (no backup to hydraulic control), and in the case of the A-340, the hydraulic actuators are controlled by digital FBW inputs. The A-380 will have a somewhat unique system of electric hydraulic controlled servos for backup, but most just have more hydraulic systems instead of a different system.
 
Hi

as a 330 driver I would like to give my 2 cents worth,
the 330 has NO trimdevices on the wing, all trimming is done
in the computer.
when no hydraulics present both ailerons are hanging down.
during taxi both sidesticks have to be checked for proper function.
full up/down and full left right. there is one full deflection.

also the ailerons droop when flaps are selected also done by the comp, pilots have no influence in this.

Question, did you also see the other aileron position?

also, airbus advises not to put in any aileron for T/O as done on many other aircraft.
the reason, sidestick deflection demand roll rate. which in plain terms means.if you have a small deflection of the stick, the aircraft will keep on rolling with the rate of roll which equals the stick deflection. to the max roll limit.

there is an advantage to this , but not for the TO roll, but for Xwind landings. in de landing when you put in rudder to decrab the wings stay level, no need to roll bank.

as to the ailerons snapping back to neutral, i have no answer to that. the only thing I can think of is that the computers went into another phase.
ie. there are multiple phases, in which the flightcontrol laws change in priorities.

I have never paid attention to the wing when sitting in the back.

if you have more questions let me know.

Homer...........mmmmmmmmm........donuts
 
reason i asked is
if the were both down, it would be as part of the flap selection.

if the were opposite of each other the only thing I can come up with is input from the flightdeck.

I searched in the books and I could not find anything about it.

There is a small posibility that it only applies to the 340, I am only familiar with the 330

H.
 
It has been awhile but in Feb I was also on a turkish airline a340 out of Istanbul. dont remember the particulars but on T.O. roll I too was thinking"What the Fxxx" as ailerons were doing things I had never seen.. that and after t.o. I have never seen a wing flex so much!
 
hello again,

on the subject of wingbending,
the FBW also has MLA=maneuver load alleviation.
the ailerons+ some spoilers will counter the bending moment

this can t be the answer for your question, because it works anly above 250kts and flaps at 0

H.
 

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