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n797mx
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787 Spoilers And Ailerons

Tue Jan 15, 2013 3:30 am

http://www.youtube.com/watch?v=gtMwzToEj1k

If you notice at 4:50-ish that the outboard spoilers and ailerons momentarily extend up and then retract. Does any one know why it did that?
Clear skies and strong tail winds.
 
CM
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RE: 787 Spoilers And Ailerons

Tue Jan 15, 2013 4:23 am

The 787 has an "autodrag" function which uses the ailerons and 2 outboard-most spoilers to automatically spoil lift / add drag when the airplane is above the glideslope. Autodrag works in conjunction with the autotrottle to automatically capture the vertical path without increasing or losing airspeed. I was under the impression this feature caused the ailerons to deflect down and the spolers up, but after looking at your video, I wonder if what we are seeing is the autodrag function in operation.

The other time outboard spoilers and ailerons will go up is when maneuver load alleviation activates, but this is obviously not the case in your video. You can see MLA active in this video as the 787 completes its turn http://www.youtube.com/watch?v=5JhbH...28&feature=player_detailpage#t=23s
 
BoeingGuy
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RE: 787 Spoilers And Ailerons

Tue Jan 15, 2013 4:32 pm

Quoting CM (Reply 1):
The 787 has an "autodrag" function which uses the ailerons and 2 outboard-most spoilers to automatically spoil lift / add drag when the airplane is above the glideslope. Autodrag works in conjunction with the autotrottle to automatically capture the vertical path without increasing or losing airspeed. I was under the impression this feature caused the ailerons to deflect down and the spolers up, but after looking at your video, I wonder if what we are seeing is the autodrag function in operation.

Where did you hear this? I asked some experienced 787 people and they've never hear of it either. The L1011 had something (DLC) like this, but to the best of my knowledge no Boeing airplane had any kind of "autodrag" function or uses the spoilers in connection with the glideslsope or autothrottle.

I'm guessing the autopilot - or manual flight - commanded a roll to the right past the point at which the spoilers augment the roll.
 
CM
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RE: 787 Spoilers And Ailerons

Tue Jan 15, 2013 5:06 pm

Quoting BoeingGuy (Reply 2):
Where did you hear this?

Well, I've known about it for a long time. Now that you ask this question, you've made me go back and look for it... I don't have a public source for you. However, if you have access to one, you can read about it in section 9.20.19 of the 787 FCOM.
 
BoeingGuy
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RE: 787 Spoilers And Ailerons

Tue Jan 15, 2013 5:11 pm

Just happen to have a 787 FCOM handy. Yep, it's in 9.20.19 like you stated. I stand corrected.

I almost posted the section, but I believe the FCOMs are Proprietary info.

However you were right the first time. The aileron deflects downward. So I don't think that's what's depicted in the video. I think it was just a spoiler deflection to augment a roll input.
 
CM
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RE: 787 Spoilers And Ailerons

Tue Jan 15, 2013 6:13 pm

Quoting BoeingGuy (Reply 2):
The L1011 had something (DLC) like this, but to the best of my knowledge no Boeing airplane had any kind of "autodrag" function or uses the spoilers in connection with the glideslsope or autothrottle.

I believe you are correct; the L1011 was the first commercial aircraft to have this kind of function. The implementation was quite different by Lockheed, however. I'm pretty sure DLC was heavily dependent on the "flying stabilizer", which (in conjunction with the spoilers) permitted the airplane to achieve a steep descent with minimal pitch change. I do not think the 787 feature uses the stab in any way, which makes me believe there is probably a greater pitch impact when the autodrag feature activates.

Quoting BoeingGuy (Reply 4):
However you were right the first time. The aileron deflects downward. So I don't think that's what's depicted in the video. I think it was just a spoiler deflection to augment a roll input.

Yes, after reading the FCOM, I come to the same conclusion; ailerons will be down if autodrag is active, so not what is happening in the OP's video.

To the OP: One thing to always keep in mind regarding the 787 flight control surfaces - because the 787 flight controls are augmented in all axis, there is no direct correlation between pilot control movements and control surface movements. This fact makes a one-time, discrete control surface movement much more difficult to interpret based solely on seeing the surface move, than it would be on a 767. On the 787, if there was a sudden, strong gust on the right side of the airplane, with the control wheel commanding zero degrees roll, the 787 flight controls will automatically respond by spoiling lift on the right wing - no pilot command required. On the 767, a control wheel input would be needed to counter it. I can't see a function reason (such as autodrag) for the surface movement shown in the video. Perhaps seeing the pilot control inputs would help sort it out, but that's not at all certain. To really understand it, you would likely need to pull the DFDR data.

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