Why do flaperons 'move down then up' on takeoff roll on a 787? The flaperons on the 787 go through certain motions once the TO/GA is pressed. I've always seen it when the 787 is viewed from behind on takeoff. The motions are completely symmetrical, i.e., not related to pilot inputs.

Video example 1
Video example 2

I'm wondering why they're programmed to do that. The primary flight computers (PFC's) go through a complete self-check after every flight. Testing the ranges of motion is done by the crew after engine start-up. Seems like the wrong time to run a test.

The drooped outer ailerons do not go through a similar 'check'. My thoughts: calibration for their position, or drag reduction during initial roll. The latter would not explain the full down deployment though.

The frames below are from an on board video for a clearer image:


1) Taxi
2) TO/GA
3) Slowly full up
4) Back to normal.
I found this 16-year old thread on airliners.net. The Boeing 777 has the same system, and at the time it was unique in that. (You can watch it here.)

When takeoff power is applied, hydraulic power is removed from the flaperons, as not to stress the actuators for an extended duration, which causes them to drop due to gravity. They are then slowly lifted by the rushing air. At around 100 knots, power is restored to them.

The stress being avoided is from the engine's jet blast. During ground runs for an engine, the flaperon is known to flutter.



The poster cites the B777 Maintenance Training Manual Rev 4 02/08/1997. The reasoning comes from this other 12-year old thread.