787 Flare

[ElNull]

Any ideas why the Spoilers bias up during 787 Autoland?

From the FCOM:

Flare

Below 350 feet radio altitude, the inboard spoilers are biased up incrementally. If the autopilot is subsequently disengaged, the spoiler bias is removed and a slight forward column control force may be required to maintain trim.

[FlyingStone]

I suspect it's similarly to flare spring bias in some other Boeing airplanes, to reduce rate of descent if the autopilot(s) are disengaged and prevent hard touchdown, as well as aid go-around.

[ElNull]

Thanks, but how would the spoilers reduce the rate of descent?

[dixi188]

Is it Direct Lift Control like the Tristar had?

[B2N2]

Auto drag function to counter ground effect?
section 9.20.19 of the 787 FCOM.

[oldmacdonald757]

dixi188

I wish it was. But sadly not. DLC is awesome

[FlyingStone]

ElNull

They don't, but the effect on the inboard spoilers seems to be that the aircraft requires a bit more nose up trim with spoilers deployed. If the autopilot is then disengaged, the spoiler bias is removed (i.e. the spoilers retract) but the nose up trim still exists, which will mean that pitch will increase slightly and that should arrest the descent to some extent.

PS: Not 787 rated, just guessing based on similar behaviour on other Boeing aircraft, as well as what the note says (slight forward pressure required means that aircraft is slightly out of trim in the NU direction).

[roundsounds]

B2N2

the auto drag function is gradually removed below 500’ RA, so wouldn’t help with the ground effect.
the spoiler bias forces a slight nose up trim condition.

[Fly3]

It may be part of the Landing Attitude Modifier system which partially deploys the inboard three spoilers in certain conditions.