787 Alternate pitch trim

[ElNull]

As I understand we use the Alternate pitch trim during "a complete loss of flight control signaling", however, I don't see any checklist that direct us to use Alternate pitch trim. Any idea under what circumstances exactly do we use it?

[ScepticalOptomist]

Originally Posted by ElNull

As I understand we use the Alternate pitch trim during "a complete loss of flight control signaling", however, I don't see any checklist that direct us to use Alternate pitch trim. Any idea under what circumstances exactly do we use it?

You would use them if the trim switches on the control column / yoke were inoperative. If you found you couldn’t trim the normal way, you could use the alternate trim switches instead.

No checklist required.

[ElNull]

Originally Posted by ScepticalOptomist

You would use them if the trim switches on the control column / yoke were inoperative. If you found you couldn’t trim the normal way, you could use the alternate trim switches instead.

Well, but as I understand there is a "Column cutout" feature that would disable the stabilizer if it senses a failed or jammed trim switches (either from the primary or alternate trim switches). So, I guess this means if the trim switches on the yoke fails, the Column Cutout feature will shutdown the stabilizer trim system and prevent trimming from the alternate trim switches. The flight control system will also revert to Direct mode which will even prevent pitch trim through the elevators.

Column Cutout
The column cutout function is designed to stop the effects of uncommanded pitch trim input from jammed or failed pitch trim switches. If a nose up or nose down pitch trim command from either the primary or alternate trim switches is opposed by either control column for more than 2 seconds, the column cutout function disables the specific pitch trim switch commands until the switch input indicates no trim input. If the column cutout function remains active for more than 20 seconds, i.e. the trim switch remains jammed or failed, the STABILIZER L2/R2 EICAS advisory is displayed, and the uncommanded input is removed by system shutdown.

[STBYRUD]

The alternate pitch trim is always active and will not be inhibited by the column cutout switches or even an engaged autopilot. That's the point really, the stabiliser must remain trimable even when everything else is stuck/disabled/inop.

[Fursty Ferret]

I don't think that's right. On the 787 if any trim input is opposed for long enough the relevant channel will be shut down. Doesn't matter if it's primary or alternate trim switches.

[sleeve of wizard]

1. Primary control of the stabilizer is from 2 pitch trim switches on each control wheel.

1. Each set of 2 switches makes arm and control signals: - The outboard switch makes the arm signal
   - The inboard switch makes the control signal.
   
2. Both switches must move in the same direction to move the stabilizer.
   
3. Switch contacts have interface with the ACES in the FCE.
   

5. The switches are on the outboard grips of the captain and first officer control wheels.



1. Alternate pitch trim switches control stabilizer movement for either of these conditions:

- Failure of the interface between the flight control electronics (FCE) and the horizontal stabilizer trim actuator (HSTA )

- The HSTA fails to respond to an FCE command.

2. There are 2 alternate pitch trim switches - ARM and CONTROL. - Outboard switch - ARM
- Inboard switch - CONTROL.
3. Both switches must be moved in the same direction to make a command signal.

4. Alternate pitch trim switch signals go directly to the electric motor control units (EMCU) - not through the FCEs.

5. The alternate pitch trim switches are on the aisle stand adjacent to the parking brake lever.


Flight Crew Operations manual (FCOM)

Alternate pitch trim is controlled by the dual pitch trim switches on the control stand. Both switches must be moved to command trim changes. The switches are linked electrically to the horizontal stabilizer trim actuator and then mechanically to the stabilizer. Alternate pitch trim switches have priority over primary pitch trim commands in all flight modes. Alternate pitch trim switches are inhibited when the autopilot is engaged. Pitch trim does not move the control column.