737 standby hyd

[Jim_12]

Recently got into a debate with a few co-workers and we were not able to come to a conclusion do to lack of resources. Will the rudder trim remain operative with a loss of sys a and b hydraulics with the standby rudder active?

[STBYRUD]

It will continue to work through the standby rudder PCU

During electric trim operation, the pilots move the rudder trim
control on the aisle stand which sends a signal to the rudder
trim actuator. The rudder trim actuator moves the rudder feel
and centering unit. This moves the rudder torque tube and the
main and standby rudder PCU input rods and it backdrives the
rudder pedals to the new commanded position. The trim
actuator movement also creates a new neutral position for the
feel and centering unit.

Reference AMM SDS 27-21-00

[aviatorhi]

The plain English way to say this is that the "trims" on all Boeing aircraft work by re-centering the artificial feel, not by moving trim tabs. So, as long as artificial feel is in play (surface is hydraulically powered) the "trims" will continue to operate.

[sky-738]

are the aileron trim usable when the Sys A and B fail?

[aozc]

aviatorhi is this only valid for the rudder? because the stab. trim still works even in manual reversion.

[Yeelep]

With A and B system hydraulics inop, you'll still have rudder and elevator trim. Aileron trim will still operate but it will be ineffective as there is no hydraulic power to hold the ailerons in the trimmed position. They will want to fair with the airstream.

[de facto]

Recently got into a debate with a few co-workers and we were not able to come to a conclusion do to lack of resources. Will the rudder trim remain operative with a loss of sys a and b hydraulics with the standby rudder active?

As long as you have electrical inputs for your rudder trim and an hydr source(stby is one),the rudder trim will power the pcu which will use the stby hydr press to move the rudder.

are the aileron trim usable when the Sys A and B fail?

The ailerons trim SWITCHES will work but the electrical input from these switches will not move the ailerons without hydraulic pressure.

So NO.

With A and B system hydraulics inop, you'll still have rudder and elevator trim

Rudder trim is working the same way as the aileron trim,electric input but hydr pressure is necessary for the pcu to move the rudder.Standby hydr is there for that reason.

Elevator trim?you mean elevator balance tabs? They are mechanical so they will help in offloading elevator forces during manual reversion.

The stabiliser trim can be electrical via the switches or manual via the handles,in any case hudraulic pressure is not needed to operate the stabiliser.

[aviatorhi]

I should have specified the aileron and rudder trims.

The trim wheel works any time it moves, electrically or manually. That is true of the 707, 727, 737, I'm not familiar with the stab trim on later Boeings.