Aileron/rudder runaway trim - Procedure/Cutout Switches ?

[David36]

As far as I know, on big jets, as an example 737, there are no procedures or cutout switches for rudder/aileron trim. Is the crew able to cope with a runaway aileron/rudder trim forces?

[flyboyike]

Yes, they are.

[David36]

I would appreciate to detail a bit.

[flyboyike]

Details like what? What actual muscles are used in said overpowering? I don't know, that may be a question for a physician.

[David36]

Too late to edit the mistyped title, I'm sorry.

Ok. The question was how large are the forces on rudder pedals or yoke due to a runaway aileron/rudder trim and if those can be overpowered by pilots.

As I know, usually there are 2 switches to prevent inadvertent operation of the aileron/rudder trim, however, there are no multiple cutout switches as you have for the THS. I figure out there may be some CBs to pull in case of rudder/aileron trim malfunction, but I guess they are not easy to reach them quickly.

So, basically the concern was how are you able to deal with these runaway trim scenarios.

[RAT 5]

Is a runaway possible on aileron or rudder? Not too sure of B737, but I suspect the elevator trim is electric and the others your reposition the PCU with hydraulics. But then again, I'm not a engineer.

[LimaFoxTango]

By no means a big jet, but in the Dash 8 it requires the crew to attempt to center the trim. If centered, the rudder or aileron trim CB is then pulled.

[David36]

In the 737, aileron trim or rudder moves the neutral position via the feel & centering mechanism.

What if you can't center them? Usually you attempt this using the same switch that malfunctioned.

Pulling the CBs I think just prevents further movement (if not already reached an extreme position). In this case, can you overpower the forces imposed by the runaway trim?

[NSEU]

Aileron and Rudder Trim have a limited authority over their respective flight controls. You should be able to manually centre with your feet and hands (and go in the opposite direction to a limited degree).

The centering cams and springs will make it awkward to fly, however, because of their non-linear behaviour.

As previously stated, there are two switches per system (arm and control), so it's very unlikely that both would fail in an active position.

Unfortunately, the trim switch electrical contacts on some aircraft are such that you may only get the trim motor to stop dead in its tracks by applying reverse trim, not reverse its direction. I would still attempt to reverse it, however ;)