Cessna's electric trim

[jtsastre]

One of the more common questions I get out of new students is why the electric trim on the left seat yoke of the newer Cessnas has two switches. My first guess was for redundancy. If one switch is pressed, it will not activate the trim. Also if one shorts or gets stuck it won't cause runaway trim. I've heard also that one switch drives the trim motor while the other activates a clutch, meaning the trim can only be activated by pressing both switches. Any validation for these? Thanks for the help.

Jtsastre

[USMCmech]

Having two switches prevents (hopefully) accidentaly runaway trim from a stuck switch.

One closes the clutch on the trim accuator

One powers the motor.


You must hit BOTH to move the trim tab.



You can check the electrical schematic in the service manual.

[jtsastre]

Thanks. I just saw too that I accidently title the thread "trip" instead of "trim".

Jtsastre

[nosehair]

It also prevents accidental activation of the switch by the pilot. A casual brushing of the edge of the 2-piece switch will not activate it. It takes a more positive thumb on the switch(es) to make it(them) activate.

[trafficinsight]

It's so that it doesn't matter how you touch the switch, one half will be broken... therefore the trim will never work no matter how hard you try

I prefer to just grab the wheel... only had that break once