G3X Trim Reverse switch trips circuit breaker

[storkeye]

Hi All,

I'm in the process of wiring my panel and all was going well until today. I had planned on having a trim reverse switch on my panel in case of trim runaway to bring the trim back in to a neutral position to ease stick forces. The idea is if the trim is running away, just reverse it towards neutral and then switch it off.

I have a Ray Allen trim motor wired into a GSA 28 AP server for auto-trim. From the servo it goes to the GAD 27 J272 pins 11 & 12 (manual calls these 'Pitch Trim Out 1&2').

From the GAD 27 (J272 pins 1 & 2) it goes to a DPDT ON-OFF-(ON) switch and then from the switch it goes to the Circuit breaker and ground.

So the overall connections are like this...

Battery -> Circuit Breaker -> Trim Rev Switch -> GAD 27 -> GSA 28 AutoPilot Pitch Servo -> RA Trim Servo

The switch is wired as follows:

Pin 1 +12v from CB
Pin 4 Ground
Pin 2 & 5 output to GAD 27
Pin 3 is connected to Pin 4 above
Pin 6 is connected to Pin 1 above

With the J272 connector disconnected from the GAD and testing with a multi meter the trim switch works a dream. I can connect the meter to pins 1 & 2 (trim input) of the J272 plug and the voltage from the circuit breaker reverses as expected when the switch is moved from the ON to the momentary ON position.

However, when the connector is attached to the GAD 27 the trim works perfectly only in the ON position. As soon as it is moved to the momentary position the Circuit Breaker trips.

To rule out the CB I hooked up an inline fuse and that pops in the same test so that confirms the breaker is working correctly.

I have already set a 1 second maximum trim run time in the G3X settings. So, this switch is really a final line of defence.

Does anyone else have a trim reverse switch in their GAD 27 system and seen a similar issue?

If this is a bad idea, I can obviously remove the switch, but is there a way to reverse the trim from the PFD during flight?

Or, is there another reason why this is tripping the CB that I might have missed at all?

Thank you in advance for any help or suggestions.

Abe

 

[mburch]

From the GAD 27 (J272 pins 1 & 2) it goes to a DPDT ON-OFF-(ON) switch and then from the switch it goes to the Circuit breaker and ground.

These two pins are not switch inputs, they are the power and ground inputs for the pitch-trim subsystem of the GAD 27. What you're actually doing here is connecting power to the ground pin and ground to the power-supply pin, so it's no wonder your breaker pops. Please don't do this, it's hard on the equipment.

 

[Walt]

I'll get right to the point; BAD IDEA!

 

[storkeye]

Thanks both, for info connector J271 is for the switch inputs. J272 is a separate 15 pin connector and for trim power in and out.

Reading through the manual in a bit more depth and, having had time to think it through, I can see that the switch should be between the GAD and the autopilot servo to achieve the desired outcome.

As you rightly say, Pins 1 and 2 are power and need the correct polarity. Pins 11 & 12 are outputs and I think these can be reversed as they will simply drive the motor in the opposite direction.

 

[lr172]

So you ask if this is a bad idea and an avionics installation expert says yes and then you proceed anyway? That is your prerogative of course, but why ask? Either way, your approach will require much concentration and possibly two hands, all at a time when you will probably need two hands on the stick to keep the trim from trying to kill you; either direction will do it - stall/spin or breakup from exceeding Vne. Instead of thinking up new ways to make your electrical system more complex, your time might be better spent going up in the plane applying full up trim and learn how to deal with it. Your approach, imho, does not eliminate the potential for runaway trim.

And what if your passenger bumps this switch and your autopilot starts aggressively trimming in the wrong direction?These things actually happen. Just the other day, i was in moderate turbulence and while reaching for an ap knob for an atc vector, I accidentally turned off my efis. Little bit of excitement there, as i didn’t realize i hit the switch. Stuff happens and we need to be able to react and adapt in a calm manner and this comes from confidence built with training. And we learn from it; that should have been a locking toggle.

 

[Richard Connell]

I’m with Walt. Bad idea.

Firstly, changing a runaway trims direction from nose up to nose down runaway isn’t going to solve any aircraft control problems. It’ll likely make it worse no matter how well you think you can pin neutral while it’s misbehaving. I can picture this being a very bad distraction in a critical phase of flight.

Secondly, all the 2 seaters are perfectly flyable at the trims extreme range with moderate stick force. No stick force if you’re happy to just fly slowly with trim stuck runaway full nose up. I’ve tested it. Once intentionally and once with a failed trim motor.

Thirdly -as you’ve found- unnecessarily monkeying with a very well engineered product in a way not intended by the designers - will probably end up being the cause of your runaway or failure of your trim system in the first place.

Lastly - the most likely culprit for a runaway will be an internal failure of the trim motor or a AP servo which will render your switch useless anyway.

Id strongly recommend wiring everything IAW the Garmin install manual. Incorporate the motor disconnect functionality and/or the separate means of disconnecting power to the entire AFCS and trim system that is in easy reach. If it’s misbehaving pull the breaker and fly the plane.

 

[Dr_Dave_MT]

Those RA servos move FAST, very sensitive. I would not find the ability to “fly the Damn Airplane!” AND be able to find a switch, breaker, CB, to flip or disconnect. Fly the airplane, fly the airplane, fly the airplane. Please!

 

[storkeye]

Thanks everyone for the replies and I apologise if I came across as ungrateful and for not spotting that feedback was from an avionics specialist.

Richard I really appreciate the detailed explanation of why it's a bad idea.

This is such a great forum with so much experience and knowledge so I've decided to take everyone's advice and bin the switch!