Garmin GAD 27 Electricity Flow (trim)

[kirkbauer]

I'm working on wiring for the Garmin GAD 27 and have some questions about how power goes in and out related to the trim servos.

First, some basics. There are two power inputs so the GAD 27 can be powered from two separate busses -- J271 pin 7 and TB273 pin 1. Technically the latter is for the "keep alive" feature but the manual says it can also power the GAD 27.

Second, the lights (presumably landing/tax) are easy -- light 1 power in, light 2 power in, light 1 power out, light 2 power out. The flaps are just as easy -- flap power in, flap power out (there is also a flap power ground, but no ground for the lights).

Now my assumptions about trim -- there are inputs for PITCH TRIM POWER IN, PITCH TRIM POWER GND, ROLL TRIM POWER IN, and ROLL TRIM POWER GND. As I understand it, if you are using the GSA 28s for autopilot, and if you want autotrim, you should connect the trim motors to those units, and then you connect ROLL TRIM OUT 1/2 and PITCH TRIM OUT 1/2 from the GAD 27 to the appropriate GSA 28. It also seems that, in normal situations, not much power flows from the GAD 27 to the GSA 28s -- the actual power for the trim motors comes directly from the GSA 28s, and the connection from the GAD 27 is just a signal.

Finally, I have this information from @g3xpert about how/when the GAD 27 does directly power the trim motors:

In normal operation the GSA 28 servos are always powered-up, and even when the autopilot is disengaged, the servos are providing speed scheduled variable voltage to the trim motors independent of the GAD 27. The GAD 27 is only providing trim direction signals to pins 11,12 of the GSA 28 the majority of the time.

It is only when the GSA 28 is powered off (a trim bypass relay drops) and non speed scheduled direct drive power is being supplied by the GAD 27 to the trim motor does the size of the GAD 27 trim power circuit breaker make any difference.

So, here is my actual question -- how do I wire two pullable circuit breakers -- one that can disable autopilot and a second one that can disable the trim servos?

I think Autopilot is easy -- I can have a single circuit breaker and wire all of the GSA 28s through that breaker. If that breaker is pulled, the trim system will work as described in the quote above.

Now how do I disable the trim system through a circuit breaker? If I interrupt PITCH TRIM POWER IN and ROLL TRIM POWER IN into the GAD 27, will that prevent it from passing along any signals to the GSA 28s regarding trim? Even though the trim is actually being powered by the GSA 28s? I'm looking for a quick way to disable both the pitch and roll trim servos quickly in a runaway situation. Pulling a breaker to disable the GAD 27 is not ideal since it does other things as well, and since I'll be using two power inputs.

 

[Watzlavick]

I put the pitch and roll servos on a single breaker on one bus, then used a 2nd breaker for the bypass trim power on the other bus. Upside is two separate power sources for trim, downside is you have to pull 2 breakers for runaway trim. But as long as the GAD 27 is up, it should limit the amount of trim with a stuck switch (I think you can configure that, haven’t got to it yet).
Edit: It appears the max run time setting for trim is handled in the GSA 28. So if you had a stuck trim switch and then pulled the breaker for the servos, you could get the continuous trim motion until you also pull the separate trim.
-Bob

 

[kirkbauer]

So I think I figured out something interesting for my dual-bus architecture. The GAD 27 has two power inputs (the normal one and the "keep alive" one, but per the manual it can power itself from both). So that lets me connect it to both of my busses.

If I have the TRIM PWR inputs connected to bus #1 and the autopilot servos connected to bus #2, then I believe that means my trim will work with either bus offline.

Now, the GAD 27 is a single point of failure, however the manual says it when it is offline it connects the pilot's trim controls directly to the trim outputs, so theoretically the trim will still work.

 

[Traash]

Use the 'motor disconnect' feature in the GAD27 plus an AP disconnect that removes power from the GSA 28's.

 

[SteveHicks]

I am based in the UK, and the authority here which oversees homebuilt aircraft (Light Aircraft Association) stipulates that any computer controlled trim and flap systems should have a means to be isolated. Additionally, in the case of trim, preferably a means to reverse a trim runaway.

In a typical Garmin G3X system, the GAD27 and GSA28 servos both have an input into the trim servo (probably a Ray Allen or equivilent). Hence to electrically isolate the trim servo from the GAD27 and GSA28, BOTH must be removed from the system. The attached diagram shows how I did this in my RV14 G-STRV. It uses the "passthrough" facility in the GSA28 which is activated when its power supply is removed.

 

[kirkbauer]

Very helpful, and it aligns with what I learned (eventually) about how the Garmin system handles trim. Thank you!

 

[Halleffect]

"It is only when the GSA 28 is powered off (a trim bypass relay drops) and non speed scheduled direct drive power is being supplied by the GAD 27 to the trim motor does the size of the GAD 27 trim power circuit breaker make any difference."

So, based on this info from @g3xpert, without a GAD-27 (using VP-X running trim through the GSA servos), removing power from the GSA servos will interrupt trim if I understand correctly. I haven't been able to test this yet.

 

[mburch]

So, based on this info from @g3xpert, without a GAD-27 (using VP-X running trim through the GSA servos), removing power from the GSA servos will interrupt trim if I understand correctly. I haven't been able to test this yet.

On the contrary - and regardless of whether or not you have a GAD 27 - if power is removed from a GSA 28 servo, its trim input and trim output pins are directly connected together, as described in section 23.13.7 of the current G3X installation manual (revision AU):

If the GSA 28 is not powered, a fail-safe feature causes the trim inputs to connect directly to the trim outputs.

 

[Halleffect]

On the contrary - and regardless of whether or not you have a GAD 27 - if power is removed from a GSA 28 servo, its trim input and trim output pins are directly connected together, as described in section 23.13.7 of the current G3X installation manual (revision AU):

Got it, thanks, Matt. I recall seeing that sentence now.