Failure Modes for Garmin GSA 28 Servos Installed to Drive 3rd Party Trim Motors

[1001001]

After reading through the G3X installation documentation, it's not clear to me what the behavior of the aircraft trim system would be if the trim motors (e.g., Ray Allen) were installed to be driven by the GSA 28 servos. Garmin states that the trim motor current is limited to 1A when connected through the GSA 28, but it's not explicitly stated if the power is simply passed through from the trim switches. The closest clue I can find is Figure 24-1.10 in the current (version AT) G3X installation manual, which shows 12V applied to the trim switches, but it's unclear whether that power is passed through in the event of a GSA 28 failure.



It's also not clear if there are failure modes of the GSA 28 that could cause a complete failure of the trim circuit.

Any thoughts? What am I missing?

 

[mburch]

After reading through the G3X installation documentation, it's not clear to me what the behavior of the aircraft trim system would be if the trim motors (e.g., Ray Allen) were installed to be driven by the GSA 28 servos. Garmin states that the trim motor current is limited to 1A when connected through the GSA 28, but it's not explicitly stated if the power is simply passed through from the trim switches. The closest clue I can find is Figure 24-1.10 in the current (version AT) G3X installation manual, which shows 12V applied to the trim switches, but it's unclear whether that power is passed through in the event of a GSA 28 failure.
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It's also not clear if there are failure modes of the GSA 28 that could cause a complete failure of the trim circuit.

Any thoughts? What am I missing?

The power from the trim switch passes straight through to the trim motor if the GSA 28 is powered off. If the GSA 28 is powered on, the trim signals from the switch are used as inputs. The AP DISC input will also electrically interrupt the GSA 28's ability to drive the trim motor (when the GSA 28 is powered on).

 

[smt6]

In this scenario, with the A/P engaged, will activating the trim via the stick, cause the A/P to disengage? (i.e. a "trim up" message on G3X)

 

[Raiz]

In this scenario, with the A/P engaged, will activating the trim via the stick, cause the A/P to disengage? (i.e. a "trim up" message on G3X)

There are actually 3 states for the autopilot: power off, power on and engaged. In the power on state, the trim switch provides input to the servo, which operates the trim motor. In the engaged state, there is autotrim (the servo controls the trim), so you wouldn't need to operate the trim switch. More detail here.

 

[rstory]

In this scenario, with the A/P engaged, will activating the trim via the stick, cause the A/P to disengage? (i.e. a "trim up" message on G3X)

This is the behavior (trim switch activation disengages AP) I have experienced with a G5/GMC 507 setup.

 

[Dugaru]

This is the behavior (trim switch activation disengages AP) I have experienced with a G5/GMC 507 setup.

Same with a G5/307.