GMA 35c and GTN 750xi help understanding failure modes

[Max Clark]

My understanding is if the GMA 35c "fails" it enters a state where the pilot is effectively hard wired to COM 1 (I think this is fairly standard for audio panels).

My question is what happens if the GTN 750 fails or is offline?

Would COM 1 actually be wired to the second device in the plane with the 750 wired as COM 2 on the audio panel?

How does this work with the audio panel integrated with the primary COM/NAV?

Thanks!

 

[Vance Harral]

54 minutes ago, Max Clark said:

Would COM 1 actually be wired to the second device in the plane with the 750 wired as COM 2 on the audio panel?

How does this work with the audio panel integrated with the primary COM/NAV?

Yes, in this kind of arrangement, the shop is supposed wire the audio lines of your "second" radio to the COM1 inputs of the GMA 35c, and the GTN750 to the COM2 inputs (i.e. wire them "backwards" relative to normal use).  The GMA 35c is then programmed to logically swap the two COM devices in software for normal operation, such that the GTN appears to be COM1.  But if power to the GMA35c is subsequently lost, the internal relays that close will wind up connecting the second radio to your headset, avoiding the problem of having a working COM2 radio that you can't select because the COM1 GTN you would otherwise use to select it is offline.  Note that this concept is often misunderstood or not correctly implemented by avionics shops, you can find several complaints about it on Beechtalk.

Having said that, the engineer in me is pretty skeptical about this stuff.  It's neat to demonstrate that when you turn off power to your audio panel, internal mechanical relays close that short radio wiring directly to your headset.  But a simple loss of power is not the only manner in which an audio panel can fail, and in fact I doubt it's even the most common failure mechanism (anecdote: I've never had an audio panel lose power, but I've logged several occurrences of headset jack failure, which requires a completely separate backup strategy).  Furthermore, this idea of making COM2 the "failsafe" instead of COM1 is all well and good, but it's not clear to me there's any kind of rollover mechanism that makes COM2 the failsafe only if COM1 loses power at the same time your audio panel loses power.  I think it's one or the other, exclusively; and I figure if you're unlucky enough to lose power to both your audio panel and only one of your two COM radios, Murphy's law dictates that whichever COM radio died will be the one that's wired as the audio panel failsafe.  Accordingly, I'm not sure why the "swap the COM inputs" trick actually buys you much.

I'd advise you to not worry too much about it and just squawk 7600 and carefully get on the ground NORDO in the event of a problem.  But if you've got the coin for a GMA35c, you probably have a GTX-345R remote transponder too, that you can't control if your GTN goes offline.

[Pinecone]

16 hours ago, Vance Harral said:

I'd advise you to not worry too much about it and just squawk 7600 and carefully get on the ground NORDO in the event of a problem.  But if you've got the coin for a GMA35c, you probably have a GTX-345R remote transponder too, that you can't control if your GTN goes offline.

A good reason to have a G3X.

 

[Max Clark]

17 hours ago, Vance Harral said:

Yes, in this kind of arrangement, the shop is supposed wire the audio lines of your "second" radio to the COM1 inputs of the GMA 35c, and the GTN750 to the COM2 inputs (i.e. wire them "backwards" relative to normal use).  The GMA 35c is then programmed to logically swap the two COM devices in software for normal operation, such that the GTN appears to be COM1.  But if power to the GMA35c is subsequently lost, the internal relays that close will wind up connecting the second radio to your headset, avoiding the problem of having a working COM2 radio that you can't select because the COM1 GTN you would otherwise use to select it is offline.  Note that this concept is often misunderstood or not correctly implemented by avionics shops, you can find several complaints about it on Beechtalk.

Having said that, the engineer in me is pretty skeptical about this stuff.  It's neat to demonstrate that when you turn off power to your audio panel, internal mechanical relays close that short radio wiring directly to your headset.  But a simple loss of power is not the only manner in which an audio panel can fail, and in fact I doubt it's even the most common failure mechanism (anecdote: I've never had an audio panel lose power, but I've logged several occurrences of headset jack failure, which requires a completely separate backup strategy).  Furthermore, this idea of making COM2 the "failsafe" instead of COM1 is all well and good, but it's not clear to me there's any kind of rollover mechanism that makes COM2 the failsafe only if COM1 loses power at the same time your audio panel loses power.  I think it's one or the other, exclusively; and I figure if you're unlucky enough to lose power to both your audio panel and only one of your two COM radios, Murphy's law dictates that whichever COM radio died will be the one that's wired as the audio panel failsafe.  Accordingly, I'm not sure why the "swap the COM inputs" trick actually buys you much.

I'd advise you to not worry too much about it and just squawk 7600 and carefully get on the ground NORDO in the event of a problem.  But if you've got the coin for a GMA35c, you probably have a GTX-345R remote transponder too, that you can't control if your GTN goes offline.

Wanting a modern, integrated, avionics suite coupled with limited panel space really drove the decisions. If you want a MFD over the passenger yoke there's only a couple paths you can take.

I'm expecting everything to fail at some point, it's just the nature of things. For example:

Is the iPad+Sentry AHRS ideal in flight? Absolutely not.

Have I navigated with it in VFR conditions to know what it would be like? You betcha.

I carry a Sporty's PJ2+ in arms reach for the same reason.

[Max Clark]

45 minutes ago, Pinecone said:

A good reason to have a G3X.

For clarity I understand that the GTX-345R is wired into the GTN 750, so if the 750 goes you lose control of the GTX with it.

[Pinecone]

1 hour ago, Max Clark said:

For clarity I understand that the GTX-345R is wired into the GTN 750, so if the 750 goes you lose control of the GTX with it.

Not sure where it goes to first, but it is controllable from the G3X.  I have to ask my avionics shop