Adding second GI 275

[sdstolworthy]

Hi all! I have an existing Garmin GI 275 in my panel serving as my AI. I want to add a second GI 275 as an HSI. I'm curious if I can simply add the second 275 to serve as a DG, or if there is something special about the AI/DG package that Garmin sells that would prevent me from doing a simple addition of a 275.

[201Mooniac]

Nothing I'm aware of that would prevent it as many people have both ADI/HSI combos with GI275.  It isn't clear what you mean by "simple addition".  As I understand the STC installation would have them wired together over HSDB which would allow the HSI to take over as ADI if the ADI fails.

[Flash]

I have a dual GI 275. As @201Mooniac said, the second GI 275 acts as an HSI but talks to the first GI 275, and the second GI 275 automatically switches to an ADI if the primary fails. It's a terrific set-up, especially if like me you have a King 150 autopilot. One of the advantages of the GI 275 is its compatibility with legacy autopilots like mine.

[jlunseth]

Yes, be sure you actually have the dual system installed. It requires purchasing a different part number as the primary AI. The one you have may not be the correct part number. There was an incident a year or so ago where a pilot got into trouble in IMC and had two 275’s, but thought

The pilot, an ATP, thought he had the dual system because he had two 275s but he did not, so part of the problem was that the system did not revert the HSI to an AI and he was without an AI.

Also, bear in mind that even if you have a dual system, it is not bulletproof. The weakness is that both your primary and your backup AI (the HSI in dual reversionary mode) are electric, so if you lose your electricity, say the alternator goes out, you will be without any AI at all. For that reason I left the vacuum pump in my aircraft and it runs a second simple vacuum AI as a backup.

You do not ever want to be without an AI in IMC. The old system of having a Turn Coordinator act as a sort of backup AI works fine so long as there is no significant turbulence to upset the aircraft, and so long as your skills are very up to date, but if the aircraft gets even several degrees off level the TC is not much help.

You then have 178 seconds to live so you might as well sit back and enjoy the roller coaster ride while you can. https://www.aopa.org/news-and-media/all-news/2022/june/pilot/asi-tips-178-seconds#:~:text=Search the aviation safety reporting,reports%2C or submit a report.&text=A classic study supported by,when flying VFR into IMC.

Edited October 13, 2023 by jlunseth

[JimB]

31 minutes ago, jlunseth said:

Also, bear in mind that even if you have a dual system, it is not bulletproof. The weakness is that both your primary and your backup AI (the HSI in dual reversionary mode) are electric, so if you lose your electricity, say the alternator goes out, you will be without any AI at all.

Both GI-275s will have a battery back up source that are certified to last a minimum of an hour. There is also a required capacitance check that is done annually to be sure they can provide power for that hour. 

[Utah20Gflyer]

I did things the reverse way were I installed the HSI first and then later added the AI. If you had the magnetometers, etc installed for the AI the HSI will be much cheaper and quicker to install than the first.  
 

You will love the HSI. I think the HSI is much more useful than than AI.  

[jlunseth]

@JimB - Been over that before in multiple threads. There are multiple issues. The manuals you get when you buy the device warn that the time can be less, as I recall, down to a half hour, but the half hour may be wrong, I just remember the manuals do not give you as much time as the marketing, which is disconcerting. My plane is in the shop with the manuals so I can't look it up for you. Further, you are in IMC, maybe you are flying from GTF to GPI over the Rockies, you can't get to an airport at all for quite awhile and once you get there it is going to take you 20 minutes to fly an approach. If you are only going to fly in your local area, know the wx, and will stay out of conditions where the backup would be challenged, ok. But Mooneys are cross country machines. 

A positive is that the 275 tests the battery and gives you a readout on its condition, and I believe a check has to be done at annual, that helps with the problem of aging batteries, i.e. that the battery you buy may have half the life five years later that it did when you got it.

As I understand it, the G5 has much more backup capacity, 4 hours according to the marketing. But as I recall, that won't work with most, maybe all non-Garmin APs.

Edited October 13, 2023 by jlunseth

[Flash]

11 hours ago, jlunseth said:

@JimB - Been over that before in multiple threads. There are multiple issues. The manuals you get when you buy the device warn that the time can be less, as I recall, down to a half hour, but the half hour may be wrong, I just remember the manuals do not give you as much time as the marketing, which is disconcerting. My plane is in the shop with the manuals so I can't look it up for you. Further, you are in IMC, maybe you are flying from GTF to GPI over the Rockies, you can't get to an airport at all for quite awhile and once you get there it is going to take you 20 minutes to fly an approach. If you are only going to fly in your local area, know the wx, and will stay out of conditions where the backup would be challenged, ok. But Mooneys are cross country machines. 

A positive is that the 275 tests the battery and gives you a readout on its condition, and I believe a check has to be done at annual, that helps with the problem of aging batteries, i.e. that the battery you buy may have half the life five years later that it did when you got it.

As I understand it, the G5 has much more backup capacity, 4 hours according to the marketing. But as I recall, that won't work with most, maybe all non-Garmin APs.

Yes, the G5 is no good with a King autopilot. Also, I think much if not all of this issue should be alleviated if you have a Dynon pocket panel as a backup. The pocket panel is advertised to have four hours of battery life. Even if that has been cut in half by the time I need it, that should be long enough to get me on the ground from almost anywhere I'm likely to go (although one day I guess I might be IMC over the North Atlantic or between Hawaii and California).