Alan

[alanglickson]

I have a GFC AUTOPILOT WITH DUAL GI 275’s . One is the ADAHARS + autopilot and the other is the the HSI. Does anyone know if the reversionary switch is a requirement or can you just rely on the HSI 275  becoming an ADI it the ADAHARS FAILS. How do you test it to see if it will actually change from A HSI TO AN ADI. 

[carusoam]

1 hour ago, alanglickson said:

can you just rely on the HSI 275  becoming an ADI it the ADAHARS FAILS. 

Welcome aboard Alan,

You can edit the title of your thread by using the three dots at the top of your first post….

 

Focussing on your question as I have cut the excess, above…

 

If you only have one ADAHARS, and it fails… (that’s your attitude reference isn’T it?)

What are you going to use for attitude reference?

 

I believe we have seen this before… one attitude source, and it fails… becomes a wild roll of the dice…

Do you have a second AI?

Just getting the conversation going… not an avionics guru…

 

Best regards,

-a-

 

[David Lloyd]

It all depends. Most 275 HSI installations are the ADAHRS version with standby battery and configured as a standby ADI.  If so, and installed as primary instruments a reversionary switch is required to comply with the STC.  You should have a AFMS in your POH that would have details on configuration.

If no reversion any switch, turn the big knob on the HSI. If configured as a standby, it will go to the next screen: ADI.

[Seymour]

Is the autopilot a Garmin GFC500? 

If so, the GI 275 ADAHRS ADI does NOT need to be the "plus autopilot" version.    The "plus autopilot" version is for control of third party (aka non-Garmin) autopilots.   

[alanglickson]

20 hours ago, Seymour said:

Is the autopilot a Garmin GFC500? 

If so, the GI 275 ADAHRS ADI does NOT need to be the "plus autopilot" version.    The "plus autopilot" version is for control of third party (aka non-Garmin) autopilots.   

I have the ADAHARS 275  for ADI and an HSI 275. I ‘ve read you  can turn the outer knob on the HSI counter clockwise and it reverts to an ADI. I have not tried this yet. Does anyone know how to test to see if the ADAHRS ADI fails , will the HSI become an ADI automatically. I’ve been told you don’t have to have the Reversionary switch

[David Lloyd]

If installed only as an HSI the switch is not required.  If installed only as an HSI, it will not show as anything except an HSI.  What does happen when you turn the outer knob?

If the HSI is configured as a HSI/standby ADI, the reversionary switch is required by the STC.

[201Mooniac]

To be a standby ADI, I believe it would require the ADAHRS and a battery.  You can test what would happen by pulling the circuit breaker for the primary ADI and see if the HSI either fails over to the ADI or can be set to ADI by the knob.

[carusoam]

If it only has one attitude sensor….

How concerned are we going to be about what display reverts to what when they are both showing red Xs?

Time to ask the Instrument installer what it takes to be IFR capable…

 

There is an important difference between the VFR only set-up… and the IFR set-up…

They both look alike… 

Having two displays seems important to the OP…

Having the second sensor to back-up the second display seems equally important…

 

Something seems to be amiss…

PP thoughts only, not an instrument guru…

Best regards,

-a-

[Flash]

I am getting dual GI275s installed. The reversion of the HSI to an attitude indicator is automatic. The HSI and the AI both must be ADAHRS. Here is what Garmin says: "You can count on the reliability of the GI 275, but primary instruments often require backups. With the installation of a GI 275 attitude indicator and GI 275 HSI, you get the benefit of auto-reversionary mode. This mode kicks in and displays attitude and heading data on the remaining HSI or MFD if an outage should occur."

[Flash]

Here is the language from the STC: "When configured as a standby instrument the GI 275 MFD and HSI include an ADI Page which is displayed automatically when faults are detected or when selected by the pilot. Both the GI 275 primary and standby ADIs have their own integral ADAHRS and internal batteries for independence and redundancy." (Emphasis added.)

It's true that the GI275 ADAHRS system gets help from an external GPS and airspeed inputs. If you lose the external GPS or airspeed input to one, you may well lose it to the other. But there is an internal GPS in each one, as well.

It includes an optional GMU 11 or GMU 44B magnetometer interface to determine magnetic heading and an OAT probe for measuring outside air temperature.

I've read about the dual failure issue. I think it's very unlikely with a proper installation.

[chriscalandro]

On 4/4/2022 at 12:41 PM, alanglickson said:

I have a GFC AUTOPILOT WITH DUAL GI 275’s . One is the ADAHARS + autopilot and the other is the the HSI. Does anyone know if the reversionary switch is a requirement or can you just rely on the HSI 275  becoming an ADI it the ADAHARS FAILS. How do you test it to see if it will actually change from A HSI TO AN ADI. 

I see you met the self appointed unofficial forum police. 
to answer your question directly - 

The switch is required according to the installation guide.  You can test the auto function by disconnecting power.