Garmin GI 275 HSI/GMU-11 Inaccuracy

[Tom 4536]

I recently replaced my old HSI with a new Garmin GI 275 that uses the GMU-11 magnetometer. It turns out that the new HSI needs a deviation card because interferences in the plane make the HSI heading up to 12 degrees off. The old HSI always read the same as the compass in straight and level flight because I set the HSI to match the compass. Can't do that with the new GI 275 HSI. Has anyone found a solution short of reinstalling the vacuum system and the old HSI? Using a deviation card with the HSI is a real pain.

[OR75]

Maybe I don’t understand what you mean but my thinking is :

the only instrument that requires a card is the whiskey compass 

any other magnetometer based compasses (HSIs) have a calibration procedure to be within 10 degrees max (I think that’s the right number for the G5 if I recall well) 

if that’s not the case , your GMU needs to be realigned (I had to do that for my G5 using washers)

Edited November 27, 2020 by OR75

[jamesm]

I can only speak of G5 and GMU 11 combination, on the G5 they have a  magnetic  interference test for the GMU-11 that you run during installation. The magnetic interference test  has you move various  flight controls and also has you cycle the various components of the electrical system. It's about a 8 minute test or so it seems to be  pretty through and sensitive .  I am surprised that the installer didn't pickup on the magnetic error during the test, if the GI-275 uses the same test. 

You may have to relocate your GMU-11.

The G5 had you do Magnetic survey before installing the GMU-11 permanently in the survey. I had mine too close to the Step and elevator pivot point and  it failed I relocated about 10" away and then it past. So in total there is 3 tests/check(s)  for the GMU-11 when paired with G5's 1 is the Magnetic survey, Magnetic interference , Magnetic calibration.

Good luck in getting this resolved.

Hope this helps,

James  '67C

 

[OR75]

Reading the previous post : 

if the magnetic heading you are getting changes when you switch on and off some other electric equipments, then you have the wrong location for the GMU and you need to relocate it to a place with no magnetic interference 

if your magnetic heading is just off (vs a compass card painted on the ground at an airport) but does not change when you switch on and off electric equipment, then your GMA is not correctly aligned 

Both cases are addressed in the install manual 

 A paper correction card (like you have for a whiskey compass) will not solve your issue 

 

[carusoam]

See if @Fly_M20R is around... he has a pair for these and is writing much about the experience...

My old version of these devices used the magnetometer to adjust the HSI... when that didn’t work, the HSI had some wear to deal with...

Best regards,

-a-

[Tom 4536]

Thanks for your input. The deviation does not change when operating the landing gear, flaps or electrical equipment. The installer saw the errors but Garmin said there wasn't a fix. If you change the GMU-11 alignment you reduce the error in one direction and increase it in the opposite direction. The only way I can get a correct heading is by reading the new GI 275 heading and add or subtract the known errors in my head. Just like using a compass deviation card. Also, Garmin says a 10 degree error is acceptable! Really??

Tom

[jamesm]

Curious to know where the installer mounted the  GMU11.  Wing ??  empennage?? or ??  Wonder if the GMU11 was mounted  to close steel tube frame  or steel component could be causing problems and may need to degauss the frame . The 10 degree mentioned is probably correctable with the modern day electronics  you put some sort of an offset  value and through magic of software  should correct for it. I would suspect there someone out that has mounted a GMU11  in a 231 Even know the GI-275 fairly new to market .... at least one with G5 setup similar to yours.

[OR75]

3 hours ago, Tom 4536 said:

Thanks for your input. The deviation does not change when operating the landing gear, flaps or electrical equipment. The installer saw the errors but Garmin said there wasn't a fix. If you change the GMU-11 alignment you reduce the error in one direction and increase it in the opposite direction. The only way I can get a correct heading is by reading the new GI 275 heading and add or subtract the known errors in my head. Just like using a compass deviation card. Also, Garmin says a 10 degree error is acceptable! Really??

Tom

A 10 degrees error would not be unusual for a standard whiskey compass 

I found it odd that changing the alignment would reduce the error in one direction and increase it in other direction. You have interference in where the GMU is located 

[Fly_M20R]

There is a standard procedure by which yiu"swing" and calibrate the GMU-11 requiring an initial heading of 360 + or - 5 Deg and having it line up on that heading. If engine vibrations are mowing it around too much it will fail that initial lineup and you may have to shut the engine down and then restart for the 360 deg swing with stops every 15 degrees or so. It is tedious and can fail if not done well. I had mine mounted where the old HSI magnetometer was located - almost midway  on the left wing. It is perfect without any deviations!! What you describe is either a poor install, interference, calibration or unit.

Edited December 1, 2020 by Fly_M20R

[JimB]

Was the magnetometer calibration done correctly? Either way, I would suggest that you have the installer re-accomplish it. Section 6.4.1 of the install manual. It's also possible there are magnetic disturbances in the area the calibration was done and that will throw it off as well. Section 6.4.5 

 

[StevenL757]

Ditto @JimB, @Fly_M20R, @jamesm, and @OR75.

Bad calibration, install, interference, or unit.  I’m betting on #1 and/or #3 with a likelihood of #2.  Not so much a bad unit.

[N201MKTurbo]

It is hard to imagine it has errors like that. The calibration procedure should take out any constant errors. The only thing that should throw it off would be something that is different than when it was calibrated. The calibration should be done with the engine and all the avionics running.

My GMU11 is dead on. It has no detectable errors at any heading.

[Tom 4536]

The magnetometer is installed about 3 feet in from the end of the left wing. The installer said he did the magnetic interference test and and the calibration and when he contacted Garmin about the inaccuracy, Garmin said there is nothing in the GI275 software to make any corrections.

[N201MKTurbo]

3 minutes ago, Tom 4536 said:

The magnetometer is installed about 3 feet in from the end of the left wing. The installer said he did the magnetic interference test and and the calibration and when he contacted Garmin about the inaccuracy, Garmin said there is nothing in the GI275 software to make any corrections.

I would just redo the calibration. It should make all the corrections for you.

Did Troy do the installation?

[Tom 4536]

Is it possible for me to do the re-calibration? If not, since you are in Tempe, do you know of someone that can redo the calibration? I know I have a GI275 but the Part 23 AML STC Maintenance Manual for the G5 (Rev 7, 5-8-20) says the reading for the G5 should be +/-6 degrees. I am not sure why 10 degrees is acceptable for the GI275 when using the same GMU 11 for both the 275 and G5. I have had many report of GI275 users having their readings right on.

 

[N201MKTurbo]

32 minutes ago, Tom 4536 said:

Is it possible for me to do the re-calibration? If not, since you are in Tempe, do you know of someone that can redo the calibration? I know I have a GI275 but the Part 23 AML STC Maintenance Manual for the G5 (Rev 7, 5-8-20) says the reading for the G5 should be +/-6 degrees. I am not sure why 10 degrees is acceptable for the GI275 when using the same GMU 11 for both the 275 and G5. I have had many report of GI275 users having their readings right on.

 

I haven’t read the manual for the 275. With the G5, it is just a few button pushes to do the procedure. If I recall, you align the plane with magnetic north, then taxi in a circle. This procedure should be done as far away from any buildings, airplanes or light poles as you can get.

I used a military compass to align the plane. You can get it less than 1 degree.

[N201MKTurbo]

43 minutes ago, Tom 4536 said:

Is it possible for me to do the re-calibration? If not, since you are in Tempe, do you know of someone that can redo the calibration? I know I have a GI275 but the Part 23 AML STC Maintenance Manual for the G5 (Rev 7, 5-8-20) says the reading for the G5 should be +/-6 degrees. I am not sure why 10 degrees is acceptable for the GI275 when using the same GMU 11 for both the 275 and G5. I have had many report of GI275 users having their readings right on.

 

If you want to come up to Chandler this week end, I can help you with it.

[MB65E]

Where is the survey procedure listed? The manual just calls for the survey, but doesn’t list the steps for the survey itself. 
Can one -11 be used to drive 2 G5 HSIs. 
 

I have an install with a G5 in the front seat and another in the back seat. Mounting the -11s is a challenge. 
-Matt

[Tom 4536]

I don't have the installation manual for the 275. Garmin said it is available only to installers. I read that other pilots have had a problem getting the manual.

Are you available only on the weekend? I was thinking of having Frank do an oil change for me; of course that has to be a weekday. Then I could do both on the same day.

[Tom 4536]

I have 1 EMU-11 driving 2 GI275 HSI's.

[N201MKTurbo]

11 minutes ago, Tom 4536 said:

I don't have the installation manual for the 275. Garmin said it is available only to installers. I read that other pilots have had a problem getting the manual.

Are you available only on the weekend? I was thinking of having Frank do an oil change for me; of course that has to be a weekday. Then I could do both on the same day.

I'm sure we could work something out, I work till 4:00 on the week days, so it would have to be after that.

[N201MKTurbo]

I just need to run out to the airport with a flash drive andIll have the install manual.

[Tom 4536]

When I did my panel upgrade I was unaware that CAN BUS 24 was an option. I am curious whether other Mooney owners have used CAN BUS 24 in their upgrades. Any reason not to upgrade to CAN BUS 24?

[JimB]

On 12/4/2020 at 1:37 PM, N201MKTurbo said:

I haven’t read the manual for the 275. With the G5, it is just a few button pushes to do the procedure. If I recall, you align the plane with magnetic north, then taxi in a circle. This procedure should be done as far away from any buildings, airplanes or light poles as you can get.

I used a military compass to align the plane. You can get it less than 1 degree.

It is very similar to the G5. Basically start the aircraft. Power on the unit in configuration mode. Navigate to the test page- calibrate magnetometer and follow the on screen prompts. 

[andrewf15]

Has this problem been solved? I have the installation manual describing how to fix this issue.