Prescott Flyer

Prescott Flyer

Member
I had to put in a replacement GSU 73 and have completed the AHRS alignment and Magnetometer Interference tests. But the Magnetometer Calibration test repeatedly fails. I believe this is for two reasons. Either the area I'm attempting to perform the test is not free from magnetic interference, or I am doing something wrong during the test. Here's a rundown of what I've done so far.

Aircraft: RV-9A
Equipment: G3X (GSU 73, GMU 44, GDU 370 (2), GDU 375)

Attempt #1 (last week)
  • Location: On my ramp (tarmac) about 30 feet away from a row of Port-a-Port hangars, and about 20-25 feet away from a couple of tied-down Cessnas
  • Wind: Breezy, occasionally gusting over 15mph
  • Engine running
  • Started the test pointed North.
    • Immediate failure was reported due to the aircraft not being still
  • I waited a few moments then restarted the test
    • Test procedure would not progress beyond the initial instruction to hold position

Attempt #2 (last week)
  • Location: Compass Rose
  • Wind: Breezy, occasionally gusting over 15mph
  • Engine running
  • Started the test pointed North
    • The test procedure would not progress beyond the initial instruction to hold position. I waited for 5 minutes, but I never got the instruction to turn the aircraft.

Attempt #3 (last night)
  • Location: On my ramp (tarmac) about 30 feet away from a row of Port-a-Port hangars, and about 20-25 feet away from a couple of tied-down Cessnas
  • Wind: calm
  • Engine OFF
  • With the help of an assistant in the cockpit, we started the test and it proceeded as expected, alternating between holding and turning the aircraft, roughly every 30 degrees.
    • Test calibration repeatedly failed over many (6+) attempts with "Calibration Error".
When performing the test with the engine off my assistant noticed the following:
  • Often where we were instructed to hold it didn't align closely with the next expected 30-degree position (usually we'd be past the expected magnetic heading)
  • We tried to slow the rate of turn as the dots "counted down" to the next expected hold position. But if I slowed the turn down too much, the dots would reset and start the countdown over. It makes me think I'm doing something wrong with how I'm turning the aircraft.
Maybe my presence at the front of the aircraft is having some influence? I emptied my pockets of anything metallic. I tried the test holding onto the prop at the hold position points, as well as keeping a few feet away from the nose of the aircraft until it was time to continue the turn. The prop is composite so it shouldn't be conducting any static from my person, right?

We plan to repeat the test with the engine off again but over the Compass Rose this evening to rule out magnetic interference that could be causing the failure on the ramp.

Any advice on things to check or do differently next time is appreciated!

Thanks!
 
IMHO you should not attempt this with only 30’ clearance from a steel hangar. Remember the earth’s magnetic field is incredibly weak. Also, do you have any headsets close to the sensor? (If you have a wet compass, watch what happens if you bring a headset close - the compass will swing way off. The headsets have small magnets in them). Since you cannot avoid having some small magnetic fields in the plane (because you have currents flowing) you should do the tests as described - engine running, avionics and other electrical stuff ‘on’. Double check that all high current wires are routed away from the sensor. Ordinary shielding (metal braid) will not help. However, a ‘twisted pair’ can greatly reduce any magnetic fields generated, as long as the pair really flows all the ‘hot’ current and all the ‘ground’ current, e.g., there is no ground path thru the airframe.
 
BobTurner said:
IMHO you should not attempt this with only 30’ clearance from a steel hangar. Remember the earth’s magnetic field is incredibly weak. Also, do you have any headsets close to the sensor? (If you have a wet compass, watch what happens if you bring a headset close - the compass will swing way off. The headsets have small magnets in them). Since you cannot avoid having some small magnetic fields in the plane (because you have currents flowing) you should do the tests as described - engine running, avionics and other electrical stuff ‘on’. Double check that all high current wires are routed away from the sensor. Ordinary shielding (metal braid) will not help. However, a ‘twisted pair’ can greatly reduce any magnetic fields generated, as long as the pair really flows all the ‘hot’ current and all the ‘ground’ current, e.g., there is no ground path thru the airframe.
Click to expand...
When you are attempting the test with the engine off, are you pulling the aircraft in a circle or pivoting on a wheel?

I think the test is designed to go in a circle…
 
Prescott Flyer said:
I had to put in a replacement GSU 73 and have completed the AHRS alignment and Magnetometer Interference tests. But the Magnetometer Calibration test repeatedly fails. I believe this is for two reasons. Either the area I'm attempting to perform the test is not free from magnetic interference, or I am doing something wrong during the test. Here's a rundown of what I've done so far.

Aircraft: RV-9A
Equipment: G3X (GSU 73, GMU 44, GDU 370 (2), GDU 375)

Attempt #1 (last week)
  • Location: On my ramp (tarmac) about 30 feet away from a row of Port-a-Port hangars, and about 20-25 feet away from a couple of tied-down Cessnas
  • Wind: Breezy, occasionally gusting over 15mph
  • Engine running
  • Started the test pointed North.
    • Immediate failure was reported due to the aircraft not being still
  • I waited a few moments then restarted the test
    • Test procedure would not progress beyond the initial instruction to hold position

Attempt #2 (last week)
  • Location: Compass Rose
  • Wind: Breezy, occasionally gusting over 15mph
  • Engine running
  • Started the test pointed North
    • The test procedure would not progress beyond the initial instruction to hold position. I waited for 5 minutes, but I never got the instruction to turn the aircraft.

Attempt #3 (last night)
  • Location: On my ramp (tarmac) about 30 feet away from a row of Port-a-Port hangars, and about 20-25 feet away from a couple of tied-down Cessnas
  • Wind: calm
  • Engine OFF
  • With the help of an assistant in the cockpit, we started the test and it proceeded as expected, alternating between holding and turning the aircraft, roughly every 30 degrees.
    • Test calibration repeatedly failed over many (6+) attempts with "Calibration Error".
When performing the test with the engine off my assistant noticed the following:
  • Often where we were instructed to hold it didn't align closely with the next expected 30-degree position (usually we'd be past the expected magnetic heading)
  • We tried to slow the rate of turn as the dots "counted down" to the next expected hold position. But if I slowed the turn down too much, the dots would reset and start the countdown over. It makes me think I'm doing something wrong with how I'm turning the aircraft.
Maybe my presence at the front of the aircraft is having some influence? I emptied my pockets of anything metallic. I tried the test holding onto the prop at the hold position points, as well as keeping a few feet away from the nose of the aircraft until it was time to continue the turn. The prop is composite so it shouldn't be conducting any static from my person, right?

We plan to repeat the test with the engine off again but over the Compass Rose this evening to rule out magnetic interference that could be causing the failure on the ramp.

Any advice on things to check or do differently next time is appreciated!

Thanks!
Click to expand...
I had a similar issue with a new GMU 22 magnetometer. Couldn't even get it to calibrate on a compass rose on a calm day. I finally gave up and got a refurbished GMU22 (original was out of warranty, even though it was unused) from Garmin and it calibrated on the first try.
 
BobTurner said:
IMHO you should not attempt this with only 30’ clearance from a steel hangar. Remember the earth’s magnetic field is incredibly weak. Also, do you have any headsets close to the sensor? (If you have a wet compass, watch what happens if you bring a headset close - the compass will swing way off. The headsets have small magnets in them). Since you cannot avoid having some small magnetic fields in the plane (because you have currents flowing) you should do the tests as described - engine running, avionics and other electrical stuff ‘on’. Double check that all high current wires are routed away from the sensor. Ordinary shielding (metal braid) will not help. However, a ‘twisted pair’ can greatly reduce any magnetic fields generated, as long as the pair really flows all the ‘hot’ current and all the ‘ground’ current, e.g., there is no ground path thru the airframe.
Click to expand...
The GMU44 was working correctly with the previous GSU 73 installed in 2013. The only thing that has changed is the installation of a "newly overhauled" GSU 73 Garmin sent me. I was getting the correct heading indications on the GDU 37x displays before I ran the AHRS alignment test. After completing the ARHS alignment test the Magenetomer Calibration test must be completed.

I'm the 2nd owner of the aircraft so I can't speak to specifics of how everything was wired up, other than the panel was designed and wire harnesses done by Stein Air. At this point, I don't have reason to suspect a wiring issue with the GMU44.
 
I once struggled trying to figure out magnetometer calibration issues on a plane...
Turned out the magnetometer was perpendicular to the aft fuselage side... in other words about 10 degrees off of correct fore/aft alignment.
Took a while to notice that!
To paraphrase Sherlock Holmes... if I explain it to you, you'll say that's obvious!
 
So many of the "usual suspects" have been mentioned, but..: Your own body won't affect the calibration. If the asphalt apron you mention is overlaid over concrete, there may be rebar in the concrete. The steel buildings sound close (30') -- I've only calibrated farther than that, don't know if 30' is too close. Diameter of your turn should not make a difference (unless it takes you close to some EM field). You might contact [email protected] -- I had repeated failures of an accelerometer, they concluded (correctly) it was the instrument & replaced it (successfully). Good luck!
 
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