Magnetic flight data error

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Clam
Posts: 40
Joined: Thu Mar 14, 2013 1:10 pm

Magnetic flight data error

Post by Clam »

I have been flying HXr with legacy dual AHRS with the magnetometer mounted in the left wing tip. This produced satisfactory performance for 275+ hours. During the last couple of flights, I have been getting heading drifts up to +/-50 degrees and gross wind dir & speed errors. Both of these conditions are present simultaneously.

I have not "re-done" a mag calibration yet or even opened up to chase wires, because I have noticed data in the FDL that I wanted to ask about.

What is the Magnetic Field Heading? I would assume this is the raw data (uncorrected) coming from the magnetometer?

The FDL is showing this number steady for periods while all is well, and then jumping around randomly with associated wind errors. The Magnetic Heading will often drift during this time as well.

Can I assume this is a sign of magnetometer failure or would you expect a mag calibration could correct it? Wanted to ask before ordering a new part.
Thanks.
Jeff
RV-8 Memphis, TN (KFYE)
jb.flynavy@gmail.com
GRT_Jeff
Posts: 802
Joined: Tue Dec 11, 2012 12:11 am

Re: Magnetic flight data error

Post by GRT_Jeff »

The "Magnetic Field Heading" can be two things. Mainly it's the calculation of your magnetic heading from the field sensors and attitude, plus your calibration correction. If the calculation results in a big jump, then this will cycle between the last calculation before the jump and the current one for up to 30 seconds. Temporary jumps like this can be caused by the wires picking up com radio transmissions. The FDL file doesn't have enough data or recording rate to tell you anything more.

The legacy AHRS uses multiple wires for the magnetometer sensors. If any one of these wires develops a bad connection it can throw off the result. You can check the STATUS page (push Next on the PFD until you find the STATUS button) for any messages about the magnetometer. Go to SET MENU, AHRS Maintenance and look at the Magnetometer X/Y/Z Raw Data to see if it looks normal and somewhat steady. It always jumps around a little but the average should remain about the same. It also flips sign about once per second for a single update. You should be able to see that happening regularly. You can ignore the flipped values. Generally when level and pointing north in the northern hemisphere, the X Raw Data should be a positive value above 1000. The Y Raw Data should be within +/- 500, and the Z raw data should be greater than positive 1500. Magnetometer Bias X/Y/Z values above +25000 or below -25000 also can be a sign of an issue with one of the sensor wires or power.

The DEMO RECORD function records the magnetometer data at 15 times per second so you can record it with that and e-mail it to us to look for patterns. Be sure to STOP the recording and let it save before disconnecting the drive or powering off the display.
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