G1000 HDG / Traffic Fail

[OriensFlight]

Here’s the non-scientific way it has been explained to me. The Magnetometer is a solid-state electronic compass which passes its heading information to the AHRS. The AHRS will also keep track of what it thinks the heading should be based on some factors that I can only describe as magic, but what’s probably some mathematical algorithm based on historical heading and roll data. When these two views of the heading world don’t match (for some period of time) the AHRS software throws an error. Now, it’s auto-correcting, which is why this issue can be intermittent and hard to troubleshoot.

Magnetometers are incredibly sensitive and one mechanic suggested hanging the thing from a string dangling 50’ behind the aircraft. He was kidding, of course, but he once had to de-magnify the empennage of a metal aircraft that had been struck by lightning in order to get the Magnetometer to work. The reason our Magnetometers are out on the right wing is to get them as far away from the rest of the airplane as possible. Out there the only things that can cause interference are magnetic screws, the tie-down hook, the strobe ballast and bulb contraption, messed up wires, the fuel tanks, or perhaps a GoPro you’ve mounted to the wing. Obviously all this stuff is designed NOT to cause interference for the Magnetometer.

Grounding. Our fiberglass airplanes don’t have natural grounding paths like metal aircraft do, so we have to pay special attention to the grounding connections. (There’s a grounding inspection every 1,000 hrs.) Have you ever wondered where the grounding point between the wing and the fuselage is? It’s a slip-joint where a curved metal fitting on the wing slips into a round metal hole on the fuselage. Ideally this joint is very tight and free of corrosion. If it’s not, then the grounding contact can become intermittent, which gets particularly aggravating when it’s firm on the ground but loose in the air, due to the wing flexing. Messed up grounding can cause problems for the Magnetometer-AHRS system.

I’ll spare you my story for another day, but if the fitting is corroded or bent (by a mechanic who doesn’t know how to properly re-connect a wing) and it’s making intermittent contact, or some other part of the grounding path to the Magnetometer is broken, then you can have this HDG failure condition. Some flight schools in humid climates go as far as to attach extra grounding straps between the wings and the fuselage. (I’ve attached a perfectly good Garmin Magnetometer to a shelf in my house to always remind me to stay grounded.)

[Charles K]

Just to chime in - yes a heading fail causes ADS-B fail because it needs to know your heading I believe to sort out relative heading of the other planes.