Beware the Cargo: Magnetic Interference in Your Aircraft

I have built and flown a lot of airplanes with modern glass cockpits, so I am not new to the care and feeding of sensors—sensors such as ADAHRS, GPS antennas, and magnetometers. I know that ADAHRS should be mounted to the most rigid structure you can find and preferably near the center of rotation (close to the CG) of the airframe. GPS antennas should be mounted up top with as clear a view of the sky from horizon to horizon all around the compass. And magnetometers need to be mounted away from moving ferrous items—steel control horns, pushrods with steel Heim bearings, and anything with a magnet or changing magnetic field. I’ve spent a lot of time and effort locating these items on all my aircraft—so it is a little embarrassing what happened to me as I left AirVenture this year.

EAA had asked me to do a photo shoot with my new F1 Rocket as I left first thing Thursday morning, and I’ll take an escort to the runway with a formation departure at 6 a.m. any day! We’d staged the airplane next to the Alpha taxiway the night before, so it was a simple matter of waiting for the “Go” sign at 5:55 a.m., hitting the starter, and following the golf cart to Runway 27. I met the camera ship coming from the north side, we were cleared for takeoff, and I slid under and into the echelon right position at about 100 feet. I didn’t look at my EFIS or get a glimpse of the lovely Wisconsin countryside at dawn for the next 45 minutes—both eyes (and all my attention) were on the A36 and the photographer’s signals.

As we finished up and they waved me off to head west (and home), I settled back and looked at my map display—and nothing made sense. I knew I was headed west, the map showed my location correctly to the north of Oshkosh, but my heading was reading south. My first fuel stop was halfway across southern Minnesota, and it was dialed—and fortunately, the NAV function of the autopilot didn’t care what the heading might be—all it knew was left or right of course, and it steered appropriately. Heading and Track modes? Fuggedaboutit …

With little to do once I had leveled off and set my power, I began thinking about what could have gone wrong. The avionics hadn’t faltered during Phase I testing once we nailed down a short in the CAN bus, so it was surprising that I was having a problem at this point, 50 hours later. Just to make sure, I opened the network diagnostics page, and sure enough—the CAN was happy with all boxes present, accounted for, and talking nicely with each other. But my map view stubbornly refused to shift—almost as if my compass needle was locked by “something” that was holding it in place. Uh-oh … wait a minute … cargo!

The F1 has a long, extended baggage compartment that goes back almost to the front of the horizontal tail. This design “gift” is due to that big honking IO-540 hanging on the nose—counterbalance weight is good! There’s a lot of space back there—enough that when a friend asked me if I could carry a couple of brand-new headsets home for him (he was airlining it … ), I said, “Sure, just throw them in the rear baggage compartment!” Of course, I forgot two things—one, the magnetometer was hanging under the floor of the rear baggage compartment, and two, headsets have magnets in them!

Garmin GMU 11 magnetometer

When I landed for my first fuel stop, I unpacked the entire airplane—sure enough, the two headsets, in their boxes, were all the way in the back, right above the magnetometer. I repacked so that soft, non-magnetic things were in the back, and anything with ferrous components was up front. Powered up the EFIS and bingo—east was east, west was west, and never the twain shall meet. Lesson learned—remember what you pack, and where it goes!

This was a nice VFR day, and to be honest, I can fly VFR from Oshkosh to the West Coast using pilotage and landmarks—I’ve done it so many times. Section lines to the Badlands, see the Black Hills, then beyond those the Bighorns … and so on and so forth. But ’tis nice to have all the expensive toys happy and contributing to the flight’s data. From Minnesota to Tahoe required only a single fuel stop, the Rocket purred like a kitten (or roared like a tiger—my helmet keeps the noise away), and all was right with the world—with a place for everything and everything in its place.

See those screws on the floor of the baggage compartment extension? The magnetometer is hanging from them …

The post Beware the Cargo: Magnetic Interference in Your Aircraft appeared first on AvBrief.com.