Lightning Knocks Out Aircraft's Compass

In the 1980s, I worked as an avionics supervisor at Hayes International Aircraft Corp. in Birmingham, Ala. A Boeing KC-135 test pilot on return from a test flight told me in a debriefing that they had been struck by lightning during the flight, and all of the compass instruments, and some of the other flight gauges, were now not working correctly.

We checked and found that the compasses were pointing in all different directions -- except the correct one.

We started the repair by replacing the simplest part -- the cockpit whiskey compass. The new one pointed in the wrong direction, as well. My crew and I were dumbfounded. We thought that maybe we had a bad part, so we replaced it again. This one also performed the same as the first replacement.

I went to my truck and got my trusty Boy Scout hiking compass. As I entered the cockpit, it too was pointing in the wrong direction. Something had to be magnetized. We brought the aircraft engineers out to try to help figure this out. When they looked at the cockpit blueprints, they found that there was a steel shield buried under the aircraft skin just above the cockpit to reduce damage from bird strikes.

That was it, and it was huge -- nearly six feet long and 11 feet across. Now the question was: How do we demagnetize something this big? We used a TV demagnetizer coil and went over every square inch of the top of the cockpit, hanging on a strap from a crane.

It actually worked! We towed the plane to a compass rose, checked the headings using a sextant, and found it was back on target. We calibrated the compass and everything was back to normal. I've heard of aircraft being struck by lightning before, but I never knew something like this could happen. I definitely think it could happen to a steel car, as well.

This entry was submitted by Dan Clark and edited by Rob Spiegel.

Dan Clark designs RF imaging coils for the highest magnetic field MRIs in the world at the National High Magnetic Field Laboratory in Tallahasse, Fla., Gainesville, Fla., and at Los Alamos, N.M.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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