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.
Dan, you mention that this could happen to a steel car, and that is may be correct. There is a bit of a difference, since the car is typically a cage, or enclosed structure, while the plate you mention is not.
Cars are said to be a safe place to ride out a lighting storm since they create this Faraday cage effect and becuase the rubber tires insulate the vehicle from the ground. This helps protect the passengers. With all the talk about cars made of composites to save weight, we may loose this safety feature.
I used to design warning equipment for cranes and the equipment was frequently damaged by lightning strikes. I had a small collection of artifacts and was amazed by the strange paths the charge would follow. I had one unit with a neat 1/8" hole blown through a filter capacitor so that you could see through to the other side. The capacitor still tested good, but the chassis behind the capacitor had a 1/2" hole of melted steel.
This is fortunate that the planes are designed with compass redundancy. If there had been only one compass and the plane was flying at night, the pilot could have been flying in the total wrong direction for a long time before seeing the error.
That was a very interesting scenario - which also hits home as to why pilots need to be trained how to fly when losing instrument orientation - those guys knew what to do!
I really appreciated the solution they came up with - an obvious fix because they related it to similar problems with a known solution and out of the box thinking to make it work for their particular situation. I would have liked to seen it implemented!
Having a 1976 Ford Pinto that was struck by lightning, seemingly nothing happened. Although, after about a month or so (memory does not recall time lapse but it seemed short) the charging system quit working and then the electronic spark module quit working. The radio never seemed to tune in stations very good. Then when I thought I had everything fixed, the engine started knocking due to a cracked piston skirt. Coincidence?
Presumably they took all the compasses out of the aircraft before they degaussed it. Otherwise they could demagnetize them and weaken or destroy perfectly good compasses.
This would be even more likely if they had built a huge coile and passed the entire aircraft fuselage through it. That would do a more thorough job of demagnetization.
This is not so far out as it seems. During World War II, submarines were degaussed (so as to eliminate vulnerability to nmagnetic-homing torpedos and mines with magnetic triggers by passing them through a huge degaussing coil.
My convertable (an older, mostly metal vehicle) has a cloth roof and a flimsy metal frame (hardley a cage) and I think about this every time I'm caught in a thunder storm. For me this isn't just a statistic, as my father was protected in a metal car that was hit by lightning in a freak tornado (and yes, his car was not touching the ground when it was hit).
I doubt the insulation the tire's provide has a measureable effect on a lightning strike (the electrical arc has already travelled thru 10's of miles of insulation, the extra few inches of rubber are negligable).
Good point about the compasses, Larry. That is very interesting about the submarines. I would think if the airplane design was such that this was a frequent problem then building a huge coil for degaussing would make sense. But then if the problem was frequent and caused instrumentation loss at critical times - a redesign is probably a better solution...
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