Dan--Very interesting post. Several years ago my wife and I were coming back from a visit to a friend of ours now living in Spain. We were in-bound over the Atlantic, about 45 or 50 minutes from NY, when our 747 was struck by lightning. The pilot indicated we had just experienced a "static discharge" and there were no issues with arriving safely. The sudden "hit" sounded like cannon had gone off inside the cabin and there was a tremendous flash that illuminated the entire right wing. After 30 minutes, the captain came on line and told us we had been cleared for immediate landing and would be on the ground in15 minutes. I always wondered what, if any, damage had been done to the instruments and the air frame. Of course the airlines would never let you know but it was an interesting moment.
Ever see the movie "Sweet Home Alabama"-? Reese Witherspoon and Josh Lucas's characters put steel rods (like 10' rebar) standing upright in the sand on the beach before a lightning storm. The result was a twisted root-like structure of glass resulting from the instant crystallization of the sand. I always wondered if that was real or just Hollywood.
The US Navy used to have a destroyer degaussing site in Newport R.I. Periodically the destroyers would steam to this site and slowly maneuver through the range. As I remember, most radio and radar was turned-off and occasionally there were some eddy-current issues that created some interesting folklore. I've seen several boats struck by lightening and while some had obvious physical damage from both the heat of the lightening and the heat of the current flowing through stainless rigging, others had no damage at all. Every single electronic component that I found damaged as a result of a lightening strike on the water was from current coming up through the negative (ground) and overstressing protection as well as parasitic diodes. Making the engine block and the entire negative side of the electrical system charged to several hundred/thousand volts cause all kinds of damage, some of which didn't appear for several months after the final insurance check had been cashed. Airplanes and cars really appear as conductive air and the current usually just passes through the skin and out the opposite side. BTW car tires are not a very good insulator (the black part of the tire is mostly carbon) and you can see photo's of cars that had energized high-tension lines draped across them where the tires are on fire.
Several effects cause ship hulls to become magnetic. Some occur at construction time, like hammering and riveting of hull plates.
Others occur in operation including cables carrying DC currents, wave motion lapping at the hull, and even the motion of the ship through the earth's magnetic field. These are compensated by running calibrated DC currents through cables provided on the hull for this purpose.
Some ships and submarines have on-board magnetism-control equipment. The magnetic signature of the vessel can be adjusted to help avoid magnetic detection and ensure proper operation of magnetically-sensitive equipment.
The KC-135 has been in service for a long time (over 50 years), so it is not surprising that some of its design features are not too clever by today's standards. Placing a large piece of steel near compasses in an aircraft just isn't smart at all. There are other (non-magnetic) materials that could have been used instead.
Fortunately, with modern electronic navigation equipment, it is not absolutely necessary to rely heavily on a compass for navigation anymore.
Steel-hulled ships have used compasses for a long time. Compasses can be compensated (historically using obvious compensating balls) for the hull magnetism, and must be adjusted periodically if accuracy is important.
Actually, the KC-135 had three double airline-type seats and, across the aisle, some fold down bunks to accomodate a second crew for very long missions. Also there was a freezer, refrigerator, and microwave oven for the in-flight meals. In addition, there were some fold down troop transport type seats. There were windows in the escape doors above the wings.
I had a long ride in such an aircraft at AFROTC summer camp in 1963. We got to watch refueling of a B-47 from the boom operator's compartment in the tanker.
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...
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).
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.
At the Design News webinar on June 27, learn all about aluminum extrusion: designing the right shape so it costs the least, is simplest to manufacture, and best fits the application's structural requirements.
For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This radio show will show what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.