Metal Fatigue
Take a small piece of wire. If you don't have wire clippers to break it, you hold it between your fingers and bend it back and forth. It heats up and eventually breaks. Thats metal fatigue in its simplest form.
Its the hidden danger you check for when you do your preflight. You look carefully at all the hinge joints. See if all the cotter keys are in place and properly fitted. If you see the beginning of a crack that is a serious sign. Know you A&E person. Listen carefully to what he or she is telling you about fatigue.
In a major air disaster an airliner lost its engine just after takeoff from a busy midwestern airport. After a very careful examination of the crash it boiled down to two things. A large hinge bolt that was the main attachment point for the engine and a powerful surge of thrust at takeoff.
This is like the example above. When too much power was applied too soon the large, heavy gross weight of the plane hesitates before it begins to move ever so slightly forward from the engine thrust. The engine moves forward slightly from the full power applied too soon. The bolt attaching the engine to the main structure of the plane begins to bend, back and forth a little, each time the plane was powered up at takeoff.
How many times has this sequence happened before the bolt broke from metal fatigue. Nobody really knows. It was a case of inertia, application of power too soon and slight movements back and forth like the wire you broke above. An inspection wouldn't show anything out of the ordinary. When it was time, it broke at a very inopportune time for all involved.
Vibrations can result in metal fatigue. Temperature changes can result in metal fatigue. The best engineering in the world can't predict metal fatigue accurately.
I was flying in the Bahamas in a 1964 S - Model Bonanza, from an earlier post on Instrument Scan, when just what we are talking about today happened. I noticed a slight position change in the needle of the ammeter gauge in the plane.
I did all the right things and landed safely at Red Aircraft Repair Facility in Ft. Lauderdale. An inspection showed all three welds that attached the rotor to the shaft of the alternator were broken. The belts that spun the rotor were all right but the shaft was not generating electricity. Metal fatigue? Bonanza Corporation was very interested in the alternator and, as a gift for letting them keep it for research purposes, outfitted the plane with a new alternator and a variety of other electrical things that virtually replaced much of everything except the engine block. I thought it was a good trade.
Try to sense what you plane is telling you. It may save your life sometime.
