Troubleshooting

A 1918 sketch of ground crew receiving instruction on hand starting. 

Marc related this story at lunch yesterday. Seems he knows a guy who has equipped his small, single engine airplane with a whole boat load of electronical gizmos. Somewhere along the way it developed starter problems.
     A little background: the engines on small planes are equipped with magnetos which provide spark for the spark plugs. The electrical system in the plane is used for lights and radios and such. The two systems are not connected, so even if the electrical system fails, the airplane will keep flying because that failure will have no effect on the engine.
     The starter problem was such that the electric starter would not reliably perform it's duty. If it worked, the engine started and ran fine. But it didn't always work. So Marc crawled in amongst the wires, rooted around, and eventually found a small fuse in some obscure location and the fuse was blown. Aha! He found the problem. They replaced the fuse, the engine started right up, and they thought they were golden.
     Golden that is, until the next time they wanted to go flying, when the starter wouldn't crank again. Hmmm, better check that fuse. Gosh darn it, it's blown again! Oh, well, fuses are cheap. They replace it, the engine starts and they go on their merry way.
     This goes of for a while but it eventually gets to be annoying. The fuse is blown again, we're all out of spares, we'll have to go to the store to get some more. Besides, fuses shouldn't just blow like that. There must be something wrong. So they decide to reinvestigate.
    This time the investigation is more thorough and involves worming your way into small places, hanging upside down and backwards and fighting with god's own rat's nest of wiring. But Marc perseveres and discovers that the fuse was for a clock that was never installed. When the latest batch of electronical gizmos were installed, the installers found this loose wire hanging behind the instrument panel and secured it to ground with a screw.
    Well, no wonder the fuse was blowing, the wire goes straight to ground. OK, that explains the fuse, but what does that have to do with the starter? More rooting around, and Marc discovers that connections to the starter switch are loose. He tightens them up. Problem solved.
    The problem with the starter was that there was an intermittent connection at the switch, any little vibration could cause it to make or break contact. The blown fuse was just a red herring. Certification is no substiture for careful, competent workmanship.