A chain is only as strong as its weakest link, and that cliché applies more in aviation than just about any other field — with the possible exception of training attack dogs. Old Bonanzas like mine have Continental E-series engines, similar to later O-470s, but with enough differences to make operating one a constant source of education. One important class involves the Thompson TF1900 engine-driven fuel pump. Experience may be the best 'professor' but when it comes to E-series Continental engines, Lew Gage is right up there on the podium alongside the school of hard knocks. From his book, E-Series Bonanzas concerning the Thompson pump:
"The pump part of this unit is very well designed and manufactured to close tolerances with high quality. However, the drive end is of marginal design and poorly manufactured. Due to this condition, the [FAA-mandated] 300-hour inspection is a very necessary maintenance procedure."
Simply and elegantly put.
The drive end is coupled to the pump end with a shaft and a shear pin, designed to break should the pump seize. That's to keep the gear drive from fouling the engine. But the pin and the rotor shaft are soft metal, and the pin has a tendency to wear the hole into an egg-shaped oval — then flop out altogether. Pilots of older Bonanzas with Thompson pumps have in the back of their minds (at least they ought to) that if the fuel pressure should drop off for no apparent reason, it's time to start working the hand pump by the pilot's left knee — and keep working it until reaching an airport.
In my case, the fix is to replace the old Thompson (the company is long out of business) with a later-model fuel pump that doesn't have the same maintenance worry. There are rebuild options to replace the soft metal of the shaft and the pin, as well. Or just keep inspecting it every 300 hours, and hope.
And just in case anyone thinks that required inspections such as this one are no more than a pain in the wallet, dreamed up by the FAA to keep us from flying our delightful old airplanes, have a look at the accompanying close-up photo of the rotor shaft and pin of my Thompson pump. And this one isn't considered too bad, compared to some others.
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