One day in the 1980s, I was discussing problems discovered during testing with our quality assurance engineer, Len. He had been a test engineer for the US Air Force years earlier. He related this story to me:
He had just received a C-130 back from a total refit. They powered it up and were putting it through a full flight control system check prior to its test flight. Len noticed that the elevators (rear control surfaces) were “twitching” down. He noted that it was hard to describe it any other way. They would start to go down and then return to level flight position after about a two-second interval. No one seemed concerned about it, but Len was convinced that this was not normal.
Of course, the wire harnesses had been through continuity check, and all of the systems on the aircraft had been checked and passed. Yet Len was still not convinced. On a whim, he reached up into the wheel well and pushed the “Gear Up” switch (this switch is aptly named, since it is actuated when the landing gear is fully stowed). The elevators went fully down and stayed that way as long as the switch was pressed.
Now he had plenty of believers. If the aircraft had taken off, it would have been fine until the landing gear was stowed. Then it would have pitched nose down, straight into the ground! Naturally, they scheduled a full tear down of the Flight Control System wiring. After many hours at this task, they discovered that a semicircular cliver of aluminum was lodged in one of the large mating connector plugs.
It was likely a remnant of the punching out of a bulkhead connector panel hole to a slightly larger size (it was less than a 1/16 of an inch wide). This effectively caused a massive, although intermittent, short circuit in the wire harness.
This entry was submitted by Dwight Bues and edited by Rob Spiegel.
Dwight Bues is a Georgia Tech Computer Engineer with 30 years' experience in computer hardware, software, and systems and interface design.
ell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Dave, that is a good point. Showing is better. Maybe we should all be from Missouri.
It is amazing that aircraft, being as complex as they are, are so reliable. You are more likely today to hear that there was a problem with a pilot than with a airplane.
This is indeed "frightening." Maybe Design News should be highlighting this kind of scary story as we approach Halloween.
What stands out to me is how the crew's attitude changed when Len hit the "gear up" switch. He was right from the beginning, but no one took his concerns seriously until he had a physical demonstration. Then he quickly won everyone to his side.
This should be a lesson to all engineers that "show" is better than "tell."
That's why it's so important to do a complete and thorough preflight, especially after any service. Thankfully I've never found any problem other than compass deviation. From time to time I've mentioned that people should do predrive with automobiles as well. At the very least, check all your tires every time you get in the car, and stomp on the brakes to see if they work.
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