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.
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.
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."
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.
The spirit (or should I say ghost) of Richard Feynmann lives on.
I'm really, really surprised that the sliver wasn't accounted for during the modification. Aircraft maintenance and fabrication is normally much more focused on accounting for everything that goes in and out of an aircraft. Lost fasteners must be found and so forth.
Good points - I agree it is always more convincing to show than tell. We as engineers are often too busy to spend the time to figure out how to show - it's not always easy. But definitely worth it - especially when it comes to mission critical issues.
It is always better to ask stupid questions before the accident than having to testify before the Accident Review Board.
If doesn't look right, investigate until you're certain there is no problem or there is one. Is someone's word that "it's good enough" equal to an aircrews' life?
The longer I am in engineering, the longer I realize intuition is as important as math and numbers. Learning to listen to your intuition is as important as learning theories. There are so many interactions in a system, is impossible for anyone to systematically troubleshoot. Sometime when you are faced with a problem, is best to stand back and ask yourself what do you think. What does your little voice tell you.
After reading the story I don't believe that Lockheed designed a single point failure mode of the primary flight control system. All aircraft are designed with redundancy based on risk. Flight control failure though low in risk is high in severity and thus qualifies for redundancy.
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