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.
This story reminds me of a physics professor that refused to give partial credit on complex problems. His reasoning was that in real life even tiny mistakes like an errant punch slug can have disastrous consequences. We as engineering students had to learn that there is no almost wrong or almost right.
The problem described would have been in the autopilot system, not the primary flight controls. As kenish correctly notes, C-130's have hydraulically-booseted manual controls, not fly-by-wire. The Functionl Check Flight crew would not have the autopilot engaged during takeoff, but could have had an unexpected surprise when they engaged the autopilot in flight. However, they could have quickly disengaged the autopilot, and most autopilots have clutches that will allow the crew to overpower the autopilot inputs with the cockpit controls.
TJ, the metallic sliver was likely introduced by drilling of the structure near the harness, which introduced shavings into the harness that were not properly cleaned up. Have seen this several times. You would be shocked at the lack of QA at some of the military overhaul depots. One major depot that one of my aircraft went through had no independant inspection. They relied on the technician performing the work to inspect his own work (all in the interest of cost savings).
I always preflight my plane, truck and motorcycle. On the C-130 in the article a preflight probably would not have found the problem. The elevator isn't visible from the cockpit, so "Flight Controls Free and Correct" would not help. The first sign of trouble would have been at gear retraction as the author pointed out.
I do wonder a bit on the veracity....the flight controls in a C-130 "Herky Bird" are mechanical...pushrods, bellcranks, cables, and pulleys. AFAIK even the latest "J" version is not fly-by-wire.
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.
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.
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?
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.
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.
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