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.
Before retiring in the Pengagon I had a USN Captain boss who firmly believed in the JDLR theory: just does'nt look right. When planes were prepared to be "shot off the pointy end of the boat", there is a collection of maintenance people who are looking at the plane on the catapult and all are giving a "thumbs up" before the catapult officer "fires" the cat and launches the plane. Any one of those "checkers" who may be very junior airmen, though highly trained, are given the authority to halt the launch if he / she sees something irregular. It is a big deal to take a plane off the cat, as it has to be taken backwards to a position where the problem can be evaluated / resolved. Making the call must be supported by the maintenance officers, as the consequence of a fault on launch can be catastrophic in terms of both costly material and lives. Very often the "JDLR" is hard to define exactly, but when it is finally figured out, it may have been something very subtle and almost impossible to explain why it triggered a concern on the part of the guy who suspended the event. Also, the system has to tolerate the occassional wrong guess: when there really is nothing wrong: the technician still has to be ready to do the same thing again the next time he sees something that JDLR. Lots of the comments above bring familiar memories to mind from my 26 years of AC maintenance in the USN, USAF and aircraft manufacturer environment. None are unbelievable to me. Thanks for the memories.
Wow, the lack of good QA at the depot level is extremely scary. I worked 12 years in the general/commercial avionics world, and EVERYTHING had to be checked by an in-house FAA approved inspector. Techs certifying their own work is a recipe for disaster.
I take my hat off to anyone who is able to diagnose any electrical problem in an automobile. Perhaps with the right equipment and schematic, but to just be able to walk in and solve seems impossible.
My wife relayed a story one of the tubing bender operators told her about a young "hot-shot" engineer that had designed a tube that he was convinced saved money, material, and maintenance cost by eliminating a few fittings. He had spent days checking that the tube could be installed in one piece as opposed to being made in 3 pieces and assembled on the aircraft. The tubing bender operator rejected the part several times, only to have it resubmitted without any changes. This resulted in a "face to face" requested by the tubing bender operator. He took the engineer, who was still adamantly defending the viability of the part, down to the bending machine and proceeded to "bend" the newly engineered part. The tubing bender started whipping the tubing around wildly to make the multitude of small bends necessary resulting in large chuncks of tubing kinking, snapping off and flying around the shop....... Obviously the engineer agreed to re-work the part......this time with fittings.
Experience does count. When I had my automotive electrical shop, I could "diagnose" a number of problems just from the customer's description... Certain GM automobiles would have the radio shut off when the driver's door was opened. This was a blown Cigarette lighter fuse, most likely caused by coins in the ashtray,,,, Certain Fords would trigger the intermittent wipers whenever the headlights were turned on, or the high beams activated. This was a loose Driver side wiring harness body ground. Imagine tracking those down the first time!
I have been in manufacturing since 1964 and I love to hear a younger voice admit that there are occasions when experience counts for something. As another poster on this thread has said, sometimes things just did not sound right. It is amazing to me how much seat-of-the-pants knowledge is rejected by people new to the field because it does not appear in a textbook nor is there an algorithm written to which it can be tested.
It goes without saying that a dinosaur like me also needs to be receptive to innovation. It is called teamwork.
"Intuition" is not just wild hunches popping out of the air! Most often it is built on both experience and understanding of a system, or systems. A complete ubderstanding does not need to come from experience, and experience alone does not bring understanding. But some few people develope both, and hence become quite valuable.
What I wonde is just what kind of test would have spotted that problem, and how often are tests like that done? A short circuit between circuits on a harness plug is not something that one person would check for with a multimeter alone. Finding the problem by dignostics would have meant looking at the circuits and finding what wires could have a problem that would cause the symptom, , and then tracing that circuit to find the problem. Not a simple task by any means.
Explained neither in the story nor in the comments I've seen is what moved the quality guy to hit the gear limit switch when the elevator motion showed a transient behavior.
The story doesn't say how many years of experience the mechanic had. After spending several years working on stone machines I could diagnose many problems by a slight difference in the noise the machine made. A new operator couldn't distinguish the difference. So the twitch that was of no concern to a less experienced mechanic, obviously caught the attention of the more experienced mechanic.
Good point about asking dumb questions, MBlazer. For that to happen, a certain attitude is needed at the top -- one that welcomes all kinds of questions, not just those that sound intelligent but may, in fact, be worthless.
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