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TESTING 1, 2—Part 1: Enduring Challenges

by George Novacek

Start • A Bit of Background • Hard-Won Experience • What’s the Worst That Can Happen? • Finding a Solution • Sources

WHAT’S THE WORST THAT CAN HAPPEN?

Consider this example. A flight-control surface began to oscillate after several months of aircraft service, and exhaustive testing of the system on the ground revealed no problem. Eventually, the culprit was found in the RVDT used for position feedback. The seal around the shaft was not perfect, allowing a small amount of moist air to be sucked in every time the aircraft descended and the air pressure changed. Condensation formed when the warm, moist air entered the still-cold RVDT.

Eventually, enough condensation was trapped inside the RVDT for the water to freeze and seize the RVDT shaft. But, it did not stop the armature completely. It allowed the shaft to slightly wind up, at which point it broke away, then skipped. The system overshot and because of the torsion-spring-like action of the RVDT shaft, the flight surface went into a flapping motion that every eagle would be proud of. Once the vendor redesigned the seal, the problem went away. The possibility of catching this problem through laboratory testing was almost nonexistent.

My second example involves a control system that, after several years of flying, would generate uncommanded movements. Once the pilots discovered that the movement would happen every time they used the radio, they had great fun with it, calling it "fly-by-mike," in contrast to the more widely known fly-by-wire. Although the solution seemed simple at the first sight, the unit was rock steady when exposed to the strongest e-fields in the test lab.

Eventually, gaps caused by corrosion and mechanical stress were discovered between the airframe and the cover where the equipment was installed. The gaps, acting like a slot antenna, effectively radiated energy into a supposedly shielded space.

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