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Flameout

Why the fire in a perfectly healthy jet engine can die.

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(Continued from page 2)

Military jets are regularly tested for restart ability—for example, after an engine upgrade or modification. In twin-engine airplanes it’s routine work, but when an airplane has only one engine to begin with, it can occasionally get tense. Art Nalls, now retired from the Marine Corps, recalls a wintertime test of a TA-4J—a training version of the single-engine A-4 Skyhawk—at Edwards Air Force Base in California. The giant lake bed, an alternate landing site for space shuttles, is normally bone dry, but recent rains had soaked the ground and left it largely flooded.

The test card called for restarts at selected points along the edge of the restart envelope; if the engine failed to relight, Nalls would move to the “heart of the envelope,” where the engine was considered sure to start. “One of the last points was a low-altitude, slow-airspeed point that left little margin for error,” he remembers. “Only a small portion of the lake bed was available for landing, and it was soft. Quite possibly the airplane could flip over. But it was legally usable and met the criteria of our test plan, so we elected to continue. We were almost done with the project, everything had worked normally so far, and get-home-itis had started to set in.”
Already at a low altitude when the test began, Nalls found it impossible to restart the engine. Only when he was below 1,000 feet, seconds away from a landing on the muddy lake bed, did the engine finally relight. It later turned out that the cause of the trouble had been a malfunctioning ram air turbine—the backup electrical source for the engine’s igniters.

Nalls was a test pilot, and test pilots feel strong pressure to bring back the ship in one piece. Under the same circumstances, a service pilot whose jet had flamed out would long since have ejected. The likelihood of making a successful dead stick landing in a jet fighter is considered so slight that the military services have wavered on whether “flameout approaches” should be taught at all.

Though the reliability of jet engines is far better than that of the reciprocating engines that they largely replaced half a century ago, the danger of flameouts hasn’t disappeared. Flameouts are a natural consequence of the way jet engines work. They live on an island of stable operation—a dynamic balance of powerful forces—ringed by a sea of instability.

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