Why the fire in a perfectly healthy jet engine can die.
- By Peter Garrison
- Air & Space magazine, September 2006
(Page 3 of 3)
Not all flameouts are accompanied by noise or vibration or by any obvious triggering event. In some cases, especially on multi-engine airplanes, one engine may spool down unnoticed by the pilot, while autopilot and autothrottle conspire to mask the thrust asymmetry. In a few instances, crews have temporarily lost control because they failed to realize that one engine has stopped producing thrust.
A fatal accident in 2004 illustrates the potentially dire consequences of inattention to engine parameters and the unexpected difficulties that can beset restart attempts. Two pilots flying a Canadair regional jet to its next departure location decided, on a lark, to take the airplane up to its 41,000-foot ceiling, where neither had ever been. They programmed the autopilot to climb at a fixed rate. As the airplane ascended into ever thinner air and the engines produced less and less thrust, the autopilot had to keep reducing speed in order to maintain the commanded climb rate. The crew did not notice anything was wrong until both engines flamed out.
The pilots turned to the restart checklist, which first required descending rapidly to a lower altitude. Meanwhile the engines spooled down, and unequal cooling of closely fitting seals in the compressor caused them to bind—a condition now dubbed “core lock.” The engines would not spool up, either from windmilling or with the help of the auxiliary power unit. By the time the crew realized that the engines would not come back, they were too low to reach the nearest landing field. The aircraft crashed a couple of miles short of a runway; both pilots were killed.
Engines that have flamed out and that have not been damaged by, say, a violent compressor surge can, in principle at least, be restarted. The difficulty of restarting, and the time it takes, depend on several factors, one of which is how much the engine has spooled down. With sufficiently high forward speed and sufficiently low altitude—generally above 250 knots and below 25,000 feet—engines can windmill up to a speed sufficient to permit ignition; then they gradually bootstrap back to operating speed and compression. Although jets, like any airplane, can glide without power—airliners can progress 10 miles or more horizontally for every mile of altitude they give up—the speed required for a windmilling start is much higher than the best glide speed, and so altitude melts away rapidly during restart efforts.
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.