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

Air & Space Magazine

(Continued from page 1)

Volcanic ash, which is highly abrasive, sandblasts an airplane’s skin and windows, requiring extensive repairs. Inside the engines, its effects are more varied. Ash grinds away compressor blades and reshapes the airfoils of turbine blades and guide vanes by filling up their concave surfaces. It melts and fuses on the perforated walls of combustors. It plugs up the delicate shrouds and vanes of injectors, whose job is to vaporize the fuel and mix it with just the right amount of air to ensure ignition in the generally over-lean atmosphere of the combustor. All these effects are most disruptive at high altitude, where air is thin and the conditions of combustion are most critical; crews were able to get relights only after long and harrowing glides.

Like all internal combustion engines, a jet engine compresses air, then adds fuel and ignites it. The burning gases in the combustion chamber rush toward the open back end at high speed. On the way, they deliver power—in large engines tens of thousands of horsepower—to a turbine that drives the compressor at the front. The Newtonian equal-and-opposite reaction to gas shooting out the back is the force pushing the engine forward. The whole process sounds somewhat chancy, and is. Getting a jet engine to run, then keeping it running and under control, is not a simple matter. The early notebooks of Frank Whittle, the Englishman who invented the modern jet engine (independently of, but simultaneously with, a German, Hans Pabst von Ohain), are full of descriptions of dramatically brief tests, shrieking runaways, overheated burners, and melting turbine blades. Air, fuel, and engine speed must remain balanced within certain limits; otherwise, the fire either goes out or consumes the engine around it. The immensely reliable modern jet engine is the fruit of millions of hours and billions of dollars spent getting all the parts just right: the shapes of compressor and turbine blades and the stator blades that guide the flow between them, the lubrication and seals, and the geometry of fuel injectors and igniters. Besides perfecting these components, research has produced materials for the “hot section” in and downstream of the burners, where small parts made of exotic alloys with a melting point of 2,200 degrees Fahrenheit survive, thanks to elaborate and ingenious methods of insulating and cooling, in a steady bath of 3,000-degree gas.

Nevertheless, seemingly small things can still make an engine quit. Very hot or disturbed intake air can do it. Adverse interactions between engines and armament have plagued many military jets. The engines of the A-10 “Warthog,” which are mounted on pylons beside the rear fuselage, suck up much of the gas that blows back from the muzzle of its 4,000-round-per-minute Gatling gun. Occasionally, as Air Force Captain Rusty Gideon learned the hard way (see “All Because of a Little Hot Air,” right), ingestion of gun gas can shut engines down.

The F-94 Starfire, a 1950s Lockheed fighter based on the F-80 Shooting Star, would sometimes flame out after firing salvos of rockets, which distorted the flow of air into its side-mounted engine air intakes. Even the relatively modern F-14 experienced interactions between its guns and its Pratt & Whitney TF-30 engines; its gun muzzles were retrofitted with special gas diffusers to alleviate the problem. Actually, the 21,000-pound-thrust TF-30 engine was notoriously prone to flameout for any number of reasons, including—rather inopportunely in the carrier-based Vought A-7, which had only one of them—the jolt of a catapult launch.
Improper inlet flow is said to be “distorted” because jet engines are happiest when the air entering the engine is going in the same direction, and at the same speed, at all points on the engine face. For the short inlets of airliner nacelles and the narrow range of flight attitudes they experience, uniform flow is easy to achieve. Fighters, however, present special challenges to designers. Their engines are normally buried within the fuselage and behind the cockpit, and air has to travel through ducts to reach them. Fighters maneuver violently. Inlets placed alongside the fuselage, like those of the F-14 and F-15, ingest distorted flow whenever the airplane’s nose swings to the right or left relative to the flight path. The F-16’s intake placement—like that of the Eurofighter, beneath the forward fuselage—tolerates maneuvering better.

Compressor stall—technically called surge—is a much more frequent phenomenon than flameout, and may lead to flameout. Surge occurs when engine speed, airflow, and fuel supply get out of balance and the required distribution of pressure throughout the engine is disturbed. Some or all of the blades in the compressor experience an aerodynamic stall, like that of the wing of an airplane when its nose is held too high. The abrupt pressure drop can generate one or more extremely loud bangs, and, in particularly dramatic cases, the flame from the combustor, no longer forced backward by incoming compressed air, can shoot out the front of the engine. Usually the engine recovers on its own. Frank Smith, a former Navy A-7 pilot, recalls a particularly startling compressor stall that happened during a practice dogfight in 1970. His opponent “went from 250 yards astern to 100 yards ahead in about one second while I experienced a complete end-swap and the biggest noise I ever heard from an aircraft that remained in one piece. But the engine kept running and rpm barely dropped before I was able to regain control. Hell, I really didn’t do anything but hang on.”
Surges do not always clear automatically; sometimes they are “locked in,” rotating in place within the compressor. Then, rising temperatures in the hot section force the pilot to shut the engine down. Loss of an engine affects other aircraft systems—hydraulic, pressurization, and electrical—all of which are supplied by engine-driven components. There are backup systems, but restarts can still be surprisingly difficult because of the distracting secondary effects of losing power.

Even within its operating envelope, however, the higher a jet flies, the narrower the “surge margins” that define how far conditions within the engine can stray from the optimum before it quits running. At sufficiently high altitudes, jet engines flame out simply because they run out of oxygen.

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.

Comment on this Story

comments powered by Disqus