THERE ARE few things as bone-chilling as the thought of an aircraft engine exploding in mid-flight, particularly if you happen to be in a seat close to the fast-flying debris. Such accidents are exceedingly rare. But two incidents involving engine failures on February 20th—one affecting a passenger plane over Denver, the other a freighter over Meerssen, in the southern Netherlands—have raised worries. What causes an aircraft engine to blow up? And why are these accidents typically not as catastrophic as instinct suggests they should be?
Today, most commercial planes fly by generating thrust from jet engines. Their encased fan blades suck air and compress it. This is mixed with jet fuel and burned in the combustion chamber, which creates a jet, propelling the aircraft forward. (The whooshing exhaust exiting from tailpipes is the “equal and opposite” force that Isaac Newton popularised.) Jet engines are more technologically advanced than piston engines, which were originally developed for steam trains and steamboats and were favoured by aircraft designers until the 1940s. Such powerplants simply convert pressure into rotating motion. For aviators, they can be used to spin propellers rapidly enough to generate forward thrust. A third type of powerplant—the turboprop—looks similar to the piston engine by dint of its external propellers, but behaves more like a jet engine.
With fewer moving parts than piston engines, jet engines are much less susceptible to mechanical failure. And even if they suffer a breakdown, parallel advances in aerodynamic engineering mean that modern planes can fly safely for several hours with just one functioning engine. This has allowed regulators to ease restrictions on flying long distances away from airports, such as for trans-oceanic journeys. Such is the reliability of modern jet engines that the requirement that transatlantic flights be powered by at least three engines was dropped in the 1970s and 1980s.
Jet engines are designed to withstand some bird-strikes (many have been tested by firing dead chickens at them); and the casings, or nacelles, are meant to contain the debris of any fan blades that break. However, mechanical failures can—and do—still occur. Modern jet engines have an “in-flight shutdown rate” of as little as one per 1m flight hours (the rate for piston engines was about 300 per 1m flight hours). This covers most situations where an engine stops functioning, whether due to a mechanical fault, an external event (eg, a bird-strike or volcanic ash) or an intervention by the pilot if an engine is not functioning correctly. Typically, these shutdowns pose little danger. But they are always treated as emergencies by pilots. And, on rare occasions, they can be sudden and dramatic. The Denver and Meerssen incidents were both “uncontained engine failures”, meaning that parts of the engine disintegrated and exited the turbine in a chaotic fashion. In contained engine failures, debris either stays within the nacelle or is dispatched neatly via the tailpipe.
Though the loss of an engine should not endanger a flight, the random dispersal of debris might well. Shrapnel can damage the wing, lighting fuel reserves or causing pilots to lose control of flaps and other systems needed for a safe descent. It can also pierce the fuselage, injuring or killing passengers and decompressing the cabin. More abstractly, the stress placed on pilots while dealing with an engine failure can result in human error, particularly if the failure occurs at a critical stage of a flight or in poor weather. After the initial investigation in the Denver incident pointed the finger at metal fatigue on a fan blade of a Pratt & Whitney engine, America’s Federal Aviation Administration ordered the checking of all Boeing 777s equipped with that powerplant. If more cracks are found on that fleet globally, aviation regulators may tighten the already stringent rules for maintenance inspections. Their response will no doubt help propel the airline industry’s safety record ever higher.
