What happens if an airplane's engines fail?

An engine failure is the scenario that most shapes the design of a commercial aircraft. FAA and EASA certification rules require that a twin-engine airplane be able to continue the takeoff with one engine shut down from the critical point on the runway, climb at a minimum gradient with the gear retracted, and fly and land normally. Every takeoff is calculated under that assumption: V1 speed, beyond which the pilot continues even if an engine fails, exists precisely for that reason.

In cruise, a twin with one engine stopped descends to a lower altitude, around 6,000-7,500 m, because the remaining engine cannot sustain cruise altitude, and heads to the nearest suitable airport. ETOPS rules set how far a twin-engine aircraft can stray from a runway under those conditions: up to 180 minutes of flight on one engine for most airplanes, and over 300 minutes for some Boeing 777, 787 and Airbus A350 models, which covers almost any oceanic route. Pilots practice engine failure in every simulator session, repeated every six months.

The simultaneous loss of all engines is extraordinarily rare and almost always has a common cause: fuel exhaustion, bird ingestion, volcanic ash or ice in the fuel system. In that case the aircraft glides, with flight controls and essential systems powered by the APU and by the ram air turbine, which deploys automatically. From cruise altitude there is over 100 km of range and enough time to attempt a restart, which succeeded in most known incidents.

A "contained" failure, in which the engine stops or loses thrust, doesn't affect anything else. "Uncontained" failures, in which rotor fragments pierce the casing, are what worry engineers: they can damage hydraulic lines, the wing or the cabin. To minimize them, turbine disks are inspected on a regular schedule and the fan casing is designed to contain a detached blade, a test that is filmed for every new engine. The result is that in today's commercial aviation, with tens of millions of flights a year, accidents caused by engine failures can be counted on the fingers of one hand per decade.