Eli5: how do passenger plane engines work redundantly?


It is expected to have redundancy in most parts of design of a plane. I wonder about the engine. Is it about balancing the load by controlling roll angle in case of a failure in one of the engines?

In: 2

It isn’t roll so much as rudder, which controls yaw. The rudder is adjusted to compensate for the uneven thrust in the event of engine failure so the aircraft can continue to fly a desired path despite “crabbing” as it points off to the side.

Well there are 2 engines which are separate. When one of them fails, the other one continues to work, which is how redundancy is achieved. Of course, a 2-engine aircraft with just 1 engine working, usually does not just continue to the destination (unless it was already very close), it diverts to the nearest suitable airport.

You can fly with a dead engine. You use the other one. The engine can take the extra load for a flight, and the plane is designed to fly straight and maneuver with a dead engine.

The other redundancy is that the engine is controlled entirely by a dual computer, called fadec. It’s two computers in one, with dual everything, including dual sensors, controls and so on. Each of the two channel does monitor the other, and send messages to the cockpit if “his twin is misbehaving”. If one of the two channel fails, the other will continue the job. When you land, you replace the failed part. The concept is that you can’t statistically have two of the same failures in a single flight. And even if, see paragraph one.

End of the redundant systems.

The rest of an engine is not duplicated, you have a single fuel system, a single combustion chamber and so on. But all this parts are monitored by sensors, in a way that will almost predict issues instead of detecting them. Then the parts (if needed) are life limited. A life limit means that after a fix amount of -flight time/cycles/calendar time- they will be replaced no matter their condition.

This is a brief summary of all the different ways the plane is protected. Basically if something bad happen on a plane, all the same-type planes will have that part modified/life limited to make it impossible to happen again. Redundancy is not the favored method to fix things, altho it is used for many vital items. Otherwise, for example, is better to have a reinforced wing than to carry a spare set of wings.