How can modern aircraft fly on just one engine if the other one fails? Shouldn’t the torque produced by the functioning engine make the aircraft yaw?
In: Engineering
You’re correct. We refer to this as asymmetric thrust. We compensate for this by using the rudder. Multi-engine aircraft are designed with this exact scenario (loss of one engine) in mind. For this reason, engine placement is a key part of aircraft design. Manufacturers want to place the engines as far out on the wing as possible to reduce cabin noise, but the engineers want to keep the engines as close to the body of the plane as possible to minimize the possible asymmetric thrust during an engine failure. So this leads to finding a happy medium. The engines aren’t right up against the body of the plane, but they’re not out at the wings tips either.
So, when an engine fails, we deflect the rudder in the opposite direction of the failed engine. That is to say if the left engine fails, we deflect the rudder to the right. This deflection allows the plane to continue flying in a straight line. Even with this rudder deflection we can still control the plane fully in all directions. Of course we won’t be able to go as fast, but that’s not a major concern. As long as the plane is kept above a minimum speed then it will fly just fine. A modern airliner can (depending on weight) easily maintain somewhere between 20,000-30,000ft and 200-300 knots of airspeed on one engine. So essentially an engine failure in cruise or in descent is a non-issue for an airliner.
During the take off though, that’s not the case. This is the worst time for an engine failure to occur (low altitude and low airspeed and high thrust setting). This is the reason that pilots practice this manoeuvre over and over again until it becomes muscle memory. The procedure if an engine fails during the take off is essentially to keep the plane flying straight (using the rudder) and fly a certain speed (we call this speed V2). This speed is the most efficient speed that we can climb at with one engine. The plane will keep climbing (even on just the one engine) and then when we get to a safe altitude (typically 1000ft) we’ll lower the nose and pick up some speed and keep climbing away.
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