Airplane engines are numbered beginning at one from the farthest left of the pilot’s point of view. In other words, the engine to the farthest left of the pilot is engine one. If it’s a four-engine airplane, the next closest engine will be engine two. And the two engines to the right of the pilot will be engines three and four, respectively.

In the case of a twinjet, such as a Boeing 777 or an Airbus A350, the engine on the left hand side wing will be number 1 while number 2 would be the one mounted under the right wing. In the case of a trijet number one will be under the left wing, number 2 will be mounted on the vertical stabiliser and number 3 under the right wing.