Let's start with a brief review of the aerodynamics that makes the aircraft fly. During normal flight, the engine(s) provide the aircraft forward motion that creates a solid flow of air around the airfoils. The wings are shaped in such a way that the air above the wings have to cover more distance and therefore have to flow faster than the air below the wings. This creates lift that keeps the aircraft in the air. Below a certain speed, however, the airflow above the wing gets turbulent. Then (almost) all lift is lost and the aircraft stalls. Remember: it is not the engine that makes the aircraft fly. It is the wing.

Multi-engine aircraft:

Straight and level flight occurs when the forces acting on the aircraft (thrust vs drag, lift vs gravity) are balanced. But this is not enough. These forces must also be symmetrical, or the aircraft will turn around one of its 3 axis (vertical, horizontal, longitudinal) Therefore the thrust produced by the left engine must equal the thrust produced by the right one and, similarly, the lift pruduced by the left wing must equal the lift produced by the right one. see illustration labelled "normal flight"

Legend: green: op. engines, red: inop. engines, blue: thrust / lift vectors, gold: motion vectors

Losing one engine:

When one of a twin-engine aircraft's motors stops, the thrust gets unbalanced and the aircraft immediately starts to turn around its vertical axis, towards the faulty engine. As the aircraft turns, the wing with the faulty engine gets slower than the wing with the good engine. The slower wing produces less lift and the aircraft starts to turn around its longitudinal axis as well banking towards the faulty engine. see illustration labelled "right engine out" While it is bad enough to lose half the engine power, what makes the situation really intimidating is this: if you don't act immediately, the wing with the faulty engine slows down too much, stalls, and the aircraft falls into an urrecoverable spin.

How to save the day:

In order to keep the blue side up, you must immediately compensate the yaw towards the faulty engine with the rudder pedals, turning your nose back towards the good engine until you fly straight again. But this is not all. For reasons beyond the scope of this article, the rudder also acts as aileron, simultaneously turning and banking the aircraft in the same direction. Many pilots apply just enough rudder to keep the wings level, but it's insufficient. see illustration labelled "insufficient left rudder" It may be enough to save you from falling into a spin but usually it's not enough to stop the good engine turning the aircraft around its vertical axis. The plane will keep turning, although at a reduced rate, causing you trouble. Flying straight will require repeated heading corrections or flying with a constant bank towards the good engine. Both is very dangerous during the approach.

Therefore you must use enough rudder to make the aircraft fly straight. see illustration labelled "sufficient left rudder" If you do so, however, the aircraft will bank towards the good engine due to the rudder's aileron effect. In order to keep your wings level, you'll have to compensate this with aileron input towards the faulty engine. Then the aircraft will fly straight and wings level, with a little sideslip towards the faulty engine. (the nose will not point towards the direction of flight) This is a normal situation that you have already been involved in several times during crosswind landings. see illustration labelled "left rudder + right aileron"

In theory, it's easy, in practice it isn't. The tricky part is finding the right amount of rudder and aileron required to maintain a straight and wings level flight. The problem is that maintaining the balance of forces requires continuous attention, as there are various factors influencing the amount of rudder and aileron needed for the straight and wings level flight:

• airspeed: the higher the airspeed, the less rudder is needed
• engine power: the higher the power, the more rudder is needed
• mechanization: the more flaps used, the more rudder is needed

In level flight, speed changes have little effect on the amount of rudder needed, as the influences of the increased speed and increased power weaken each other. Deploying the flaps, however, have a dramatic effect on the balance of forces, because the influences of the increased power and decreased speed strengthen each other. The most rudder is needed in case of a one-engine takeoff or go-around due to the high power setting combined with a low airspeed. Try to avoid such a situation if it's at all possible. Good Luck!