In a twin-engine aeroplane, losing an engine is not always equally serious. On many light twins, losing one particular engine makes the aircraft harder to control and reduces climb performance more than losing the other. That engine is called the critical engine: the engine whose failure would most adversely affect the aircraft's performance or handling.
Why one engine is worse to lose
On a conventional twin, both propellers turn clockwise as seen from the cockpit. Four aerodynamic effects then make the left engine critical:
1. P-factor (asymmetric blade effect)
At high angles of attack, such as during climb, the descending propeller blade produces more thrust than the ascending blade. With clockwise rotation, the descending blade is on the right side of each engine. The right engine's thrust therefore acts farther from the fuselage centreline than the left engine's. If the left engine fails, the right engine's long thrust arm creates a larger yawing moment, which is harder to control.
2. Accelerated slipstream
The extra thrust on the right side of each propeller accelerates more air over the wing behind it. The right engine's slipstream is farther outboard, so losing the left engine leaves more lift on the right wing, producing a stronger roll toward the dead engine.
3. Spiralling slipstream
The left engine's slipstream spirals around the fuselage and hits the tail, helping counter yaw toward the left. Lose the left engine and that help disappears.
4. Torque
The right engine's torque rolls the aircraft to the left, the same direction as the dead engine, making the roll worse.
Watch: the critical engine in 60 seconds
Counter-rotating propellers
Some twins, such as the Piper Seneca and Seminole, use counter-rotating propellers: the right propeller turns anticlockwise. Both descending blades are then near the fuselage, the effects balance out, and neither engine is critical.
VMC: minimum control speed
VMC (marked by a red radial line on the airspeed indicator of light twins) is the lowest speed at which the pilot can keep directional control after a sudden failure of the critical engine, under certification conditions. Below VMC, full rudder may not be enough to stop the aircraft yawing and rolling toward the dead engine. This is why multi-engine training emphasises never letting the speed decay toward VMC with an engine out.
The engine-failure drill
- Maintain control: pitch for the blue line (best single-engine rate of climb) and use rudder to stop the yaw.
- Identify: "dead foot, dead engine". The foot that is not pressing the rudder is on the side of the failed engine.
- Verify: close the throttle of the suspected engine and confirm there is no change.
- Feather the propeller of the failed engine to reduce drag, then secure it.
- Bank about 5° toward the operating engine to reduce sideslip and improve performance.
Critical engine theory is a core topic for the multi-engine rating. It ties directly into V1, VR and V2 on airline jets.
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