Engine failure in a multi-engine aircraft is serious, but it is a planned and trained event rather than an automatic disaster. Twin-engine aircraft are designed so that the remaining engine can help maintain control and, under required conditions, allow continued climb or safe landing. The key physics idea is asymmetric thrust, where one side produces forward force and the other side does not.
Pilots use rudder, bank angle, airspeed control, and checklists to keep the aircraft stable.
Key Facts
- Asymmetric thrust creates a yawing moment: torque = thrust difference x engine distance from centerline.
- Lift must still balance weight in steady level flight: L = W.
- During climb on one engine, excess power is reduced: rate of climb = excess power / weight.
- The operating engine produces thrust, but also creates yaw toward the failed engine.
- Minimum control speed, Vmc, is the lowest speed at which directional control can be maintained after critical engine failure.
- Takeoff planning uses accelerate-stop distance and accelerate-go distance to decide whether the runway is long enough.
Vocabulary
- Asymmetric thrust
- Asymmetric thrust is the unbalanced forward force that occurs when one engine produces more thrust than the other.
- Yaw
- Yaw is rotation of an aircraft around its vertical axis, causing the nose to swing left or right.
- Vmc
- Vmc is the minimum control speed at which a multi-engine aircraft can still be controlled after failure of the critical engine.
- Critical engine
- The critical engine is the engine whose failure most strongly reduces aircraft performance or controllability.
- Accelerate-go distance
- Accelerate-go distance is the runway distance needed to accelerate, lose an engine at a decision point, and continue the takeoff safely.
Common Mistakes to Avoid
- Ignoring yaw after an engine failure is wrong because the working engine creates a turning moment that can quickly point the aircraft away from the runway centerline.
- Letting airspeed fall below Vmc is wrong because the rudder may no longer have enough airflow to counter asymmetric thrust.
- Using a large bank toward the failed engine is wrong because it can increase drag and reduce climb performance when the aircraft is already power-limited.
- Assuming two engines mean twice the safety in all situations is wrong because losing one engine often removes more than half the climb performance due to added drag and control limits.
Practice Questions
- 1 A twin-engine aircraft has engines 3.0 m from the centerline. One engine produces 18,000 N of thrust and the other fails. What yawing torque is produced about the centerline?
- 2 An aircraft weighs 60,000 N and has 120,000 W of excess power after one engine fails. Estimate its rate of climb in m/s using rate of climb = excess power / weight.
- 3 Explain why a pilot must maintain airspeed above Vmc after one engine fails, even if the aircraft is still producing enough lift.