What Is the Critical Engine?
Definition
The critical engine on a multi‑engine, fixed‑wing aircraft is the engine whose failure would most adversely affect the aircraft’s handling and performance. In simpler terms, it’s the engine that causes the most trouble if it stops working.
Why It Matters
When an engine fails, several aerodynamic and mechanical effects emerge:
- Asymmetrical thrust: The operating engine creates yaw (turning) and roll toward the side of the failed engine due to an imbalance.
- Control challenges: The pilot must use rudder and aileron to counter yaw and roll. The ‘critical engine’ failure results in the most significant control difficulty.
- V<sub>MC</sub> (Minimum Control Speed): Aircraft performance data—like V<sub>MC</sub>—are established assuming failure of the critical engine to ensure safety margins.
Which Engine Is Usually Critical—and Why?
Typically, on conventional twins (both propellers rotating clockwise, as seen from the cockpit), the left engine is critical. This is due to four aerodynamic factors, commonly summarized by the mnemonic PAST:
1. P-Factor
- The descending blade (right side of each prop) produces greater thrust, shifting the center of thrust laterally.
- The right engine’s thrust is farther from the aircraft’s center of gravity, creating a larger yaw moment if the left engine fails—making the left engine critical.
2. Accelerated Slipstream
- Propeller wash (slipstream) generates more lift farther from the fuselage on the right wing.
- If the left engine fails, the stronger right-side lift imbalance causes a more severe roll toward the failed side.
3. Spiraling Slipstream
- Clockwise rotating propellers produce spiraling airflow.
- On a left engine failure, the right engine’s slipstream does not hit the vertical stabilizer, providing no yaw correction; whereas if the right engine fails, the left engine’s slipstream helps counteract yaw.
4. Torque
- Propeller rotation induces a rolling motion opposite to rotation due to Newton’s third law.
- When the left engine fails, the torque from the right engine adds to yawing tendency, making control even harder. Conversely, if the right engine fails, torque helps counter the roll.
Exceptions: When There Is No Critical Engine
- Counter‑rotating props: In twins designed so that each prop rotates in opposite directions, the adverse effects balance out—making no single engine critical.
- Turbojet/turbofan aircraft: Modern jets with engine pods under the wings typically experience similar yawing moments whether the left or right engine fails—there is no defined critical engine.
- Special configurations: Some aircraft with asymmetric systems (like hydraulic or electrical systems driven by one engine) can introduce non‑aerodynamic criticality—making that system-dependent engine critical.
