Engine-Out Performance

Key Definitions and Regulatory Context

Engine-out performance, also called one engine inoperative (OEI) performance, is the set of requirements and data describing how an aircraft performs when one engine fails. For transport category (Class A) aeroplanes, regulations demand that both all-engines-operating (AEO) and OEI scenarios are evaluated for take-off, climb, and en-route flight. The critical engine is the one whose failure most adversely affects performance or handling.

Take-Off and Climb Performance

During take-off, performance calculations must consider both AEO and OEI cases. The take-off distance required (TODR) is determined by comparing the factored AEO distance (usually multiplied by 1.15 for net performance) and the unfactored OEI distance. For twins, the OEI distance is often limiting because losing half the thrust significantly increases the required field length. For four-engine jets, the AEO case can sometimes be limiting due to the smaller proportional thrust loss.

Climb performance after engine failure is specified in gradients and rates of climb, ensuring the aircraft can clear obstacles and maintain safe flight paths. OEI climb is typically calculated using maximum available thrust for a limited duration (e.g., 2½ minutes at OEI power), then continuous OEI power for en-route segments.

V1, Accelerate-Stop, and En-Route Considerations

V1 is the decision speed for rejecting or continuing take-off after an engine failure. Errors in V1 selection can compromise the ability to stop safely or achieve OEI climb. Accelerate-stop distance is the runway length needed to accelerate to V1 and then stop, or to continue take-off with one engine inoperative. En-route, drift-down procedures ensure obstacle clearance if an engine fails at altitude, using net flight path data.

Reduced Thrust and Performance Margins

Using reduced (flex) or derated thrust on take-off affects OEI performance by reducing initial climb gradients and increasing take-off distances. Pilots must ensure that even with reduced thrust, regulatory OEI performance criteria are met for the actual take-off mass and conditions.