Performance - en-route and other flight phases (including obstacle clearance and contaminated runway)
Performance - en-route and other flight phases covers obstacle clearance away from the take-off and landing paths, driftdown, depressurisation and contaminated-runway operations. The exam tests whether you keep a legal margin when normal performance is degraded.
En-route and degraded cases
- Required obstacle gradient = (obstacle height above reference + required clearance) ÷ horizontal distance; use net performance.
- Driftdown: with one engine inoperative, maintain MSA and terrain clearance at driftdown speed; terrain may force a lower cruise altitude.
- Depressurisation: plan an emergency descent, oxygen use and terrain avoidance.
- Contaminated runway: use AFM contaminated data; contamination affects acceleration, braking and directional control, and may prohibit take-off above stated depths.
Worked example
Dry take-off distance required is 1,850 m. A contamination factor of 1.25 gives 2,313 m. TORA is 2,400 m and ASDA is 2,350 m. If dry accelerate-stop is 1,800 m, a 1.20 factor gives 2,160 m, so the take-off is legal with 87 m TORA margin. If updated slush depth raises the take-off factor to 1.35, the distance becomes 2,498 m and the take-off is not legal.
Traps
Do not use take-off obstacle gradients for en-route clearance. Contamination can invalidate a flex take-off. Do not apply dry reverse-thrust credit on a contaminated runway. Recheck ASDA and brake energy, not just TORA.