Engine Performance and Limitations
What this domain tests
Performance questions test ratings, derates, altitude and temperature effects, and recognition of failure modes such as surge, stall and flameout.
Core rules
- Ratings: takeoff (typically 5 minutes), maximum continuous and cruise. Derates reduce takeoff thrust to extend engine life; an assumed temperature derate uses a higher fictitious OAT.
- Thrust falls with altitude and temperature because air density falls. Hot and high conditions reduce mass flow and increase EGT for the same thrust.
- Bleed air, anti-ice and accessory loads reduce available thrust.
- Failure modes: compressor surge or stall (bang, EGT rise, N1 fluctuation), flameout, bird strike, FOD and bearing failure.
Worked example
At sea level ISA, an engine produces 20,000 lb thrust. At 30°C (ISA +15°C) thrust is about 2% lower per 10°C, so roughly 20,000 × 0.97 = 19,400 lb. Reasoning: warm air is less dense, so mass flow falls; the FADEC may schedule more fuel, but EGT limits cap the recovery. If the runway is also at 5,000 ft pressure altitude, the loss is larger.
Traps
- Confusing pressure altitude with density altitude.
- Assuming a derate is a physical modification; it is a thrust setting.
- Ignoring bleed air when calculating performance.
- Treating a single surge as harmless; repeated surges require investigation.
Key points
- Ratings: takeoff, maximum continuous and cruise; derates reduce thrust.
- Thrust falls with altitude and temperature; density altitude matters.
- Hot and high conditions raise EGT for the same thrust.
- Surge, stall, flameout, FOD and bird strike are key failure modes.
- Bleed air and accessory loads reduce performance.