A twin-engine transport jet experiences a flameout of the right engine while cruising at FL410. The crew is unable to achieve an air start at that altitude. According to standard procedures, why must the crew typically descend to a lower altitude before a windmill air start will be successful?
- ALower altitude decreases indicated airspeed, which reduces structural loads on the engine mounts during restart
- BLower altitude increases air density, providing sufficient airflow and core rotation (N2) for combustion to be sustained
- CLower altitude reduces the cabin pressurization differential, allowing the outflow valve to fully close for restart
- DLower altitude reduces Mach number below the critical Mach number, preventing shock-induced separation on the engine inlet
Show answer & explanationAnswer & explanation
Correct answer: B. Lower altitude increases air density, providing sufficient airflow and core rotation (N2) for combustion to be sustained
At high altitude, low air density limits the mass airflow through the engine core, resulting in insufficient windmilling N2 rpm and fuel/air mixture to sustain combustion. Descending into the windmill relight envelope (a specific altitude/airspeed combination defined in the AFM) increases air density and ram airflow, restoring adequate core rotation for a successful air start.
Why the other options are wrong
- A. Airspeed reduction alone does not address the fundamental density/airflow requirement for combustion.
- C. Cabin pressurization and outflow valve position have no bearing on engine core airflow or relight capability.
- D. Mach number and shock effects on the airframe are unrelated to engine core airflow requirements for relight.
Windmill Relight Envelope
An altitude/airspeed range published in the AFM within which sufficient air density and ram airflow allow a flamed-out engine to windmill fast enough for a successful air start.
- High altitude has low air density, limiting windmilling N2 and combustion sustainability.
- Crews typically descend and may need a specific airspeed to enter the relight envelope.
- APU-assisted starts can supplement or substitute for windmill starts at higher altitudes.
Memory trick: Thin air up high starves the fire; dive down where the air is thick enough to feed the flame