What are the two most critical factors in determining the in-flight relight or restart of a jet engine?

• A. Altitude and Airspeed
• B. Temperature and Humidity
• C. Throttle position and fuel flow
• D. Engine RPM and oil pressure

Answer: (A)

The main concept is that in-flight relight depends on having enough air mass flow through the engine to sustain combustion and allow the compressor to re-spool. The two factors that best govern this are altitude and airspeed because they determine how much air is available and how effectively it can be delivered to the engine.

At a higher altitude, the air is thinner, so even with the starter and ignition ready, there may not be enough oxygen and mass flow to support a flame and achieve stable combustion. Conversely, sufficient airspeed provides ram pressure that boosts the compressor inlet pressure, helping to maintain the necessary pressure and flow for relight. If the aircraft isn’t flying fast enough, the engine may not receive the required air mass flow to sustain ignition.

Fuel delivery and ignition are still essential, but without adequate air density and momentum to drive the flow through the engine, relight cannot be established or sustained.