Definition
In a turbine engine start sequence, the moment fuel introduced into the combustion chamber ignites and sustained combustion begins, marked by a rapid rise in exhaust gas temperature (EGT).
Plain English
The point during engine start when the fuel actually catches fire and the engine begins to burn fuel on its own.
Context Anchor
Seen in turbine engine start procedures, especially when monitoring whether the start is normal or must be stopped.
Derivation
From the everyday phrase 'lighting off' a fire or burner — the instant the flame takes hold. Aviation borrowed the term to describe the moment combustion begins inside the engine.
Why Pilots Care
Confirms successful ignition; failure to achieve light-off within time or temperature limits requires an immediate aborted start to avoid damage.
Grounding Statement
During start, the engine is first spun by the starter; light-off is the moment fuel actually begins burning inside the engine.
Intuition Check
Light-off does not mean turning aircraft lights off. In this context, it means ignition has occurred during engine start.
Example Sentence 1
After introducing fuel, the pilot watched for light-off and confirmed EGT rise within five seconds.
Example Sentence 2
A slow light-off can signal low fuel pressure or weak igniters and may require the start to be discontinued.