Definition
Spring-loaded or pressure-actuated auxiliary air doors fitted to a turbine engine inlet that open automatically when the engine demands more air than the normal inlet can supply, and close again when the demand drops.
Plain English
Extra air doors on a jet engine intake that pop open when the engine needs more air, then close when it doesn't.
Context Anchor
Seen in aircraft engine air-intake and powerplant discussions, especially where an engine may need added airflow during high-power operation or when the normal inlet airflow is restricted.
Derivation
Named for what they do: the doors are pushed open by air being drawn (or 'blown') in when inlet pressure drops below outside pressure. The name describes the action, not a separate technical concept.
Why Pilots Care
Prevents compressor stall or surge by maintaining adequate airflow when the normal inlet cannot supply enough air.
Analogy
Think of a flap on a vent that opens when a fan pulls more air than the main opening can supply. The flap does not power the airflow; it simply opens a backup path for air to enter.
Intuition Check
Do not read “blow-in” as meaning the door blows air into the engine. It means the door opens inward so outside air can be drawn into the engine.
Example Sentence 1
During the high-power ground run, the blow-in doors opened to supply the extra air the engine needed at low forward speed.
Example Sentence 2
The technician verified that the blow-in doors moved freely during the preflight inspection.