Definition
The act of injecting a small amount of fuel directly into the engine's intake or cylinders before starting, to provide an immediately combustible fuel-air mixture that allows the engine to fire and run on its first cranks. Priming is typically performed using a manual primer pump or, in fuel-injected engines, by briefly running the electric boost pump with the mixture rich. The amount of priming required varies with engine temperature -- cold engines need more, warm engines need little or none.
Plain English
Squirting a small amount of fuel into the engine before you try to start it, so there's something ready to burn the moment the engine turns over.
Context Anchor
Seen in engine-start procedures and in discussions of Full Authority Digital Engine Control, where the system may manage priming automatically.
Derivation
From the older mechanical sense of 'prime' meaning to prepare a pump or device for operation by filling it with fluid first -- the same idea as priming a water pump so it can draw water. Here, you're filling the intake or cylinders with fuel so combustion can begin.
Why Pilots Care
Proper priming prevents hard starts, reduces wear on the starter and cylinders, and avoids flooding that can delay departure or damage the engine.
Intuition Check
Priming does not mean starting the engine. It means preparing the engine with fuel before or during the start attempt.
Example Sentence 1
On a cold morning, the pilot gave the engine three shots of primer before engaging the starter.
Example Sentence 2
The FADEC system automatically meters engine priming fuel based on temperature and throttle position.