Definition
A small calibrated passage or port that allows a controlled amount of air to escape from, or enter into, a closed system such as a hydraulic line, oil passage, or pneumatic circuit. In a feathering propeller system, an air bleed is a small port that lets trapped air slowly escape from the propeller dome or governor passages, preventing the propeller from accidentally feathering during normal shutdown while still allowing intentional feathering when commanded.
Plain English
A tiny built-in leak that lets a small, steady amount of air out of (or into) a sealed system on purpose, so the system behaves the way it should.
Context Anchor
Seen in discussions of feathering and unfeathering propellers, where oil pressure is used to change blade position.
Derivation
‘Bleed’ comes from the Old English blēdan, meaning to let blood out in a controlled way. The same idea carries over to engineering: a deliberate, small release from a closed system. Pairing it with ‘air’ tells you what is being let out.
Why Pilots Care
Correct air bleed function ensures the propeller can feather without hydraulic lock, reducing drag and protecting the engine and airframe after power loss.
Intuition Check
Do not read air bleed as an accidental air leak. Here it means a deliberate, controlled release of air from the system.
Example Sentence 1
The air bleed in the propeller dome lets pressure equalize slowly, so the blades stay in their normal range during a routine engine shutdown.
Example Sentence 2
During preflight the mechanic checked that the air bleed valve opened freely so the blades would not bind when feathering was commanded.