Definition
A valve in the induction system of a fuel-injected reciprocating engine that allows the engine to draw air from inside the engine cowling when the normal external air intake becomes blocked, typically by impact ice. The valve may open automatically when the intake is restricted or be opened manually by the pilot from the cockpit.
Plain English
A backup air door for the engine. If the main air intake gets blocked — usually by ice — this valve lets the engine breathe in warmer air from inside the cowling so it keeps running.
Context Anchor
Seen in engine system descriptions, cockpit controls, and abnormal procedures for a blocked engine air inlet or filter.
Derivation
Alternate' comes from the Latin alternare, meaning 'to do by turns' — to switch between two options. The valve provides an alternate (second) source of air when the primary one is unavailable.
Why Pilots Care
Maintains engine power and prevents failure when the primary air path is blocked, directly affecting safety of flight.
Grounding Statement
If the engine’s normal air path is blocked, the alternate air valve gives the engine another way to breathe.
Intuition Check
Alternate does not mean an alternate airport here. It means a second source of engine air when the normal source is blocked or not suitable.
Example Sentence 1
When ice began forming on the air intake, the alternate air valve opened automatically and the engine continued running smoothly.
Example Sentence 2
During preflight, the mechanic verified that the alternate air valve moved freely so it would function if the intake became blocked in flight.