Definition
A secondary intake that allows engine induction air to be drawn from inside the engine cowling when the normal external air filter or intake becomes blocked, typically by induction icing or other obstruction. On fuel-injected engines it is often pilot-selectable via an alternate air control; on some installations it opens automatically when the primary intake is restricted.
Plain English
A backup way for the engine to get the air it needs to keep running if the normal air inlet gets blocked. The pilot (or the system itself) opens a second opening so the engine can keep breathing.
Context Anchor
Seen in induction icing and engine power-loss discussions, especially when ice can block the normal airflow into the engine.
Derivation
‘Alternate’ comes from Latin alternus, meaning ‘one after the other’ — here, an alternative path for air. It signals a backup route, not a preferred one.
Why Pilots Care
Allows continued engine operation and prevents power loss or failure when ice blocks the primary intake.
Analogy
It is like having a second doorway into a room. If the main door is blocked, people can still get in through the other door.
Grounding Statement
During icing conditions, the engine may need a different air path so it can keep running even if ice forms at the normal intake.
Intuition Check
Do not read “alternate air source” as cabin air, oxygen, or air for the pilot to breathe. Here it means backup intake air for the engine.
Example Sentence 1
When the engine started running rough in visible moisture, the pilot suspected induction icing and selected the alternate air source.
Example Sentence 2
The preflight checklist included verifying that the alternate air source valve moved freely.