FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A technician is inspecting the induction system of a fuel-injected reciprocating engine equipped with a spring-loaded alternate air door. Under what condition does this door automatically open to admit unfiltered air?
- AWhen a pressure differential develops across a blocked or ice-clogged induction air filter
- BWhen oil temperature exceeds normal operating range
- CWhen the magneto switch is moved to the 'both' position
- DWhen the throttle is fully closed during descent
Show answer & explanationAnswer & explanation
Correct answer: A. When a pressure differential develops across a blocked or ice-clogged induction air filter
If the primary induction air filter becomes blocked by ice or debris, a low-pressure area (vacuum) forms downstream, pulling the spring-loaded alternate air door open so unfiltered, often warmer, air can bypass the filter and maintain engine airflow.
Why the other options are wrong
- B. Oil temperature has no mechanical connection to the alternate air door.
- C. Magneto switch position is unrelated to induction airflow.
- D. Throttle position does not directly actuate the alternate air door.
Alternate Air Door
A spring-loaded door in the induction system that opens automatically to supply unfiltered air if the primary air filter becomes restricted, preventing induction system icing or starvation.
- Opens due to differential pressure across a blocked filter
- Provides unfiltered, often warmer air as backup
- May also be manually selectable by the pilot in some installations
Memory trick: Blocked filter = vacuum pulls the backup door open.