FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationEasy
A student pilot asks a mechanic why the manifold pressure gauge on a normally aspirated reciprocating engine reads well below the local atmospheric pressure when the engine is idling on the ground. What is the correct explanation?
- AThe engine-driven fuel pump is drawing down manifold pressure
- BCarburetor heat is automatically applied at idle, lowering pressure
- CThe propeller governor reduces manifold pressure at low RPM
- DThe nearly closed throttle valve restricts airflow, creating a partial vacuum in the intake manifold
Show answer & explanationAnswer & explanation
Correct answer: D. The nearly closed throttle valve restricts airflow, creating a partial vacuum in the intake manifold
At idle the throttle plate is nearly closed, restricting airflow into the manifold. The piston's downward intake stroke still tries to draw a full charge, so pressure downstream of the restricted throttle drops well below atmospheric, producing a low manifold pressure reading.
Why the other options are wrong
- A. The fuel pump moves fuel, not air, and does not affect manifold air pressure.
- B. Carburetor heat is pilot-selected, not automatic at idle, and affects intake air temperature, not the basic pressure drop.
- C. A propeller governor controls RPM via blade pitch, not manifold pressure directly.
Manifold Pressure at Idle
On a normally aspirated engine, manifold pressure at idle is far below atmospheric because the nearly closed throttle valve restricts airflow, creating a vacuum downstream.
- Idle throttle plate is nearly closed
- Low MP reading indicates high throttle restriction
- At full throttle, sea level, MP approaches atmospheric pressure
- Supercharged engines can exceed atmospheric pressure
Memory trick: Closed throttle = closed door = low pressure inside