FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is overhauling a carburetor equipped with an automatic mixture control (AMC) unit. What is the primary function of this component?
- AIt cuts off fuel flow completely when the mixture control is placed in idle cutoff
- BIt supplies a temporary burst of extra fuel during rapid throttle openings
- CIt automatically enriches the mixture during high-power climbs to prevent detonation
- DIt automatically leans the fuel/air mixture as altitude increases to compensate for decreasing air density
Show answer & explanationAnswer & explanation
Correct answer: D. It automatically leans the fuel/air mixture as altitude increases to compensate for decreasing air density
The automatic mixture control uses a bellows or bimetallic element sensitive to air density (temperature and pressure) to automatically lean the mixture as the aircraft climbs, maintaining a proper fuel/air ratio without pilot input. Enrichment for power (economizer) and acceleration (accelerator pump) are separate systems.
Why the other options are wrong
- A. This describes the idle cutoff function of the mixture control lever, a separate mechanism.
- B. This describes the accelerator pump.
- C. That describes the economizer or power enrichment system, not the AMC.
Automatic Mixture Control (AMC)
A density-sensing device (bellows or bimetallic) in the carburetor that automatically leans the mixture as altitude increases, compensating for reduced air density.
- Senses air temperature and pressure
- Automatically leans mixture with altitude
- Reduces pilot workload versus manual leaning
Memory trick: AMC is a built-in altitude 'brain' that thins the fuel as air gets thin.