FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A technician is inspecting a turbocharged reciprocating engine equipped with an absolute pressure controller (APC) in addition to the standard wastegate actuator. What is the function of the APC?
- AIt senses oil temperature and modulates oil flow to the turbocharger bearings
- BIt senses manifold absolute pressure and overrides wastegate control to prevent overboost regardless of throttle position
- CIt measures exhaust gas temperature and retards ignition timing to prevent detonation
- DIt meters fuel flow to the injector nozzles based on compressor discharge pressure
Show answer & explanationAnswer & explanation
Correct answer: B. It senses manifold absolute pressure and overrides wastegate control to prevent overboost regardless of throttle position
The absolute pressure controller senses manifold absolute pressure (MAP) and, if it senses an impending overboost condition, it overrides the pilot's throttle-driven wastegate control by directing oil pressure to open the wastegate further, protecting the engine from excessive manifold pressure.
Why the other options are wrong
- A. Oil temperature control is not the APC's function; that would be an oil cooler bypass valve.
- C. Retarding ignition timing based on EGT is not performed by the APC.
- D. Fuel metering based on compressor discharge is a function of a turbine fuel control unit, not the APC.
Absolute Pressure Controller (APC)
A turbocharger safety device that senses manifold absolute pressure and automatically overrides wastegate control to prevent the engine from being overboosted.
- Protects against overboost regardless of throttle input
- Works with, not instead of, the wastegate
- Common on turbocharged reciprocating engines
Memory trick: APC is the 'safety cop' that grabs the wastegate wheel if MAP gets too high.