FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A mechanic is troubleshooting a Bendix RSA continuous-flow fuel injection system and needs to identify the component that senses the differential air pressure created by airflow through the venturi and uses it to meter fuel flow proportional to engine airflow demand.
- AThe float chamber needle valve
- BThe mixture control idle cutoff jet
- CThe servo (regulator) diaphragm within the air metering/fuel control unit
- DThe economizer power enrichment valve
Show answer & explanationAnswer & explanation
Correct answer: C. The servo (regulator) diaphragm within the air metering/fuel control unit
In an RSA continuous-flow fuel injection system, impact air pressure and venturi (suction) pressure are applied to opposite sides of a servo regulator diaphragm within the fuel control unit. The resulting pressure differential positions a fuel metering valve, delivering fuel flow proportional to the airflow through the induction system, ensuring the correct fuel/air ratio.
Why the other options are wrong
- A. Float chambers are used in carburetors, not in Bendix RSA continuous-flow injection systems.
- B. The idle cutoff jet stops fuel flow when the mixture control is placed in idle cutoff; it does not meter fuel proportional to airflow.
- D. The economizer valve enriches mixture at high power but does not sense the primary airflow-fuel metering pressure differential.
RSA Servo Regulator Diaphragm
In a Bendix RSA continuous-flow fuel injection system, the servo diaphragm balances impact and venturi air pressures to meter fuel proportional to engine airflow.
- Used in continuous-flow fuel injection systems
- Balances impact vs venturi (suction) air pressure
- Positions fuel metering valve accordingly
- Ensures fuel flow matches airflow demand
Memory trick: Servo diaphragm is the referee balancing two air pressures to set fuel flow