FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is troubleshooting a light aircraft's fuel quantity indicating system that uses a float-type sender. As the fuel level changes, the float arm moves a wiper across a resistive element inside the sender. What does this changing resistance directly control?
- AThe position of the fuel selector valve
- BThe pressure output of the engine-driven fuel pump
- CThe speed of the electric fuel boost pump
- DThe current flow in the gauge circuit, which the cockpit indicator interprets and displays as fuel quantity
Show answer & explanationAnswer & explanation
Correct answer: D. The current flow in the gauge circuit, which the cockpit indicator interprets and displays as fuel quantity
In a resistance-type float fuel quantity system, the float's position varies the resistance of the sender, which changes the current flowing through the cockpit indicator's circuit. The indicator is calibrated so that this varying current is displayed as a corresponding fuel quantity reading, typically from empty to full.
Why the other options are wrong
- A. The fuel selector valve is a mechanically operated valve, unrelated to the electrical fuel quantity sender.
- B. Engine-driven fuel pump output pressure is unrelated to the resistive fuel level sender circuit.
- C. The fuel quantity sender has no electrical connection to or control over the boost pump's speed.
Float-Type Resistance Fuel Quantity Sender
A fuel quantity sensing device where a float-actuated wiper arm varies resistance on a rheostat, changing current in the indicator circuit to display fuel quantity in the cockpit.
- Float position varies sender resistance
- Resistance change alters circuit current
- Cockpit gauge interprets current as fuel quantity
- Common in light, piston-powered aircraft
Memory trick: Float rises, resistance changes, current tells the tale of fuel level