FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A pilot operating a fuel-injected engine at high cruise altitude on a hot day is advised to switch on the electric fuel boost pump. What problem is this action primarily intended to prevent?
- AExcessive oil consumption at altitude
- BDetonation caused by an over-rich mixture
- CFormation of fuel vapor in the lines that could interrupt fuel flow
- DOvercharging of the induction manifold
Show answer & explanationAnswer & explanation
Correct answer: C. Formation of fuel vapor in the lines that could interrupt fuel flow
High altitude combined with high temperatures lowers the boiling point margin of fuel, increasing the risk of vapor forming in fuel lines. The boost pump maintains positive fuel line pressure, keeping fuel in liquid form and ensuring uninterrupted flow to the engine-driven pump and injectors.
Why the other options are wrong
- A. The boost pump is a fuel system component and does not affect oil consumption.
- B. An over-rich mixture is unrelated to boost pump activation and would not cause detonation.
- D. Manifold pressure is controlled by the throttle/turbocharger, not the boost pump.
Electric Fuel Boost Pump
An auxiliary pump that pressurizes fuel lines to prevent vapor formation and assist the engine-driven pump during high altitude, hot weather, takeoff, and engine starting.
- Maintains positive fuel pressure to prevent vaporization
- Used during takeoff, landing, high altitude cruise, and priming
- Backs up the engine-driven fuel pump if it fails
Memory trick: Boost pump keeps fuel a liquid, not a bubble.