FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A mechanic is troubleshooting an aircraft fuel tank venting system after a pilot reports fuel starvation symptoms despite an indicated full tank. Which condition would most likely cause this problem?
- AA blocked fuel tank vent creating a vacuum that prevents fuel from flowing to the engine
- BA malfunctioning fuel quantity indicator giving a false full reading
- CA leaking fuel cap allowing outside air to enter the tank freely
- DAn oversized fuel vent allowing excessive airflow into the tank
Show answer & explanationAnswer & explanation
Correct answer: A. A blocked fuel tank vent creating a vacuum that prevents fuel from flowing to the engine
Fuel tank vents allow outside air to enter the tank as fuel is consumed, preventing a vacuum from forming. If the vent becomes blocked (by ice, insects, or debris), a vacuum forms above the fuel as it is drawn out, eventually restricting or stopping fuel flow to the engine, causing symptoms of fuel starvation.
Why the other options are wrong
- B. A faulty quantity indicator affects the displayed reading, not actual fuel delivery.
- C. A leaking cap allowing extra air in would not cause a vacuum-related starvation problem.
- D. An oversized vent would not restrict fuel flow; if anything, it allows too much air in without issue.
Fuel Tank Venting System
A system that allows outside air to enter the fuel tank as fuel is consumed, preventing a vacuum from forming that would restrict fuel flow.
- Blocked vents can cause fuel starvation due to vacuum formation
- Vents can be blocked by ice, insects, or debris
- Some aircraft have vent line heating or check valves to prevent icing
Memory trick: Blocked vent = a straw with your thumb over the top, fuel won't flow