FAA Aviation Mechanic Airframe (AMA)Fuel, Cabin and Ice Control SystemsEasy
A technician is inspecting the fuel system of a light aircraft and notes a small vent line running from the top of the fuel tank to the outside atmosphere. What is the primary purpose of this vent system?
- ATo remove water that has settled at the bottom of the tank
- BTo provide a path for fuel vapor to be burned in the engine
- CTo equalize pressure inside the tank with outside atmospheric pressure as fuel is consumed or altitude changes
- DTo allow excess fuel to be jettisoned overboard during flight
Show answer & explanationAnswer & explanation
Correct answer: C. To equalize pressure inside the tank with outside atmospheric pressure as fuel is consumed or altitude changes
Fuel tank vents allow outside air to enter or exit the tank as fuel level and altitude change, preventing a pressure differential that could collapse or rupture the tank.
Why the other options are wrong
- A. Water removal is done through sump drains, not the vent system.
- B. Vapor is not routed to the engine through the vent system.
- D. Jettisoning fuel requires a separate dedicated system, not the vent line.
Fuel Tank Vent System
A vent line or valve that allows outside air to enter or leave a fuel tank so internal pressure stays close to ambient pressure.
- Prevents tank collapse from vacuum as fuel is used
- Prevents overpressure as altitude decreases
- Often includes a check valve and vent line routed outboard
Memory trick: Vent lets the tank breathe with the sky.