FAA Aviation Mechanic Airframe (AMA)Fuel, Cabin and Ice Control SystemsHard
A technician explains to an apprentice why manufacturer fueling instructions specify filling an integral wing fuel tank to only 98 percent of its rated volume rather than completely full. What is the primary reason for leaving this unfilled space, known as ullage?
- ATo provide room for thermal expansion of the fuel as its temperature rises
- BTo prevent fuel starvation to the engine during negative-G maneuvers
- CTo reduce the total weight of fuel carried for takeoff performance
- DTo allow proper operation of the OBIGGS nitrogen distribution system
Show answer & explanationAnswer & explanation
Correct answer: A. To provide room for thermal expansion of the fuel as its temperature rises
Ullage is the vapor space intentionally left above the liquid fuel to accommodate thermal expansion; if a tank were filled completely and the fuel later warmed (for example, on a hot ramp or during climb), the expanding fuel would have nowhere to go and could rupture the tank or discharge excess fuel overboard through the vents.
Why the other options are wrong
- B. Negative-G fuel feed is managed by flapper valves/collector tanks, unrelated to ullage.
- C. Weight reduction is achieved by planning fuel load, not by leaving expansion space.
- D. OBIGGS uses the existing ullage space but is not the reason tanks are underfilled during fueling.
Fuel Tank Ullage
The vapor space intentionally left above the liquid fuel in a tank to allow for thermal expansion of the fuel and to accommodate fuel/air vapor mixture.
- Tanks are not filled to 100% capacity
- Prevents overpressure/venting from thermal expansion
- Also the space where OBIGGS introduces inert gas
Memory trick: 'Ullage is the extra room fuel needs to stretch when it's hot.'