Definition
A vent or outlet on a fuel tank that allows excess fuel to escape overboard if the tank is overfilled or if fuel expands due to heat, preventing pressure buildup and structural stress on the tank.
Plain English
A small opening that lets extra fuel spill safely out of the tank instead of building up pressure inside it.
Context Anchor
Seen during fueling and preflight, especially around fuel tank vents, drain openings, or places where fuel may appear after the tanks have been filled full.
Why Pilots Care
Prevents tank over-pressurization and reduces the risk of fuel spillage near hot engine components.
Analogy
It works like the small overflow opening in a sink: if the level gets too high, the extra fluid has a controlled place to go instead of spilling where it should not.
Grounding Statement
If an aircraft is filled to the top in the morning and then sits in the sun, the warmed fuel can expand and some may leave through the overflow drain.
Intuition Check
Do not assume an overflow drain is a drain used to empty the fuel tank or take a fuel sample. In this context, it is mainly a safe path for excess fuel to leave the tank.
Example Sentence 1
After topping off the tanks on a hot afternoon, a small amount of fuel dripped from the overflow drain as the fuel warmed and expanded.
Example Sentence 2
The pilot checked the overflow drain for obstructions before the next flight.