Definition
Gravity-actuated valves used in some aircraft fuel systems to ensure that fuel is drawn from the lowest point of the tank during maneuvering flight. The valves consist of a weighted arm that swings freely under gravity, opening the fuel pickup port that is closest to the bottom of the tank as the aircraft's attitude changes, while closing ports that would otherwise draw air.
Plain English
Small swinging valves inside a fuel tank that always direct fuel from whichever spot in the tank is currently lowest, so the engine keeps getting fuel even when the aircraft is climbing, descending, or banking.
Context Anchor
Seen in aircraft fuel system descriptions, especially when discussing fuel tank construction, fuel flow in turns, and keeping fuel near the tank outlet.
Derivation
From the Latin pendulum, meaning 'hanging' or 'swinging.' The same root gives us the swinging weight of a clock's pendulum. The name fits because the valve's weighted arm hangs and swings under gravity, just like a clock pendulum.
Why Pilots Care
Prevents the engine from starving for fuel or flooding during takeoff, climb, or abrupt maneuvers.
Intuition Check
Do not think of a pendulum valve as a clock part. In a fuel tank, “pendulum” means the valve swings on a hinge so fuel can move mainly one way.
Example Sentence 1
The aerobatic trainer uses pendulum valves in its fuel tank so the engine continues to receive fuel during inverted and high-bank maneuvers.
Example Sentence 2
The mechanic checked the pendulum valves for free movement while overhauling the carburetor.