Definition
The unintended discharge of fuel from an aircraft, typically caused by overflow from a fuel tank vent or an improperly secured fuel cap, where fuel is drawn out of the tank during flight or on the ground due to airflow, pressure differences, or fuel expansion.
Plain English
Fuel leaking or being pulled out of the tank when it shouldn't be — usually through a vent or a loose fuel cap.
Context Anchor
You may encounter this term during preflight inspection, after noticing a loose fuel cap, or in flight if the fuel quantity drops faster than expected.
Derivation
Siphon comes from the Greek siphōn, meaning a tube or pipe used to draw liquid. In aviation, it describes fuel being drawn out of the tank in the same way — through an opening that lets air pressure or airflow pull the fuel out.
Why Pilots Care
Unnoticed fuel loss reduces available range and can cause engine failure or fuel imbalance emergencies.
Intuition Check
Do not assume siphoning only means someone intentionally draining fuel with a hose. In aviation, fuel siphoning can mean fuel leaving the aircraft unintentionally during flight or after a poor seal.
Example Sentence 1
After takeoff, the pilot noticed fuel streaming from the left wing and realized the fuel cap had not been seated properly, causing fuel siphoning.
Example Sentence 2
Strong airflow over the wing vent can cause fuel siphoning if the cap seal is worn.