Definition
A gasoline fuel that contains no tetraethyl lead (TEL) compounds added to raise its octane rating. In aviation, unleaded gasoline refers to fuels formulated to meet aircraft engine octane and performance requirements without the use of lead-based anti-knock additives.
Plain English
Gasoline that has no lead added to it. Lead used to be mixed into aviation fuel to help engines run smoothly at high power, but unleaded versions achieve the same performance through other means.
Context Anchor
Seen in aircraft fuel approvals, pilot operating handbooks, fuel placards, airport fuel pumps, and discussions of automotive gasoline or unleaded aviation gasoline.
Derivation
"Unleaded" simply means "without lead added." Lead (as tetraethyl lead) was historically blended into gasoline starting in the 1920s to prevent engine knock. "Unleaded" became a standard term in the 1970s when automotive fuels began removing lead for environmental and health reasons. Aviation has been the last major holdout, which is why this distinction still matters in flying.
Why Pilots Care
Correct unleaded fuel prevents engine damage and supports regulatory efforts to reduce lead emissions.
Intuition Check
Unleaded does not mean “approved for any airplane.” It only means the fuel has no lead additive; the aircraft still must be approved for that fuel and its required grade.
Example Sentence 1
Before refueling, the pilot confirmed that the aircraft's engine was approved for the unleaded gasoline being offered at the FBO.
Example Sentence 2
Switching to unleaded gasoline reduced harmful deposits inside the cylinders on the next flight.