Definition
A heavy, oily lead-based compound (chemical formula Pb(C2H5)4) added in small quantities to aviation gasoline to raise its octane rating and prevent detonation in high-compression piston engines. It is the antiknock agent that gives 100LL avgas its characteristic blue dye color and the 'LL' (low lead) designation.
Plain English
A small amount of a lead chemical mixed into avgas to stop the fuel from exploding too early inside the engine's cylinders. Without it, high-power piston aircraft engines would knock, overheat, and damage themselves.
Context Anchor
Seen in discussions of avgas grades, piston-engine fuel requirements, fuel handling, spark plug fouling, exhaust residue, and leaded-fuel environmental concerns.
Derivation
From 'tetra' (Greek, four) + 'ethyl' (a chemical group with two carbon atoms) + 'lead' (the metal). The name simply describes its structure: four ethyl groups attached to a single lead atom. Knowing this helps a pilot recognize that even 'low lead' fuel still contains real lead, just in smaller amounts than older fuels.
Why Pilots Care
Engines designed for this additive can suffer detonation and failure if run on fuel without it; its environmental impact is driving the shift to unleaded alternatives.
Intuition Check
Tetraethyl lead is not added to make fuel heavier, and its main job is not to lubricate the engine. In this context, it is a poisonous lead-based additive used mainly to increase the fuel’s resistance to knock.
Example Sentence 1
The blue color of 100LL avgas comes from a dye added to identify fuel containing tetraethyl lead.
Example Sentence 2
New regulations are phasing out tetraethyl lead in avgas, prompting owners to verify engine compatibility with unleaded options.