Definition
A lead-based chemical compound added in small quantities to aviation gasoline (avgas), most commonly 100LL, to raise its octane rating and prevent detonation in high-compression piston aircraft engines.
Plain English
A lead additive mixed into avgas in tiny amounts to stop the fuel from burning too violently inside the engine. It is what gives 100LL its anti-knock properties and the faint blue color of the fuel.
Context Anchor
Seen in aircraft fuel, engine, and maintenance discussions, especially when talking about leaded aviation gasoline, fuel grade, spark plug deposits, or safe fuel handling.
Derivation
From the chemistry: 'tetra' (four) + 'ethyl' (a small carbon-hydrogen group) + 'lead' (the metal). The name simply describes the molecule — four ethyl groups attached to a single lead atom. Knowing the 'lead' part matters operationally because lead deposits are what cause the spark plug and valve fouling pilots and mechanics deal with.
Why Pilots Care
Without adequate TEL, high-compression engines can suffer destructive detonation, limiting safe power output and engine life.
Intuition Check
TEL is not a fuel by itself. It is an additive mixed into some aviation gasoline to change how that fuel behaves in the engine.
Example Sentence 1
The mechanic explained that the rough-running magneto check was caused by TEL deposits bridging the electrodes on the lower spark plugs.
Example Sentence 2
Piston aircraft operating at high power settings require fuel with sufficient TEL to prevent detonation.