Definition
A grade of leaded aviation gasoline with a minimum octane rating of 100, containing a reduced amount of tetraethyl lead compared to earlier high-octane aviation fuels. It is dyed blue for visual identification and is the most widely used piston-engine aviation fuel in general aviation today.
Plain English
The standard blue-dyed aviation gasoline used by most piston-engine aircraft. The 100 refers to its octane rating, and the LL means it has less lead in it than the older high-octane fuel it replaced.
Context Anchor
You will see AVGAS 100LL on fuel trucks, airport fuel pumps, fuel caps, and in the fuel section of an aircraft flight manual.
Derivation
AVGAS is simply a contraction of 'aviation gasoline.' The '100' is the octane rating — a measure of the fuel's resistance to detonation. 'LL' stands for 'Low Lead,' indicating it contains less tetraethyl lead than the older 100/130 grade it replaced. Despite the name, 100LL still contains lead — it is just lower than its predecessor.
Why Pilots Care
Using the wrong fuel grade can cause detonation, engine damage, or power loss in flight.
Intuition Check
Low-lead does not mean unleaded; AVGAS 100LL still contains lead. The 100 is an octane grade, not a percentage or a fuel quantity.
Example Sentence 1
Before departure, the pilot confirmed the fuel truck was dispensing 100LL and checked that the sample drained from the sumps was clear and blue.
Example Sentence 2
Before departure the pilot verified that the FBO had AVGAS 100LL available rather than jet fuel.