Definition
The most volatile, low-boiling-point hydrocarbon fractions in a fuel — the components that vaporize first and at the lowest temperatures during the refining or use of petroleum products such as aviation gasoline.
Plain English
The parts of a fuel that turn to vapor most easily. They evaporate at low temperatures and are the first to boil off when the fuel is heated.
Context Anchor
Seen in aviation fuel discussions, especially when talking about fuel evaporation, vapor pressure, engine starting, and vapor forming in the fuel system.
Derivation
‘Light’ refers to low molecular weight and low boiling point — these molecules are physically lighter and leave the liquid first. ‘Ends’ comes from refining, where distillation separates fuel into fractions at each ‘end’ of the boiling range. The ‘light ends’ are the fractions that come off the top (low temperature) end of the distillation.
Why Pilots Care
Excess light ends raise vapor pressure and can cause vapor lock; too few can make cold starts difficult.
Intuition Check
Light ends are not the physical ends of a fuel tank or container. In this context, they are the easiest-to-evaporate parts of the fuel.
Example Sentence 1
On a hot day at altitude, the light ends in the fuel can vaporize in the lines and cause vapor lock.
Example Sentence 2
Fuel blenders adjust the amount of light ends to keep vapor pressure within safe limits for the season.