Definition
The separate groups of hydrocarbon products produced when crude oil is processed in a refinery. During refining, crude oil is heated and separated by boiling point into distinct ranges, each containing hydrocarbons of similar size and weight. Each range is called a fraction, and each fraction becomes the basis for a different finished product such as aviation gasoline, jet fuel, kerosene, lubricating oil, or asphalt.
Plain English
Crude oil is a mixture of many different oily substances. When it is heated at a refinery, the lighter ones boil off first and the heavier ones boil off last. Each batch that comes off at a particular temperature range is called a fraction, and each fraction is used to make a different fuel or oil product.
Context Anchor
Seen in aviation maintenance discussions of fuels, oils, lubricants, and other petroleum-based materials used on aircraft.
Derivation
Fraction comes from the Latin fractio, meaning a breaking or piece broken off. In refining, the crude oil is literally broken apart into pieces by boiling temperature, which is why each piece is called a fraction.
Why Pilots Care
The specific fractions chosen determine fuel properties such as volatility and freezing point, directly affecting engine reliability and safety.
Intuition Check
Do not read “fractions” here as a math problem. Here it means separated parts of crude oil, grouped by how they behave when heated.
Example Sentence 1
Aviation gasoline and lubricating oil come from different petroleum fractions, which is why they have very different boiling points and uses.
Example Sentence 2
Different petroleum fractions are refined into avgas for piston engines and jet fuel for turbine engines.