Definition
A fire involving flammable liquids or gases such as gasoline, oil, hydraulic fluid, grease, or fuel vapors. Class B fires are extinguished by smothering or interrupting the chemical reaction, typically using carbon dioxide, dry chemical, or foam agents. Water is not used because it spreads the burning liquid.
Plain English
A fire fueled by burning liquids like fuel, oil, or grease. You put it out by cutting off the air or the chemical reaction, not by spraying water on it.
Context Anchor
Seen on fire extinguisher labels, hangar safety procedures, aircraft maintenance training, and discussions of fuel or oil fires.
Derivation
The U.S. fire classification system (Classes A, B, C, D, K) was developed to match extinguishing agents to fuel types. 'Class B' specifically designates flammable liquid and gas fires, since these behave very differently from ordinary combustibles.
Why Pilots Care
Using water or the wrong extinguisher spreads the fire, while the correct agent stops it quickly and prevents damage or injury.
Intuition Check
Do not read Class B as meaning the fire is second-rate, less serious, or a certain size. In this context, the class tells you what kind of material is burning.
Example Sentence 1
A fuel spill ignited near the engine, creating a Class B fire that the technician extinguished with a CO2 extinguisher.
Example Sentence 2
Hangar training includes drills on identifying and suppressing a Class B fire safely.