Definition
A measure of an engine's fuel efficiency, expressed as the amount of fuel burned per unit of power produced over time. In piston aircraft engines it is typically given in pounds of fuel per horsepower per hour (lb/hp/hr). A lower value means the engine produces more power for less fuel.
Plain English
A number that tells you how much fuel an engine needs to make a given amount of power for one hour. The smaller the number, the more efficient the engine.
Context Anchor
Seen in range performance discussions when comparing how efficiently an engine turns fuel into usable power.
Derivation
"Brake" comes from the prony brake, an old device used to measure an engine's actual usable power output at the shaft. So "brake" power means the real power the engine delivers, not the theoretical power inside the cylinders. "Specific" means "per unit of" -- in this case, per unit of power. So the term reads as: fuel used, per unit of actual delivered power, per hour.
Why Pilots Care
It directly affects how far an aircraft can fly on a given fuel load, guiding selection of efficient cruise power settings.
Analogy
Think of it like miles per gallon for a car, but flipped: instead of distance per fuel, it's fuel per work done. Lower is better.
Intuition Check
Brake does not mean the wheel brakes are being used. Specific does not mean “exact” here; it means fuel use measured per unit of usable engine power.
Example Sentence 1
At cruise power with the mixture properly leaned, the engine's brake specific fuel consumption drops, extending the aircraft's range.
Example Sentence 2
At the power setting with the lowest brake specific fuel consumption, the aircraft covered the greatest distance on available fuel.