Definition
Brake horsepower is the actual power delivered by an engine at its output shaft, measured by applying a load (a brake) to the shaft and recording the work the engine performs against that load. It represents the usable power available to turn the propeller after internal mechanical losses such as friction and accessory drag have been subtracted from the engine's total developed power.
Plain English
The real, usable power coming out of the engine's shaft — the power left over after the engine has overcome its own internal friction and is ready to turn something useful, like the propeller.
Context Anchor
Seen in engine specifications, powerplant maintenance, aircraft performance discussions, and engine test records.
Derivation
The 'brake' part comes from the original test method: a mechanical brake (a Prony brake) was clamped onto the engine's output shaft to measure how much work it could do against resistance. The name stuck even though modern testing uses a dynamometer instead of a literal brake.
Why Pilots Care
It indicates the real power available for takeoff, climb, and cruise performance calculations.
Intuition Check
“Brake” does not mean the airplane’s wheel brakes here. It means the engine is being loaded by a test device so the power delivered at the output shaft can be measured.
Example Sentence 1
The engine is rated at 200 brake horsepower at 2,700 RPM.
Example Sentence 2
At higher altitudes the available BHP drops, so the pilot leans the mixture to maintain performance.