Definition
The actual horsepower delivered by an engine at its output shaft, measured by applying a braking load to the shaft and recording the power required to hold it at a given speed. Brake horsepower represents the usable mechanical power available to drive a propeller, after internal engine losses such as friction and pumping have already been subtracted.
Plain English
The real, usable power coming out of the engine shaft — the power that actually turns the propeller, not the theoretical power produced inside the cylinders.
Context Anchor
Seen in aircraft engine specifications, aircraft manuals, and performance discussions about takeoff, climb, and cruise power.
Derivation
Named for the way it is measured. Early engineers used a device called a Prony brake — essentially a friction brake clamped onto the engine's output shaft. By measuring how hard the engine had to work to overcome the brake at a set speed, they could calculate the actual delivered power. The 'brake' in the name refers to that test method, not to aircraft brakes.
Why Pilots Care
It gives the true power available for takeoff, climb, and cruise calculations, directly affecting performance predictions and safety margins.
Intuition Check
Brake does not mean the airplane’s wheel brakes are involved. Here, brake refers to the resistance used to measure how much power the engine can deliver.
Example Sentence 1
The engine is rated at 180 brake horsepower at 2,700 RPM.
Example Sentence 2
Performance charts list brake horsepower so pilots can calculate expected climb rate.