Definition
A unit of mechanical power equal to 550 foot-pounds of work per second, or 33,000 foot-pounds per minute. In aviation, horsepower describes the rate at which an engine can perform work, such as turning a propeller to produce thrust.
Plain English
A measurement of how much work an engine can do in a given amount of time. The higher the horsepower, the more work the engine can perform each second.
Context Anchor
Seen in engine specifications, aircraft performance discussions, and comparisons between airplane models.
Derivation
The term was coined by James Watt in the late 1700s to compare the output of his steam engines to the work a draft horse could do. He chose a value he believed represented the sustained pulling power of a strong horse, and the unit stuck. Knowing this helps make sense of why the term sounds so ordinary for a precise engineering measurement.
Why Pilots Care
Engine horsepower directly affects takeoff distance, climb rate, and high-density-altitude performance.
Intuition Check
Horsepower does not mean physical strength like a horse pulling once. It means the rate at which the engine can keep producing useful power.
Example Sentence 1
The Cessna 172S is powered by a Lycoming engine rated at 180 horsepower.
Example Sentence 2
On a hot day the pilot noted the reduced horsepower available at the higher density altitude.