Why are thrust and horsepower difficult to compare directly for aircraft engines?

Comparing thrust and horsepower for aircraft engines is difficult because they measure fundamentally different physical outputs and depend on varying flight conditions.

Thrust is a force—the reaction to accelerating a mass of air—governed by Newton’s second law of motion. In contrast, horsepower is a measure of power, or the rate at which work is performed. While thrust can be expressed as "thrust horsepower" by multiplying the thrust in pounds by the aircraft's speed and dividing by 375, this value is highly dependent on the flight velocity.

Because a jet engine's efficiency is tied to how effectively it accelerates an airstream relative to the aircraft's speed, the "power" produced by an engine changes as the flight speed changes. Furthermore, jet engines are designed to optimize propulsive efficiency by matching jet velocity to flight speed, making a static horsepower rating an inadequate metric for comparing engines across different operational profiles. Consequently, jet engine performance is more accurately assessed through specific fuel consumption per unit of thrust rather than a fixed horsepower value.