Plain English Meaning

An engine that breathes outside air just as it is, with no pump or blower forcing extra air in. The higher you fly, the thinner the air, and the less power the engine can make.

Full Definition

Describes a piston aircraft engine that draws in air at ambient atmospheric pressure, without any mechanical device to compress the intake air before it enters the cylinders. Power output therefore decreases as altitude increases and air density falls.

Context Anchor

Seen in engine performance, takeoff planning, climb performance, and go-around discussions, especially for hot days, high airports, or higher-altitude .