Plain English Meaning

An engine that breathes outside air at whatever pressure the atmosphere happens to be, with no pump or compressor helping it. The higher you fly, the thinner the air, and the less power the engine can make.

Full Definition

A reciprocating aircraft engine that draws its intake air at ambient atmospheric pressure, without the use of a turbocharger or supercharger to compress the air before it enters the cylinders. Because intake pressure depends entirely on the surrounding atmosphere, engine power output decreases as altitude increases and air density falls.

Context Anchor

Seen in performance and operating discussions, especially when comparing engine power at , , hot , or - airports.