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.
Plain English
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.
Context Anchor
Seen in performance and operating discussions, especially when comparing engine power at sea level, high altitude, hot weather, or high-elevation airports.
Derivation
Aspirate' comes from the Latin 'aspirare,' meaning 'to breathe.' 'Normally aspirated' simply means the engine breathes in the normal way -- pulling air in through the intake using piston suction, without anything boosting or pressurizing that air first.
Why Pilots Care
Power output falls steadily as altitude rises because thinner air reduces the amount of oxygen available for combustion.
Analogy
Think of breathing through your own lungs versus having air pushed in by a pump. A normally aspirated engine is like the first case: it takes in what the surrounding air can provide.
Intuition Check
Normally does not mean “better” or “in good condition” here. It means the engine is taking in air without a pressure-boosting device. Aspirated is not a medical term here; it simply means the engine is drawing in air.
Example Sentence 1
Because the trainer has a normally aspirated engine, the instructor briefed a longer takeoff roll at the mountain airport on the warm afternoon.
Example Sentence 2
A pilot transitioning from a normally aspirated trainer to a turbocharged model must learn new procedures for managing manifold pressure.