Definition
In a turbofan engine, the flow of air that is moved by the fan and bypasses the engine core, traveling around the combustion section rather than through it. This bypass air is called the cold stream because it is not heated by combustion. It mixes with or flows alongside the hot exhaust from the core to produce thrust.
Plain English
The air a jet engine's big front fan pushes around the outside of the engine instead of through the burning part. It stays cool because it never gets near the fire, and it provides a large share of the engine's thrust.
Context Anchor
Seen in turbofan engine descriptions, especially when learning how fan airflow, bypass airflow, and thrust are produced.
Derivation
Called 'cold' because this airflow never passes through the combustion chamber, so it remains close to outside air temperature. 'Stream' refers to a continuous flow of air. The pairing distinguishes it from the 'hot stream' — the gases that have gone through combustion.
Why Pilots Care
Supplies most thrust in high-bypass engines, directly affecting fuel efficiency, range, and noise levels.
Grounding Statement
Picture the large fan at the front of a turbofan engine splitting the incoming air: some air goes through the hot center of the engine, and some becomes the cold stream flowing around it.
Intuition Check
“Cold” does not mean icy, weak, or safe. It means cooler than the engine’s hot core exhaust, and the cold stream can still be a fast, powerful blast of air.
Example Sentence 1
On a high-bypass turbofan, the cold stream provides most of the thrust, while the hot stream from the core contributes a smaller share.
Example Sentence 2
The mechanic inspected the cold stream nozzle for damage after the flight.