Definition
In a turbofan engine, the portion of air drawn in by the fan that flows around the engine core rather than through the combustion chamber and turbine. This air provides a large share of the engine's thrust and helps cool and quiet the engine.
Plain English
Air that the big front fan pushes around the outside of the engine instead of through the burning part. It still produces thrust, just by being shoved backward rather than burned.
Context Anchor
Seen in gas turbine engine discussions, especially when learning how a turbofan engine makes thrust.
Derivation
Bypass means to go around something instead of through it. Here the air literally bypasses the engine core, flowing around it on its way out the back.
Why Pilots Care
Bypass air supplies most of the thrust in modern high-bypass engines, improving fuel efficiency, reducing noise, and increasing overall performance.
Analogy
Think of two air paths through an engine: one path goes through the hot center, and another path goes around it. The air going around is bypass air.
Intuition Check
Bypass air is not air that leaks out or is wasted. It is intentionally routed around the engine core and still helps produce thrust.
Example Sentence 1
In a high-bypass turbofan, most of the thrust comes from bypass air rather than from the engine's hot exhaust.
Example Sentence 2
During engine start, the fan draws in large volumes of bypass air even before combustion begins.