Definition
A turbofan engine in which the majority of thrust is produced by air accelerated by the fan around the outside of the engine core, rather than by the hot exhaust from the core itself. The bypass ratio compares the mass of air flowing around the core to the mass of air flowing through it, and a high-bypass engine typically has a ratio of 4:1 or greater.
Plain English
A jet engine with a very large fan at the front. Most of the air the fan moves goes around the engine, not through it, and that bypass air provides most of the push. These engines are quieter and use less fuel than older designs.
Context Anchor
Seen in turbine engine descriptions for many modern transport and business aircraft, especially when discussing engine design, fuel efficiency, noise, and thrust production.
Derivation
Bypass means to go around rather than through. In this engine, air bypasses the hot core of the engine. A high ratio simply means a lot more air goes around than through.
Why Pilots Care
These engines deliver better fuel efficiency, lower noise levels, and higher thrust suitable for large commercial aircraft.
Analogy
Think of it as a very powerful fan wrapped around a smaller jet engine. The smaller engine keeps the fan turning, and the fan moves a large amount of air to create most of the forward push.
Intuition Check
“Bypass” does not mean the air is wasted or unused. In a high-bypass ratio engine, the air that goes around the core is a major source of thrust.
Example Sentence 1
The Boeing 787 uses high-bypass ratio engines, which is part of why it is quieter and more fuel-efficient than older airliners.
Example Sentence 2
During the preflight walk-around the mechanic inspected the large fan blades of the high-bypass ratio engine.