Definition
A turbofan engine in which most of the air drawn in by the large front fan bypasses the engine core (the section that burns fuel) and is accelerated rearward through an outer duct to produce thrust. Only a small portion of the incoming air passes through the core itself. The ratio of bypass air to core air is high — typically 5:1 or greater — which gives the engine its name and makes it fuel-efficient and relatively quiet at cruise speeds.
Plain English
A jet engine with a big fan in the front. Most of the air that fan moves goes around the hot center of the engine, not through it. That moving air is what produces most of the thrust. These engines are quieter and use less fuel than older jet designs.
Context Anchor
Seen in discussions of turbine-powered airplanes, especially airline-type airplanes and large business jets, when comparing thrust, fuel use, noise, and engine response.
Derivation
‘Bypass’ describes air that goes around the engine core rather than through it. ‘High-bypass’ simply means a large share of the air takes that route. ‘Turbofan’ combines ‘turbine’ (the spinning bladed wheel driven by hot exhaust) and ‘fan’ (the large bladed wheel at the front that the turbine drives). Knowing this helps because the name itself describes how the engine works: a turbine spins a fan, and most of that fan’s air bypasses the burning core.
Why Pilots Care
High-bypass turbofans deliver better fuel economy, lower noise, and greater reliability than low-bypass or pure turbojet engines on commercial and transport aircraft.
Grounding Statement
Picture a large airliner engine: the big fan at the front moves a wide outer stream of air around the hot center, and that outer stream does most of the pushing.
Intuition Check
High-bypass does not mean the engine bypasses a normal procedure or safety system. It means most of the incoming air bypasses the hot core and still helps make thrust.
Example Sentence 1
Most modern airliners are powered by high-bypass turbofan engines because they offer good fuel economy on long flights.
Example Sentence 2
Maintenance crews inspected the fan blades of the high-bypass turbofan engine during the overnight check.