Definition
A turbofan engine in which the volume of air moved by the fan around the engine core is roughly two to four times the volume of air passing through the core itself. This bypass ratio sits between low-bypass turbofans (typical of older or military engines) and high-bypass turbofans (typical of modern airliners), giving a balance of thrust, fuel efficiency, and speed suited to mid-size jets and regional aircraft.
Plain English
A jet engine with a large fan at the front. Some of the air the fan pushes goes through the hot core of the engine, and the rest goes around it. In a medium-bypass engine, the air going around is two to four times more than the air going through the core. That ratio is the trade-off point between raw speed and fuel economy.
Context Anchor
Seen in turbine engine descriptions, aircraft performance discussions, and comparisons between different types of jet engines.
Derivation
Bypass' refers to air that goes around (bypasses) the engine core rather than through it. 'Turbofan' combines the turbine that powers the engine with the fan at the front. 'Medium' here describes the ratio of bypass air to core air, sitting between the low and high ends.
Why Pilots Care
The bypass ratio directly affects fuel burn, noise levels, and climb performance, which influence route planning, operating costs, and airport noise compliance.
Grounding Statement
Picture the front fan splitting incoming air into two paths: some goes into the hot engine core, and a moderate amount flows around the core to help push the aircraft forward.
Intuition Check
“Medium-bypass” does not mean the engine is medium-sized. It means the engine sends a medium proportion of its airflow around the core compared with the air that goes through the core.
Example Sentence 1
The business jet's medium-bypass turbofan engines gave it a good balance of cruise speed and fuel efficiency on longer legs.
Example Sentence 2
Cruise fuel flow was lower than expected because the medium-bypass turbofan engines operated efficiently at the planned altitude.