Definition
A turbofan engine in which the mass of air routed around the engine core (bypass air) is roughly two to four times the mass of air passing through the core. This places it between low-bypass engines (typical of older or military fighter applications) and high-bypass engines (typical of modern airliners).
Plain English
A jet engine where a moderate amount of air is pushed around the hot core rather than through it. Less air is bypassed than on a big airliner engine, but more than on a fighter-style engine.
Context Anchor
Seen in powerplant descriptions, aircraft systems training, and maintenance manuals when identifying the type of turbofan engine installed on an aircraft.
Derivation
Bypass refers to air that goes around (bypasses) the core of the engine. The bypass ratio is the comparison of that bypassed air to the air going through the core. Medium simply places this engine between low and high bypass designs.
Why Pilots Care
Determines thrust, fuel burn, noise, and the aircraft speeds for which the engine is best suited.
Grounding Statement
Picture air entering the front of the engine: some air goes through the hot center, while a larger amount is guided around that center to help produce thrust.
Intuition Check
“Medium” does not mean the engine is medium-sized. It means the bypass airflow is in the middle range compared with low-bypass and high-bypass turbofan engines.
Example Sentence 1
The aircraft is powered by a medium-bypass ratio engine, giving it better fuel efficiency than older low-bypass designs.
Example Sentence 2
Business jets often use a medium-bypass ratio engine to maintain good high-speed performance with reasonable fuel efficiency.