Definition
A turbofan engine in which the volume of air routed around (bypassing) the engine core is roughly equal to or only slightly greater than the volume of air passing through the core. Bypass ratios for these engines are typically around 1:1 or lower, meaning most of the thrust is produced by the hot exhaust from the core rather than by the fan-driven bypass air. Low bypass ratio engines are commonly found on military fighter aircraft and older commercial jets where high speed and compact size matter more than fuel efficiency.
Plain English
A jet engine where the amount of air going around the hot core is about the same as the amount going through it. This design favours speed and compactness over fuel economy.
Context Anchor
Seen in turbine engine and turbofan discussions, especially when comparing older, military, or high-speed jet engines with quieter fuel-saving turbofan designs.
Derivation
‘Bypass’ literally means to go around something. In a turbofan, some of the air ‘bypasses’ the combustion core entirely. ‘Ratio’ is the comparison between the bypassed air and the core air. ‘Low’ here means that comparison is small — not much extra air is being routed around the core.
Why Pilots Care
Low bypass engines produce high thrust in a small package and perform well at high speeds, but they are noisier and burn more fuel than high bypass engines. Knowing the engine type helps a technician or pilot understand performance characteristics, fuel consumption, and noise behaviour.
Analogy
Think of two paths for air: one path goes through the hot engine core, and one path goes around it. In a low bypass ratio engine, the around-the-core path carries a smaller share of the air.
Intuition Check
“Low” does not mean weak or low power here. It means the engine has a low amount of bypass air compared with the air going through the core.
Example Sentence 1
The fighter was fitted with a low bypass ratio engine, giving it the high thrust and high-speed performance the mission required.
Example Sentence 2
During preflight the mechanic noted the distinctive roar typical of the aircraft's low bypass ratio engine.