Definition
A turbofan engine design in which the mass of air moved by the fan around the engine core is many times greater than the mass of air passing through the core itself, typically with bypass ratios well above those of conventional high-bypass turbofans (often 10:1 or greater). Most of the thrust is produced by the large fan rather than by the hot exhaust from the core.
Plain English
An engine where a very large fan moves most of the air around the outside of the engine, and only a small portion goes through the hot center. The big fan does most of the pushing, which makes the engine quieter and more fuel-efficient.
Context Anchor
Seen in jet engine descriptions, aircraft performance discussions, and maintenance or training material about modern turbofan engines.
Derivation
Bypass' refers to air that goes around (bypasses) the engine core instead of through it. 'Ultrahigh' simply means the bypass ratio is taken further than the high-bypass designs that came before it -- more air around the core, less through it.
Why Pilots Care
These engines improve fuel efficiency, reduce noise, and meet stricter environmental standards on large airliners.
Intuition Check
“Bypass” does not mean the engine is being skipped or turned off. It means much of the air moved by the fan goes around the hot center of the engine while still helping produce thrust.
Example Sentence 1
The new airliner uses ultrahigh-bypass engines, which is why it sounds noticeably quieter on takeoff than older jets.
Example Sentence 2
Preflight checks confirmed the ultrahigh-bypass fan blades showed no damage after the flight through turbulence.