Definition
A turbofan engine in which the bypass ratio — the mass of air that flows around the engine core compared to the mass of air that flows through the core — is very high, typically greater than about 10:1. Most of the thrust is produced by the large fan at the front rather than by the hot exhaust from the core, resulting in greater fuel efficiency and lower noise than conventional turbofans.
Plain English
An engine where a big fan up front pushes most of the air around the outside of the engine instead of through it. Because so much air goes around the core, the engine uses less fuel and runs more quietly than older jet engines.
Context Anchor
Seen in turbine engine design, maintenance, and aircraft powerplant discussions, especially for modern large transport aircraft.
Derivation
Bypass refers to air that 'goes past' (bypasses) the engine core. 'Ultra-high' simply means the ratio of bypass air to core air is much larger than in earlier designs. Knowing this makes the name self-explanatory: a lot of air bypasses the core.
Why Pilots Care
UHB engines reduce fuel consumption and noise, lowering costs and meeting stricter environmental regulations.
Analogy
Think of it like using a large, slow-moving fan instead of a small, very hot blast. The engine gets much of its push from moving a lot of air smoothly, not just from a fast stream of hot exhaust.
Intuition Check
“Bypass” does not mean the engine is being skipped or avoided. It means some air is routed around the hot core while still helping produce thrust.
Example Sentence 1
The aircraft's UHB engines produce most of their thrust from the large front fan rather than from the core exhaust.
Example Sentence 2
Technicians inspect the fan section of UHB engines during routine powerplant checks.