Definition
A turbofan engine in which a reduction gearbox is installed between the low-pressure turbine and the fan, allowing the fan to spin at a slower, more efficient speed than the turbine that drives it.
Plain English
A jet engine where the big front fan is connected to its driving turbine through a set of reduction gears, so the fan can turn slowly while the turbine inside spins fast. Each part runs at the speed it works best at.
Context Anchor
Seen in turbine engine and aircraft performance discussions, especially when comparing newer turbofan engine designs.
Derivation
Geared' refers to the reduction gearbox sitting between the turbine and the fan. In a conventional turbofan, the fan and the low-pressure turbine are bolted to the same shaft and must spin at the same rpm. Adding a gearbox lets each component run at its own optimal speed.
Why Pilots Care
Enables higher bypass ratios for better fuel economy and lower community noise on takeoff and approach in modern airliners.
Analogy
Like a 10-speed bicycle: the rider's legs spin at a comfortable pace while the wheel spins much faster. The gears let each side run at its best speed instead of forcing both to match.
Intuition Check
Do not read “geared-fan” as meaning the fan is a separate propeller. It is still part of a turbofan jet engine; the gear system is inside the engine drive path.
Example Sentence 1
The aircraft's geared-fan engines are noticeably quieter than the older turbofans on the previous generation of the type.
Example Sentence 2
During preflight the pilot noted the geared-fan engine's gearbox temperature was within limits.