ギアドターボファン (GTF: Geared Turbofan)

What Is a Geared Turbofan?

A geared turbofan (GTF) is an advanced turbofan engine that interposes a planetary reduction gearbox between the large front fan and the low-pressure turbine (LPT) shaft. This mechanical decoupling is the defining innovation: the fan and turbine can each rotate at their individually optimal speeds rather than being locked to the same shaft speed.

How It Works

In a conventional turbofan, the fan, low-pressure compressor, and low-pressure turbine are all mounted on a single shaft. Because the large fan must turn slowly to avoid supersonic blade-tip speeds, the LPT is forced to operate below its aerodynamically ideal speed — a fundamental efficiency compromise.

The GTF's gearbox, typically with a reduction ratio of approximately 3:1, allows the LPT to spin roughly three times faster than the fan. The LPT can therefore be designed with fewer stages (typically 3 vs. 7 in equivalent conventional engines), reducing weight and parts count. The fan, freed from shaft-speed constraints, grows larger — enabling bypass ratios of 12:1 or higher — while spinning at speeds that minimize aerodynamic losses.

Performance Specifications

• Gearbox reduction ratio: approximately 3.0:1 (Pratt & Whitney PW1000G series)
• Bypass ratio: 12:1 on PW1100G (A320neo), 12.2:1 on PW1500G (CSeries/A220)
• Fuel burn improvement: 15–20% over CFM56/V2500 predecessors
• Noise reduction: approximately 75% reduction in noise footprint vs. prior generation
• LPT stages: 3 (vs. 6–7 in ungeared equivalents)

Aircraft Examples

• PW1100G-JM on the Airbus A320neo — entered service 2016, rated at 120–147 kN (27,000–33,000 lbf)
• PW1500G on the Airbus A220-300
• PW1900G on the Embraer E195-E2
• PW1700G on the Embraer E175-E2

The geared turbofan concept, pioneered by Pratt & Whitney with over $10 billion in development investment spanning two decades, entered revenue service in January 2016. By 2025 the engine family had accumulated over 30 million flight hours across more than 1,500 aircraft.