Hello, and thank you for reading. Early jet airliners flew behind turbojets, engines that pushed every bit of air through a hot core.

But almost every modern airliner you board now hangs a turbofan under the wing instead. The difference sounds small. It reshaped fuel bills, airport noise rules, and how far a plane can fly.

The fan does the heavy lifting

A turbofan splits incoming air into two paths. A large fan at the front sends most of it around the core as bypass air, and only a smaller share goes through the compressor, combustor, and turbine. On a modern engine like the CFM56 that powers the Airbus A320 family and Boeing 737 Classic and Next Generation jets, the bypass ratio sits around 5 to 6 to 1. That means for every pound of air burned in the core, five or six pounds simply get accelerated backward by the fan. You get propulsion from moving a lot of air a little, rather than a little air a lot.

Why the old turbojets faded

A pure turbojet puts all its air through the core, so it runs hot and fast and drinks fuel. The Boeing 707 and Douglas DC-8 of the 1950s and 60s used engines like the Pratt & Whitney JT3D, and their fuel burn per seat was far higher than what you see now. Airlines noticed on long routes. A turbofan's bypass air also wraps the hot exhaust in a cooler, slower stream, which cuts the sharp jet noise that made early airports so loud. That noise drop is why older jets faced curfews that newer ones mostly avoid.

The tradeoffs engineers juggle

Bigger fans move more air, so they burn less fuel per unit of force. The catch is that a wide fan creates drag and needs ground clearance, which is why the 737 has that flattened engine nacelle sitting so low. Weight climbs too. The GE90 on the Boeing 777 has a fan roughly 128 inches across, and that size brings real structural demands. Engineers balance bypass ratio, fan diameter, and weight against the range the airline actually needs.

Newer designs push the ratio higher

The trend keeps going. Pratt & Whitney's geared turbofan, which flies on the Airbus A320neo family and the Embraer E-Jet E2, adds a gearbox so the fan turns slower than the turbine, letting each run at its best speed. That pushes the bypass ratio past 12 to 1. CFM's LEAP engine, on the 737 MAX and A320neo, lands in a similar range. Higher ratios mean lower specific fuel consumption, and that is the number airlines watch most closely.

What it means when you fly

Next time you look out the window at the wing, the engine you see is mostly fan. The core is a comparatively small part tucked inside. That layout is why a flight from New York to Los Angeles burns far less fuel per passenger than the same trip did in 1970, and why you can hold a conversation near the runway today. It's also why engine makers keep chasing bigger fans and higher ratios. So the next time you board, take a second to notice the size of that front fan. The quiet, cheap flight you're on starts right there.