Welcome, everyone. A jet engine looks like it's doing something mysterious up there, but the physics behind it is surprisingly plain.

Most people assume the engine pushes against the air behind the plane, the way you'd push off a wall. That's not it.

The engine grabs air, speeds it up, and hurls it out the back. The plane moves forward because the air moves backward. That's Newton's third law doing the heavy lifting.

Pull air, then squeeze it

A modern turbofan starts by pulling in a huge amount of air. On a big engine like the GE90 that powers a Boeing 777, the front fan moves well over a ton of air every second. Most of that air bypasses the core entirely and just gets accelerated backward by the fan. Only a slice of it enters the compressor, where rows of spinning blades squeeze it tighter and tighter. By the time the air reaches the combustor, it's been compressed to roughly 40 times normal atmospheric pressure and heated to several hundred degrees.

Heat is what actually does it

Inside the combustor, fuel mixes with that compressed air and burns continuously. The temperature jumps to around 1,400 to 1,700 degrees Celsius, hot enough that the turbine blades downstream need internal cooling channels and ceramic coatings to survive. Here's the part people miss: the engine doesn't need the explosion to push the plane. The heat makes the gas expand violently, and that expansion is what accelerates it. A jet engine is really a machine for heating air and letting it escape fast.

The turbine pays the bills

Right behind the combustor sits the turbine, and it has a job that seems backwards at first. It sits in the exhaust path and extracts energy from the hot gas. That energy spins the compressor and fan up front through a shaft running the length of the engine. So the turbine isn't there for propulsion at all. It's there to keep the whole cycle running. Without it, the compressor would stall and the engine would quit within seconds.

Why the fan does most of the work

On a high-bypass turbofan like the CFM56 on an Airbus A320, the fan pushes far more air around the core than through it. The bypass ratio on that engine runs about 5 to 1. On newer designs like the Pratt & Whitney PW1000G, it climbs past 12 to 1. All that bypassed air moves slower than the core exhaust, but there's so much of it that it produces most of the forward force. It's also quieter and more fuel-efficient than pushing everything through the hot section.

Force you can measure

A Boeing 737's CFM56-7B produces around 27,000 pounds of force at takeoff. The GE90-115B, one of the most powerful engines ever certified, hit 127,900 pounds on a test stand. You can feel the difference from the ground. Stand behind a running engine at a safe distance and the wake alone will knock you over. That's the same mass of air, moving backward, that's pushing the plane forward.

Reverse force isn't what it sounds like

When a plane lands, you hear the engines roar and assume they've flipped into reverse. They haven't. The core keeps spinning the same direction. What happens is that clamshell doors or cascade vanes redirect the fan airflow forward and outward, which creates a backward push on the plane. It only works at low speeds and it's mostly there to save the brakes, not to stop the aircraft on its own.

The next time you're on a runway and the plane surges forward, remember what's actually happening. The engine is throwing air backward at hundreds of miles per hour, and the aircraft is just the equal and opposite reaction. It's not pushing against anything. It's trading momentum with the air, and that trade is what gets you off the ground.