When a gas turbine start up jet lands, it’s not like parking a car. There is some downtime in between passengers loading, and crew unloading cargo in the plane’s hull. The engines can’t idle during this time, because it would burn far too much fuel.
The engines may shut down, but the plane does not completely turn off. Passengers can hear it humming as they pass through the hallway to be seated. There is also exhaust coming through the plane, which is most apparent when you sit down and open the vent over your head.
This is thanks to the portable power supply that works below the plane to keep the engines primed for start up. This power supply is a lot like a miniature jet engine that sits at the rear of the plane. It contains a compressor to provide the plane with air, a combustor to burn fuel and a turbine to help generate electricity. A jet engine turns at thousands of RPMs, no simple task for an object at rest. The power unit provides the boost needed to start that spinning, which eventually becomes self-sustaining enough to power the plane through flight.
The APU isn’t typically triggered until after passengers are seated and buckled. If you have a keen ear, you can actually hear this process after the flight attendants finish their orientation speeches. The power unit forces air through the compressor, and this spins the turbine via a large rod that connects the two.
If you were standing outside the plane, you would notice the fans at the wings begin to spin. Behind those fans is a smaller fan that begins spinning until fuel is added. An electric spark is used to ignite the fuel, the exhaust is passed through the combustor and the jet slowly gains the speed it needs to take off.
The entire process relies on Newton’s third law of physics, which states that every action is met with an equal and opposite reaction. The air and gas mixture expand and cause force in the form of forward momentum. When this is down with enough power behind it, the engines spin and the jet moves forward. Aerodynamics help the plane stay in the air, but getting it off the ground is a simple matter of supplying enough power to the turbines to get the engines going.
This method has been in use on commercial aircraft since the 1960s.