Moving aircraft around

I was sitting in the departure lounge at Johannesburg OR Tambo Airport waiting for my flight watching the ground support staff move the aircraft to the departure gates. The people next to me talking and trying to work out how they did it.

To me the answer was simple, the pushback tractor releases the brakes of the aircraft and tows the aircraft around. There was no crew in the aircraft that we could see, clearly ground support staff have to gain access to the flight deck to release the brakes, but the rest isn't difficult to comprehend.

They could not see how such a small tractor is able to pull an Airbus A340 around but it isn't too much of a mystery. It is all about torque and not size and the tractor develops plenty of wheel torque. A diesel engine, powershift transmission and torque converter assembly coupled to low range transmission develops substantial torque.

It is not unreasonable to incorporate a 9:1 reduction in the low gears of the transmission. The torque converter is basically a fluid clutch that can increase output torque by a factor of 2:1 depending on design and application.

However, this isn't enough so a single reduction differential carrier increases torque approximately by a factor of 4:1 before dividing torque to the wheel ends. The hub reduction gearing increases torque again at the wheels by a factor of 10:1 so the torque developed at the engine flywheel is increased proportionally at each stage.

The aircraft isn't running, there is no pilot available to crank up the turbines and taxi the aircraft around under its own power, the movement is all the work of the pushback tractor. For me, listening to people chat is very interesting, I had imagined people knew all this and no explanations were required.