After what can seem like a long wait, it’s time for pushback. The aircraft reverses away from the stand and taxies off to the end of the runway. Unlike a car, there’s no power to the wheels in a plane; most of the maneuvering on the ground is driven by the aircraft engines. This is not a very efficient way to travel when the plane isn’t in the air, particularly in reverse, so to get away from the terminal an aircraft tug (sometimes called a pushback tractor) is usually brought in.
The tugs used on 747s are typically 200-300 horsepower—less than a high-performance car. In principle, an airliner can back away from the terminal using reverse thrust. This involves the crude technique of placing a deflector behind the jet engines, so the blast of air is pushed towards the front of the plane. Reverse thrust is usually deployed on landing to slow the aircraft down—this is what is being engaged when you hear the engines suddenly surge as you touch the ground. But it isn’t practical to use reverse thrust when close to a terminal (‘on stand’ in airline parlance). The blast from the engines is liable to send any debris on the ground hurtling towards the glass of the building, which is why tugs are used instead.
You may wonder, given the inefficiency of taxiing on jet engines, why the tug doesn’t take the plane all the way to the runway. Virgin Atlantic did come up with the idea of doing just this in 2006. The idea was to pull the plane to a ‘starting grid’ at the end of the runway. This would have produced significant fuel savings—Virgin reckoned that they could save two tonnes of CO2 per flight, as well as reducing noise and cleaning up the air near the terminal.
Unfortunately, despite its green credentials, the technique soon had to be shelved. This was partly because airports were not willing to provide the starting grid locations, which would have produced delays while tugs were decoupled and moved clear of the jet blast. But more significantly, the aircraft manufacturers warned that increasing the amount of towing would put too much strain on the undercarriage, meaning the struts that hold the wheels would have to be replaced more frequently.