In aviation, pushback is an airport procedure during which an aircraft is pushed backwards away from an airport gate by external power, when there isn't enough room for the aircraft to turn around under its own power (which requires some degree of forward motion). Pushbacks are carried out by special, low-profile vehicles called pushback tractors or tugs.
Although many aircraft can also move backwards on the ground using reverse thrust (a procedure referred to as a powerback), the resulting jet blast or prop wash may cause damage to the terminal building or equipment. Engines close to the ground may also blow sand and debris forward and then suck it in to the engine, causing damage to the engine. A pushback using a tractor is therefore the preferred way. Powerbacks and taxiing are also very noisy and fuel-inefficient, and some airlines, notably Virgin Atlantic, are now advocating towing aircraft to the holding point of the runway to save fuel and reduce environmental impact.
Pushbacks at busy aerodromes are usually subject to ground control clearance to facilitate ground movement on taxiways. Once clearance is obtained, the pilot will communicate with the pushback tractor driver (or a ground handler walking alongside the aircraft in some cases) to start the pushback. To communicate, a headset may be connected near the nose gear.
Since the pilots cannot see what is behind the aircraft, steering is done by the pushback truck driver and not by the pilots. Depending on the aircraft type and airline procedure, a bypass pin may be temporarily installed into the nose gear to disconnect it from the aircraft's normal steering mechanism.
If a bypass pin is required but is not inserted, the nose gear steering mechanism may be damaged when the pushback tractor attempts to turn the aircraft, or the towbar may break or shear away from the nose gear, or a steering mechanism under power may swing the towbar and cause injury to ground personnel and equipment.
Once the pushback is completed, the towbar is disconnected, and the bypass pin (if used) is removed. The ground handler will show the bypass pin to the pilots to make it absolutely clear that it has been removed. The pushback is then complete, and the aircraft can start taxiing forward under its own power.
There are two types of pushback vehicles, towbarless (TBL) and conventional.
Conventional tugs use tow bars to connect to the nose wheel of the aircraft, then they push the aircraft to a position from which it can safely move under its own power before disconnecting it. The tow bar can be connected at the front or the rear of the tractor, depending on whether the aircraft will be pushed or pulled. The towbar has a shear pin. The shear pin prevents the aircraft from being mishandled by the tug—when overstressed the shear pin will (usually) snap, disconnecting the bar from the nose gear to prevent damage to the aircraft and tug.
To have enough traction, the tractor itself needs to be heavy, and most models can have extra ballast added. A typical tractor for large aircraft weighs up to 54 metric tons and has a drawbar pull of 334 kN Often, the driver's cabin can be raised for increased visibility, and lowered to fit under aircraft.
Towbarless tractors do not use a towbars but instead scoop up the nose wheel of an aircraft and lift it up off the ground, allowing the tug to manoeuvre the aircraft. This allows more secure control of the aircraft, allowing greater speeds, and lets aircraft be moved without anyone in the cockpit. However, a towbarless tractor may be usable for fewer aircraft types than a conventional tractor.
Very small aircraft may be moved around by human muscle power alone. A short pushbar is attached to the nose gear to allow the aircraft to be steered without anyone inside the aircraft at the controls.