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Pushback

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Article Information
Category: Ground Operations Ground Operations
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary

Description

Pushback means the movement of an aircraft from a nose-in parking stand using the power of a specialised ground vehicle attached to or supporting the nose landing gear. It is commonly the second part of a ‘Taxi In Push Out’ (TIPO) procedure at airport terminal gates and will be necessary to depart from all except self manoeuvring parking stands unless the aircraft type is capable of powerback and local procedures allow this. Occasionally, a pushback may need to be followed by an engines-running pull forward to a position where local procedures allow aircraft to move forward under their own power, but usually, ground vehicle disconnection will occur after the completion of a pushback.

The Procedure

Once the aircraft commander (or other person in charge on the flight deck if the aircraft is not in service) has given their confirmation of ‘brakes released’ to the person in charge of the ground crew who are to carry out the pushback, the ground crew become temporarily responsible for the safe manoeuvring of the aircraft in accordance with either promulgated standard procedures or as specifically agreed beforehand.

Unless the manoeuvre is taking place outside the movement area controlled by ATC, an RTF clearance to carry it out will be required. Usually but not always, this will be obtained by the aircraft commander or other person in charge in the flight deck. The prescribed RTF phraseology for pushback is contained in ICAO PANS-ATM.

Formerly, almost all aircraft types required that the ground locking pin be installed in the nose landing gear during any pushback; however, this is now no longer always the case. If a ground locking pin is installed for the pushback, it will need to be removed after the completion of the ground vehicle manoeuvre if the aircraft has been pushed back prior to intended flight.

The ‘traditional’ method of allowing the ground vehicle to move an aircraft is to attach it to the aircraft nose landing gear by means of a towbar. These must be approved for use with a particular aircraft type and clearly marked as such, since there is no universal towbar specification. The same towbar attachment and ground vehicle may also be used for Aircraft Towing in the forward direction. An alternative method which is becoming more common for pushback is the use of a specialised vehicle called a ‘towbarless tug’. This positions two low level ‘arms’ either side of the aircraft nose landing gear and these are used to engage with the aircraft gear leg and raise it slightly off the ground. These specialised vehicles can also be used to tow aircraft forward,

Both pushback methods are subject to the observance of any aircraft limits for maximum nose landing gear steering angle, but these are not usually especially restrictive.

The responsibilities of the ground crew team carrying out a pushback include ensuring that no part of the aircraft structure will impact any fixed object or other aircraft and may include giving clearance to start one or more engines just before, during or immediately after a pushback. The number of people assigned to a ground crew team for a pushback may vary according to aircraft size, but in most cases will be at least three. One will be driving the pushback vehicle, one will be walking in the vicinity of one of the aircraft wingtips and looking beyond the aircraft tail and one will be in charge of the manoeuvre and in communication with the person with aircraft responsibility in the flight deck. Communication between the ground crew supervisor is usually by means of a plug in to an aircraft ground intercom circuit; if so, this is facilitated by a ground crew microphone which acquires the voice of the user whilst excluding background noise, which if the aircraft engines are running can be considerable. If only two ground crew are used for pushback of a smaller aircraft then it is important that the procedure takes full account of the roles of each ground crewmember and that the person in charge of ground crew communications on the flight deck is aware of the number of ground crew being used and the physical location of the supervisor.

Effective communication between the person in charge in the flight deck and the person in charge of the ground crew, and between the members of the ground crew team is critical. If the aircraft is being pushed back prior to intended flight and the person in charge of the flight deck is therefore an aircraft commander, the procedures of the aircraft operator may require that the designated Pilot Flying, who may be the co pilot, should oversee the pushback and in this case all communications with the ground crew will be undertaken by that person rather than necessarily by the aircraft commander. If it is considered that communication by hand signals rather than intercom is acceptable then it is essential that the applicable procedures are comprehensive and thoroughly understood by both parties and that they cover all possible abnormal and emergency circumstances.

The case of engines-running pull forward as a supplementary action prior to ground vehicle disconnection after a pushback should be considered as part of the pushback procedure and trained accordingly since it bears little practical resemblance to the towing for longer distances of empty out-of-service with engines stopped.

Engine Starts may be routinely accomplished immediately before or during pushback. Where they are carried out when the aircraft is moving, it is essential that the ground crew supervisor does not allow the checks and communication required in connection with engine starting to interfere with their primary responsibility to control the pushback and remain in full communication with those on the flight deck using the means available. Many aircraft operators require that when push back is accomplished without headset communications, engine starts do not take place whilst the aircraft is being pushed, preferring instead to require that engine starting takes place before or after completion of the pushback. Observations of abnormal circumstances in connection with engine starts or any other matter affecting, or potentially affecting the safety of the aircraft during a pushback are of great importance to those on the flight deck but it is essential that any descriptions of external observations during engine starts are imparted accurately; this may sometimes be demanding using ground intercom but can be extremely difficult with only hand signals available.

The key threat to aircraft safety

If damage is caused to the aircraft on pushback, or to another aircraft by the aircraft on pushback, this must be identified and technically assessed before that aircraft flies. Unfortunately, this is not always the case. It is important to recognise that when part of one aircraft impacts part of another aircraft, the degree of resultant damage may vary between negligible and major, even if the aircraft are identical. Ground Crews must be effectively briefed on this as well as other aspects of the operation. This is especially important when the ground crew are not employed directly by the aircraft operator or if they do not speak the same language fluently for operational communications.

Risk Management

The evidence of accidents and incidents is that there are a number of recurrent features of aircraft damage during pushback:

  • Use of hand signals rather than intercom communication,
  • Lack of intercom clarity in communications between the aircraft and the ground crew supervisor,
  • Ground crew totalling less than three people,
  • Departure from non-airbridge gates,
  • Failure of vehicle driver to maintain adequate communication with supervisor,
  • Lack of clearance between horizontal stabilisers of adjacent ‘T’ tail aircraft,
  • Pushback commenced from a parking position different from that marked,
  • Ground crew poorly trained or unfamiliar with the immediate pushback environment,
  • Unserviceable towbars attributable to lack of ownership clarity,
  • Surface contamination obscuring pavement markings.


Accidents and Incidents

The following events in the SKYbrary database occurred during Pushback:

  • A332, Karachi Pakistan, 2014 (On 4 October 2014, the fracture of a hydraulic hose during an A330-200 pushback at night at Karachi was followed by dense fumes in the form of hydraulic fluid mist filling the aircraft cabin and flight deck. After some delay, during which a delay in isolating the APU air bleed exacerbated the ingress of fumes, the aircraft was towed back onto stand and an emergency evacuation completed. During the return to stand, a PBE unit malfunctioned and caught fire when one of the cabin crew attempted to use it which prevented use of the exit adjacent to it for evacuation.)
  • ATP, Jersey Channel Islands, 1998 (On 9 May 1998, a British Regional Airlines ATP was being pushed back for departure at Jersey in daylight whilst the engines were being started when an excessive engine power setting applied by the flight crew led to the failure of the towbar connection and then to one of the aircraft's carbon fibre propellers striking the tug. A non standard emergency evacuation followed. All aircraft occupants and ground crew were uninjured.)
  • B738 / B738, Toronto Canada, 2018 (On 5 January 2018, an out of service Boeing 737-800 was pushed back at night into collision with an in-service Boeing 737-800 waiting on the taxiway for a marshaller to arrive and direct it onto the adjacent terminal gate. The first aircraft’s tail collided with the second aircraft’s right wing and a fire started. The evacuation of the second aircraft was delayed by non-availability of cabin emergency lighting. The Investigation attributed the collision to failure of the apron controller and pushback crew to follow documented procedures or take reasonable care to ensure that it was safe to begin the pushback.)
  • B738, London Stansted UK, 2008 (On 13 November 2008, a Boeing 737-800 with an unserviceable APU was being operated by Ryanair on a passenger flight at night was in collision with a tug after a cross-bleed engine start procedure was initiated prior to the completion of a complex aircraft pushback in rain. As the power was increased on the No 1 engine in preparation for the No 2 engine start, the resulting increase in thrust was greater than the counter-force provided by the tug and the aircraft started to move forwards. The towbar attachment failed and subsequently the aircraft’s No 1 engine impacted the side of the tug, prior to the aircraft brakes being applied.)
  • B738, Singapore, 2015 (On 6 December 2015, a Boeing 737-800 was being manoeuvred by tug from its departure gate at Singapore to the position where it was permitted to commence taxiing under its own power when the tug lost control of the aircraft, the tow bar broke and the two collided. The Investigation attributed the collision to the way the tug was used and concluded that the thrust during and following engine start was not a contributory factor. Some inconsistency was found between procedures for push back of loaded in-service aircraft promulgated by the airline, its ground handling contractor and the airport operator.)
  • B742, Stockholm Arlanda Sweden, 2007 (On 25 June 2007, a Boeing 747-200F being operated by Cathay Pacific on a scheduled cargo flight from Stockholm to Dubai had completed push back for departure in normal daylight visibility and the parking brakes had been set. The tow vehicle crew had disconnected the tow bar but before they and their vehicle had cleared the vicinity of the aircraft, it began to taxi and collided with the vehicle. The flight crew were unaware of this and continued taxiing for about 150 metres until the flight engineer noticed that the indications from one if the engines were abnormal and the aircraft was taxied back to the gate. The tow vehicle crew and the dispatcher had been able to run clear and were not injured physically injured although all three were identified as suffering minor injury (shock). The aircraft was “substantially damaged” and the tow vehicle was “damaged”.)
  • B752 / CRJ7, San Francisco CA USA, 2008 (On 13 January 2008, a Boeing 757-200 and a Bombardier CL-600 received pushback clearance from two adjacent terminal gates within 41 seconds. The ground controller believed there was room for both aircraft to pushback. During the procedure both aircraft were damaged as their tails collided. The pushback procedure of the Boeing was performed without wing-walkers or tail-walkers.)
  • B763 / A320, Delhi India, 2017 (On 8 August 2017, a Boeing 767-300 departing Delhi was pushed back into a stationary and out of service Airbus A320 on the adjacent gate rendering both aircraft unfit for flight. The Investigation found that the A320 had been instructed to park on a stand that was supposed to be blocked, a procedural requirement if the adjacent stand is to be used by a wide body aircraft and although this error had been detected by the stand allocation system, the alert was not noticed, in part due to inappropriate configuration. It was also found that the pushback was commenced without wing walkers.)
  • B772 / A321, London Heathrow UK, 2007 (On 27 July 2007, a British Airways Boeing 777-200ER collided, during pushback, with a stationary Airbus A321-200. The A321 was awaiting activation of the electronic Stand Entry Guidance (SEG) and expecting entry to its designated gate.)
  • B789 / A388, Singapore, 2017 (On 30 March 2017, a Boeing 787 taxiing for departure at night at Singapore was involved in a minor collision with a stationary Airbus A380 which had just been pushed back from its gate and was also due to depart. The Investigation found that the conflict occurred because of poor GND controlling by a supervised trainee and had occurred because the 787 crew had exercised insufficient prudence when faced with a potential conflict with the A380. Safety Recommendations made were predominantly related to ATC procedures where it was considered that there was room for improvement in risk management.)
  • CRJ7 / CRJ2, Charlotte NC USA, 2008 (On 28 June 2008, a Bombardier CRJ 700 operated by PSA Airlines, during daytime pushback collided with a stationary CRJ 200 of the same company at Douglas International Airport Charlotte, North Carolina.)
  • E190 / Vehicle, Paris CDG France, 2014 (On 19 April 2014, an Embraer 190 collided with the tug which was attempting to begin a pull forward after departure pushback which, exceptionally for the terminal concerned, was prohibited for the gate involved. As a result, severe damage was caused to the lower fuselage. The Investigation found that the relevant instructions were properly documented but ignored when apron services requested a 'push-pull' to minimise departure delay for an adjacent aircraft. Previous similar events had occurred on the same gate and it was suspected that a lack of appreciation of the reasons why the manoeuvre used was prohibited may have been relevant.)

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Further Reading