Back-Taxi

Definitions

Backtrack. To taxi on a runway-in-use, in the opposite direction to the aircraft’s take-off or landing direction. (Source: UK CAA - CAP1430)

Back-taxi. A term used by air traffic controllers to taxi an aircraft on the runway opposite to the traffic flow. The aircraft may be instructed to back-taxi to the beginning of the runway or at some point before reaching the runway end for the purpose of departure or to exit the runway. (Source: US FAA - Pilot/Controller Glossary)

Both expressions basically mean the same. While the term backtrack is routed in official ICAO documentation, the term back-taxi was obviously introduced by the US FAA.

Description

At aerodromes with no parallel taxiway or when the parallel taxiway is unusable, aircraft may need to backtrack for both arrival and departure operation.

Figure 1 shows aA basic airport layout with just a short taxiway connecting the ramp with the middle of the runway. This will require ATC or, in case of an uncontrolled airport, the pilots to arrange for backtracking of the runway by most of the arriving and departing traffic. In this example as shown in figure 1, most aircraft (both departing and arriving) will have to backtrack if they need more than half of the runway.

To prevent a runway incursion a departing aircraft on the taxiway will wait outside the runway protected area for any landing or take-off aircraft to pass in front of it and will then enter the runway and backtrack. Upon reaching the runway end, it will make a 180 turn and be ready to start the take-off roll.

In case of a landing aircraft which cannot safely decelerate down to taxi-speed until past the taxiway it will also have to make a 180 turn - depending on its size either on the runway at any convenient point or at the runway end where some airports might provide a wider turning pad or area available for safely executing a 180 turn in order to avoid any runway excursion during the turn procedure.

The presence of taxiways parallel to the runway with several joining taxiways, or runway intersections as shown in figure 2, greatly reduces the need for backtrack and facilitate expeditious operations. Landing aircraft may vacate the runway at a convenient point while the pilots focus on optimal deceleration rather than on trying to use a specific taxiway. Runway vacation normally happens before the last taxiway therefore enabling aircraft departing in the opposite direction to use the full length without the need to backtrack.

Phraseology

An example of instruction for backtrack:

Controller: [Callsign], backtrack and line up runway [designator]

Backtrack instruction are to be read back by the pilot.

Pilot: Backrack and line up runway [designator], [callsign]

As mentioned before, Backtrack may happen on both pilot's and controller's initiative or may be required by airport design.

Risk Factors

Backtracking on runways is used widely in certain airfields. As mentioned in the Global Action Plan for the Prevention of Runway Incursions (GAPPRI) studies have shown that backtracking, when combined with a short taxi distance from apron to runway, can present an increased risk of Runway Incursions. There are two factors to be considered: 

• Workload and task sequence — The normal predeparture task flow is based on aircraft taxiing from the apron, along the taxiway system, then arriving at the runway holding point. When the routing requires the aircraft to enter the runway for backtrack, the time available for tasks is shorter. The sequence of tasks may need to be altered, with lineup actions happening before or during the taxi tasks. The cabin may not be secure, and other usual pre-takeoff checks may not be complete before entering the runway. This out-of-sequence task completion can lead to errors and omissions as well as reducing pilot/crew capacity for situational awareness.
• Increased time on runway. It is logical that more time on the runway increases exposure to traffic conflict. Visual scan for pilots/crews is critical but the aircraft is now further from the approach end of runway. Requirement for backtrack also makes the ATC traffic planning and sequencing more difficult, and there is potential for time pressure on the departing aircraft to prevent arriving aircraft having to go-around. 

As mentioned in the Global Action Plan for the Prevention of Runway Excursions (GAPPRE) backtracks also bear the risk of runway or taxiway excursions, especially when turning pads alongside or at the end of the runway are missing. Making a 180 degree turn on a standard runway (width of 45m) with a medium or large commercial air transport aircraft is not an easy task to do as most aircraft are not fitted with gear cameras so that good judgement by the pilot/crew is required to prevent the wheels from leaving the paved surface during the maneuver. The following should be considered: 

• Depending on the runway’s surface and visibility conditions the backtrack procedure requires significant time and special carefulness by the pilot/crew. This is especially the case when the turning procedure must be performed on wet or contaminated runways which may, especially on surface markings or within areas with rubber abrasion, also be (partly) slippery, and/or during darkness or in situations with reduced visibility. Missing runway centerline lighting or masking of taxiway guidance lines due to runway contamination such as snow/ice may further aggravate the risk for wrong line-up and thus excursions.
• Obvious or subtle time pressure enforced by time restrictions, e.g. airport night curfews, given take-off times or just by too tight or too optimistic traffic sequencing of ATCOs might lead to pilots/crews rushing the backtrack and turn procedures thereby increasing the risk for a runway excursion. 

Defences 

The preventive strategies for mitigating the risk of runway incursions and runway excursions during backtrack or back-taxi procedures are very similar and include the following: 

• Aircraft Operators (e.g. airlines) should provide policy that prioritizes safety above efficiency, and pilots/crews should take the time to complete all pre-departure tasks without undue commercial or time pressure.
• Aircraft Operators should provide pilots with policies and SOPs ensuring that pilots feel psychologically safe to refuse difficult ATC clearances. Crews should use the word “unable”, if they anticipate that the spacing will be too tight.
• Where relevant to their operation, aircraft operators should include policy and procedures for pilots/crew to deal with short taxi and/or long backtracks (e.g. half the runway or more) including guidance on:
  • Workload/task management during taxi and back-track.
  • Before entering the runway for a backtrack, ensure pilots/crews complete all relevant tasks related to a runway entry and lineup. Include guidance on cabin secure procedures.
  • Ensure pilots/crews build a high level of situational awareness with respect to traffic before entering the runway and while on the runway.
  • Ensure the preflight briefing includes the workload and task management aspects of the taxi and backtrack phase, as well as the potential for disorientation and wrong-runway departure associated with backtracks.
• To support compliance with pilot sterile cockpit SOP air traffic controllers should pass departure (enroute) clearances and information before the pilot begins to taxi.
• To avoid flight crew distractions (head down) during taxi, controllers should consider passing any revision to the departure (enroute) clearance whilst the aircraft is stopped.
• If a late-notice tactical change to the clearance has to be issued when the aircraft is close to the runway (at the holding position) or on the runway (at the line-up position), controllers should always use the phrase “HOLD POSITION” before or after passing a revised clearance. This will ensure that the pilot has no doubt that the ATC communication does not constitute a clearance to line up, take off or cross the runway. Incident reports prove that this is of particular relevance when the aircraft has already lined up and the flight crew confused the re-clearance with the take-off clearance.
• In case a departing aircraft is cleared to backtrack behind an aircraft that has just landed, the ATCO should clear the departing aircraft to “backtrack line-up and wait.”
• The controller should also provide the departing flight crew with additional traffic information about arriving aircraft backtracking to vacate behind them. This would increase the situational awareness of the flight crew taxiing for departure and help prevent commencement of the take-off run by the departing aircraft immediately after backtrack completion. 

For more detailed information see respective recommendations for Aircraft Operators and Air Navigation Service Providers in GAPPRI (AO21, ANSP13, 14, 21) or GAPPRE (OPS28).

Accidents and Incidents