Taxiing. Movement of an aircraft on the surface of an aerodrome under its own power, excluding take-off and landing.
Air-taxiing. Movement of a helicopter/VTOL above the surface of an aerodrome, normally in ground effect and at a ground speed normally less than 37 km/h (20 kt).
Note. The actual height may vary, and some helicopters may require air-taxiing above 8 m (25 ft) AGL to reduce ground effect turbulence or provide clearance for cargo slingloads.
Source: ICAO Doc 4444 PANS-ATM
For departing aircraft, the taxi phase starts at (or in the vicinity of) the aircraft parking stand on the apron and finishes at the runway holding position (or at line-up completion). It may sometimes involve moving on part (or even the complete length) of the active runway, i.e. backtrack. Depending on the aerodrome layout, taxiing may sometimes commence directly after start-up. In other cases, it may be necessary to reposition the aircraft using pushback before initiating the taxi manoeuvre.
For arrivals, the taxi phase starts when sufficient decelleration is achieved and ends when the parking stand is reached.
Taxiing can also be used for aircraft repositioning from one part of the [[movement area]] to another.
Taxiing requires that an ATC clearance is obtained prior to the manoeuvre. This also applies to situations where the movement only takes place on the apron and the tower/ground controller is only responsible for the manoeuvring area. Taxi clearances contain concise instructions and adequate information so as to assist the flight crew to follow the correct taxi routes, avoid collision with other aircraft or objects and minimize the potential for the inadvertent entry on an active runway. When a taxi clearance contains a taxi limit beyond a runway, it must contain an explicit clearance to cross or an instruction to hold short of that runway.
Taxxing is not restricted to taxiways. In order to expedite the traffic flow, the controller may clear it to taxi on the runway-in-use, provided no delay or risk to other aircraft will result.
While a number of situations can arise during or in relation to taxiing, the following are the most frequent:
- Ground collision with another aircraft, a vehicle or an obstacle. This may happen due to e.g.:
- Insufficient wingtip clearance (e.g. due to an aircraft being inappropriately parked or the taxiing aircraft using an inappropriate apron taxiway or if the taxiing crew does not follow the required ground path). Examples: B738 / B738, Seville Spain, 2012, A124, Zaragoza Spain, 2010
- Sudden movement of another aircraft (e.g. pushback). Example: MD82 / MD11, Anchorage AK USA, 2002
- Vehicle not giving right of way to an aircraft. Examples: DH8C, Toronto Canada, 2019, SB20, Stockholm Arlanda, 2001
- Taxiway take off. This usually happens when a taxiway that is parallel to a runway is mistakenly identified as such. Example: B733, Amsterdam Netherlands, 2010
- Runway incursion. This may happen due to e.g. misunderstood or ambiguous taxi clearance (e.g. when the taxi path crosses a runway but the controller does not explicitly state that the aircraft needs to hold before crossing). Exemple: A319 / A320, Paris CDG France, 2014
- Air-taxiing causes significant turbulence (due to the ground effect). The effect is magnified with larger and heavier helicopters.
The following (non-exhaustive) list contains factors that are often cited as contributors in accident investigation reports related to events involving taxxing aircraft:
- Aerodrome layout (e.g. confusing/ambiguous signs or markings, complex taxiway system, etc.)
- Night/low visibility conditions (due to e.g. fog, snowfall, etc.)
- Distraction (e.g. the controller transmitting the departure clearance)
- Inadequate procedures (e.g. for pushback)
The risks associated with taxxing can be mitigated in a number of ways. In general, these include equipment improvement, development of sound procedures and adherence to those and vigilance on part of everyone involved. Examples of such measures include:
- Installation of SMR/A-SMGCS greatly enhances the controller's situational awareness, especially during night/low visibility conditions.
- Transmission of the departure clearance before issuing the taxi clearance. Additionally, this reduces the risk of the crew incorrectly copying the clearance.
- Inclusion of explicit statement to cross or hold short of a runway when issuing a taxi clearance.
- While more demanding and time-consuming for the controller, the use of progressive taxi instructions reduces the risk of the crew performing inappropriate manoeuvres at aerodromes with complex layout.
- Strict adherence to speed limits and right of way rules.
- Vigilance and maintaining situational awareness at all times.
- Instructions to small aircraft to taxi in close proximity to taxiing helicopters should be avoided.