Background
An incident occurred at Hobart Airport involving a Boeing 737 aircraft (YFT) during a power-out taxi from a parking bay. The airport operator reported that the parking bays were surveyed regularly to ensure they met required standards. A schematic diagram of the aircraft's taxi path was constructed from CCTV footage and aircraft dimensions on surveyed apron markings.
Parking Bay Layout
The flight crew commented that the design standard for the parking bays meant they were far too close to be comfortably safe to conduct a power-out taxi when an aircraft is in the adjacent bay. The captain noted that a bit more spacing between the bays may prevent a similar incident. The captain also commented that consideration of operational acceptability criteria, in addition to technical design specifications, prior to commissioning any parking bay and its associated taxi guidance line would facilitate required changes and mitigate additional threats peculiar to the local environment.
The captain observed that the power-out bays in Hobart are not aligned at right angles to taxiway H. This tends to give a false sense of being clear of adjacent aircraft once the cockpit is well past the aircraft and heading towards taxiway H, as the wingtips are far behind the flight deck and outside the normal viewing arc of the pilots.
Aircraft Positioning
Aircraft VUP was parked on bay 2 with its nose wheels approximately 450 mm aft of the stop mark, and the main landing gear offset longitudinally about 380 mm left of the bay centreline. These were within allowed tolerances, and ground crew assessed VUP as correctly parked. VUP had been guided to the parking position by ground marshals. However, the position of VUP's tail slightly left of centreline and aft reduced the available clearance between the two aircraft.
Turning Geometry
When YFT stopped, the nose wheels straightened, and the aircraft subsequently taxied outside the marked turning circle. On a Boeing 737, releasing the tiller during taxi tends to centre the nose-wheel steering and straighten the nose wheels. When the aircraft starts to move, the captain cannot immediately turn the nose-wheel steering before the aircraft moves without considerable use of thrust and excessive load on the nose landing gear.
When YFT stopped, the nose wheels were inside the lead-out line. If the nose wheels track on or inside the lead-out line, the wingtip will clear a correctly parked aircraft on the adjacent bay, but it is necessary to keep a continuous turn going. The check captain commented that if the aircraft stops, particularly if heavy, significant thrust is required to keep the aircraft on the line. The captain noted that they used caution with thrust setting because of passengers boarding on the adjacent bay. The check captain also stated that there was not a lot of manoeuvring room with power-out bays, and there is a risk that the aircraft has to go straight ahead for 2-3 m to get the turn going again, with no allowance for that in the bay geometry.
The captain commented that there is an inherent error in determining where the nose wheel is tracking in relation to a taxi guidance line during turns, as the nose wheel is behind and laterally offset from the captain's seating position. On power-out bays, flight crew cannot see the position of the aircraft on the lead-out line at any time. After stopping, the captain could no longer see the lead-out line, which was then beneath and behind the cockpit. The aircraft taxied forward across the lead-out for about 1–2 seconds then turned right towards taxiway H. There was no marked line connecting the lead-out line with taxiway H.
Ground Crew Observations
When YFT stopped, the dispatcher noticed the nose wheels were inside the line, normally indicating the aircraft would stay clear of an aircraft on bay 2. The aircraft then proceeded for about 2 seconds at an angle that was not normal. Both the ramp supervisor and dispatcher thought the aircraft may have been heading towards taxiway A via the exit behind bay 2 rather than continuing the turn onto taxiway H. The dispatcher reported they were not initially concerned because momentum was needed to turn and get moving.