A Boeing 737-376 aircraft was being taxied for a departure from runway 20 at Christchurch on a scheduled service to Auckland. The Automatic Terminal Information System indicated that the surface wind was from 200 degrees at 10-15 kts, the temperature was 6 degrees Celsius and the dewpoint was 5 degrees Celsius. The visibility was 10 km reducing to 3,000 metres in rain and drizzle. During the taxi, the crew observed that the weather was fine and that the taxiway was wet after recent rain.
The route to the threshold of runway 20 included a 1,600-metre straight section of the main taxiway which ran parallel to runway 02/20, and which had a slight downhill grade. Analysis of flight data recorder (FDR) data revealed that the aircraft was on that section of the main taxiway for 2.3 minutes and that the speed of the aircraft gradually increased from 11 kts until it reached a maximum of 29 kts. The operator's flight crew training manual (FCTM) contained the following information regarding taxi procedures:
"To the pilot, the airplane appears to be moving slower than it actually is due to the flight deck height above the ground. Consequently, the tendency is to taxi faster than desired.
"Taxi speed should be closely monitored during taxi out, particularly when the active runway is some distance from the departure gate.
"Avoid taxi speeds greater than 30 kt on long taxi routes.
"Allow for decreased braking effectiveness on slick surfaces."
Although it was not included in the most recent edition of the FCTM, previous text stated that when approaching a turn, the aircraft should be slowed to the appropriate speed for the conditions. On a dry surface, approximately 8 kt to 12 kt was recommended. The aircraft manufacturer inadvertently removed that maximum recommended cornering taxi speed, when the Boeing 737 FCTM was extensively rewritten.
At the end of the main taxiway, the route to the threshold of runway 20 required that the aircraft be steered to the left, through approximately 60 degrees, onto taxiway "alpha". The pilot in command began braking the aircraft before reaching that turn point, but the aircraft had only slowed to 25 kt when the turn was commenced.
During the turn the nosewheel started "scrubbing", indicating excessive steering angle and/or taxi speed for the surface condition. Lacking traction, the nosewheel moved sideways and directional control was lost. That was followed by loss of traction on the main gear. As the aircraft moved sideways, the right main gear slid off the paved surface and onto the grassed area adjacent to the taxiway. The aircraft came to a halt with the right main gear settled slightly in the soil.
A maintenance engineer attended the aircraft, assessed that there was no obvious damage and then marshalled the aircraft back onto the taxiway. The aircraft was then taxied back to the terminal where a detailed inspection of the landing gear, the brakes and the engines was carried out. That inspection did not reveal any damage and the aircraft was released back into service.
The technical crew of the Boeing 737 consisted of a pilot in command and a co-pilot; both of whom were very experienced on the aircraft type. The airline operator had only recently commenced operations from this airport; the crew's local experience was therefore limited. The Boeing 737 nose wheel steering system was only controllable from the left control seat. Therefore, the pilot in command always taxied the aircraft. Either pilot could have used the ground speed readout on the pilots' electronic attitude director indicator (EADI) to monitor the taxi speed. The airline's normal procedure was for the co-pilot to bring to the attention of the pilot in command, any unsafe speed. During the airline operator's subsequent investigation of the incident, both pilots expressed surprise at the high speed recorded by the FDR during the turn.
The taxiway in question was treated during February 2001 with a surface enrichment treatment that resulted in making the surface more slippery. A notice to airmen (Notam) was issued on 20 March 2001 that advised poor braking action was possible when the surface was wet. That Notam was subsequently cancelled on 27 May 2001, when it was considered that the taxiway surface had weathered sufficiently for friction levels to return to normal.
During August 2001, a different airline operator reported another two minor skid events at that same location. Neither of those incidents resulted in the aircraft leaving the sealed area. As a precautionary measure, the airport operator re-issued the Notam that warned of the possibility of poor braking action. The taxiway surface was subsequently roughened by water blasting. That action removed the smooth bitumen top layer and exposed the aggregate for a better friction surface.
Although the act of taxiing an aircraft to and from an active runway may have become a routine task, ensuring that it was done safely required planning and constant vigilance by both members of the technical crew. In this particular instance, the aircraft was safely taxied a considerable distance in accordance with company procedures. However, as the aircraft approached the turn at taxiway "alpha", the crew was apparently unaware that the aircraft's speed was high. The aircraft entered the turn at an inappropriate speed and on a surface that was noticeably wet. The probability of maintaining directional control under those circumstances was doubtful.
It was considered likely that both pilots became accustomed to the relatively high taxi speed owing to the extended time spent on the long, straight taxiway that preceded the turn. The crew relied solely upon their visual estimation of speed in circumstances that required cross-reference to the EADI ground speed readout.
Although the taxiway treatment carried out by the airport operator resulted in a surface that was slippery, it was considered that the aircraft would have been unlikely to have left the taxiway if a speed appropriate for the existing conditions had been used.
As a result of the occurrence, the airline operator issued a Flight Standing Order applicable to the Boeing 737 fleet. The following policy regarding taxi speed was stated:
"1. Maximum taxi speed on long straight smooth taxiways is 30 kts.
" 2. Maximum taxi speed entering a turn is 10 kts.
" 3. If the taxiway surface is other than dry crew judgement will dictate a lower speed."
|Date:||22 July 2001||Investigation status:||Completed|
|Time:||0215 hours UTC|
|Release date:||04 February 2002||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Christchurch, NZ|
|Role||Class of licence||Hours on type||Hours total|