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.
