What happened
On 21 September 2012, a Boeing 737-35B, registration LY-SKA, arrived at Birmingham International Airport on a scheduled passenger flight from Nice, France. The aircraft, operated by a UK-based carrier, landed on a wet runway with a following aircraft approaching for landing.
In an effort to vacate the runway quickly and prevent the following aircraft from needing to perform a go-around, the commander decided to exit at Taxiway Bravo. During the landing roll, the commander manually disconnected the autobrake and cancelled reverse thrust to facilitate a faster exit. As the aircraft approached the turn onto Taxiway Alpha, the commander began the turn at a groundspeed of approximately 20 kt.
During the maneuver, the aircraft became uncontrollable and began to skid toward the left edge of the taxiway. The pilot applied full braking, but the nosewheel and right main gear left the paved surface, causing the aircraft to come to rest on the adjacent grass at roughly 2/0 kt groundspeed. The following aircraft was subsequently instructed to go around. There were no injuries among the 137 passengers or 6 crew members, though the nosewheels and tyres sustained damage.
The investigation
The AAIB investigation examined the aircraft's flight data, which confirmed the deceleration profile and the timing of the braking override. Engineers inspected the aircraft and found no hydraulic leaks or faults in the anti-skid system; however, they discovered cuts in the tyre treads caused by the contact with the grass.
Investigators also assessed the friction levels of the runway and taxiway surfaces. While the runway friction was found to be within acceptable limits, a localized area of reduced friction was identified on the taxiway. The investigation also reviewed the operator's manual, which recommended a ground speed of approximately 10 kt for turns onto non-high-speed taxiways in dry conditions.
Findings
- The aircraft departed the taxiway because the turn was initiated at a speed inappropriate for the wet surface conditions.
- The commander's primary motivation was to avoid delaying the following aircraft, though the specific distance and timing of the following aircraft's approach were unknown to the crew.
- The area of reduced friction on the taxiway was too small to have been the primary cause of the excursion, as the wheels only traversed the low-friction zone for approximately 5 metres.