Background
Resurfacing and re-profiling work was taking place at Bristol International Airport (BIA) as part of a major project to resurface the manoeuvring area pavements. Sections of the runway surface were ungrooved ‘base course’ asphalt. From 14 November 2006, flight crews reported problems related to the friction characteristics of the temporary runway surface. No serious incidents occurred until 29 December 2006.
Incidents on 29 December 2006
On 29 December 2006, at 1150 hrs, the flight crew of Boeing 737-81Q G-XLAC (operated by XL Airways UK Ltd) experienced poor stopping performance during landing on Runway 27.
At 1215 hrs, the flight crew of Avions de Transport Regional ATR-72-202 G-BWDA (operated by Aurigny Air Services Ltd) experienced stopping and lateral control difficulties during landing. The aircraft departed the runway surface and came to rest on the grass area at the side of the runway. The only damage was to G-BWDA, which suffered damage to its left propeller.
At 2133 hrs, the flight crew of Embraer EMB-145EU G-EMBO (operated by British Airways CitiExpress PLC) experienced lateral control difficulties during landing. The aircraft partially left and then regained the runway.
Incident on 3 January 2007
On 3 January 2007, at 1832 hrs, another flight crew operating Boeing 737-81Q G-XLAC experienced poor stopping performance during landing.
Runway Conditions and Actions
The longest stretch of ungrooved base course was the central runway portion, 295 m long and covering the full width. Marshall asphalt is not porous and, when used as a surface course, is usually grooved to allow water to drain. The surface friction of the ungrooved base course had not been assessed using a Mu‑meter with self-wetting in dry conditions predominantly due to the prevailing weather. Mu-meter runs in wet conditions revealed that the ungrooved base course had significantly less friction than the grooved runway sections. Mu-meter runs of the central ungrooved section in natural wet and damp conditions indicated that the friction was probably below the Minimum Friction Level (MFL) of 0.50.
Following the incidents, the airport operator closed the runway on 7 January 2007 and cut temporary grooves in the ungrooved base course. The runway re-opened on 8 January 2007; no further runway excursion or braking difficulty reports were received after this date.
Investigation Findings
The investigation found no evidence of the aircraft involved having experienced a technical fault. The tyre tread depths and pressures were within allowable limits. The airport operator’s risk assessment plan had not adequately addressed the hazards presented to aircraft operating on the temporary surfaces in wet and windy weather. Runway surface contractors believed the temporary ungrooved base course did not represent a significant risk and were more concerned about limiting the length of ungrooved surface course to 100 m; no length limitation was specified for the ungrooved base course. The information promulgated by NOTAM, that braking action information would be available during wet conditions, was incorrect. The instruction to air traffic controllers in the MATS Part 2, that they should provide runway friction value information based on Mu-meter measurements in wet conditions, was incorrect. The use of the snow and ice table for conversion of Mu-meter reading into braking action in wet conditions was incorrect.
Causal and Contributory Factors
The investigation identified the following causal factors: 1. Reduced friction on the wet ungrooved base course sections of the runway caused flight crews to experience reduced braking action and reduced lateral controllability on landing in strong crosswinds. 2. The Flight Operations Communication Department (FODCOM) advice published by the CAA regarding operations on runways notified ‘slippery when wet’, in wet conditions, was not communicated by operators to flight crews. 3. The passing, by ATC, of braking action reports based on Mu-meter friction assessments, gave flight crews a false confidence in the braking action available on the wet runway.
The investigation identified the following contributory factor: G-BWDA landed in a crosswind outside the operator’s published limits and the subsequent use of reverse thrust was contrary to the advice contained in the company’s Operations Manual.