What happened
Between late December 2006 and early January 2007, a series of serious incidents occurred at Bristol International Airport involving three different aircraft. On 29 December 2006, a Boeing 737-81Q, registration G-XLAC, experienced poor stopping performance during landing. Later that same day, an ATR-72-202, registration G-BWDA, struggled with lateral control and braking on the wet runway, eventually departing the paved surface and coming to rest on the grass. The aircraft sustained damage to its left propeller. On the evening of 29 December 2006, an Embraer EMB-145EU, registration G-EMBO, also experienced lateral control difficulties, partially leaving the runway before the crew regained control. The issues recurred on 3 January 2007, when G-XLAC again suffered from inadequate stopping performance.
The investigation
The AAIB investigation focused on the runway's condition during a major resurfacing and re-profiling project. It was established that parts of the runway consisted of an ungrooved Marshall asphalt base course. While reports of friction issues had been noted since November 2006, the serious incidents occurred when the surface was wet.
Investigators found that the ungrooved asphalt lacked the necessary porosity to allow water to drain, leading to reduced friction. Testing with a Mu-meter in wet conditions confirmed that the friction of this ungroered section was likely below the Minimum Friction Level of 0.50. Furthermore, the investigation found that the airport's risk assessment for the temporary surface had not adequately addressed the hazards of operating in wet and windy weather. The investigation also noted that air traffic controllers were providing braking action reports based on Mu-meter friction assessments, which provided flight crews with a false sense of security regarding available braking.
Findings
- Reduced friction on the wet ungrooved base course caused the loss of braking and lateral control.
- The CAA's Flight Operations Communication (FODCOM) advice regarding 'slippery when wet' runways was not communicated to flight crews by operators.
- Air traffic control reports based on Mu-meter assessments were misleading in wet conditions.
- The operator's risk assessment for the temporary runway surface was insufficient.
- In the case of G-BWDA, the landing was performed in a crosswind exceeding the operator's limits, and the use of reverse thrust deviated from the company's Operations Manual.