What happened
On 8 July 2005, a Boeing 737-436, registration G-DOCT, was preparing for a passenger flight from Aberdeen to Gatwick. To maximize the available runway length due to performance limitations, the commander lined the aircraft up close to the runway threshold. During the takeoff roll, the crew applied high thrust while holding the brakes. As the aircraft accelerated, large sections of asphalt from the adjacent blast pad were lifted by the engine exhaust and thrown into the aircraft's path.
The debris struck the aircraft's tail section, causing a large dent in the underside of the left tailplane and causing a significant portion of the left elevator to detach. Despite the damage, the crew was unaware of the impact and continued the flight to Gatwick. The damage was only discovered later during a routine engineering inspection.
The investigation
Investigators examined the construction of the blast pad and the takeoff procedures used by the crew. The investigation found that the asphalt surface of the blast pad was thinner than recommended by international standards and that the bond between the asphalt and the sub-base may have been inadequate.
Analysis of the engine thrust and wind conditions suggested that as the aircraft maneuvered to line up, the left engine's path over the blast pad, combined with a cross-wind, may have created a suction vortex. This vortex, along with the high-power jet blast, likely enabled the exhaust to penetrate the surface and peel the asphalt away. Additionally, the investigation noted that the commander used a standing takeoff technique—holding the brakes while increasing thrust—which is not the standard rolling takeoff procedure recommended by the manufacturer, and which may have contributed to the surface breakup.
Findings
- The primary cause of the damage was the dislodgement of asphalt sections from the blast pad by the force of the engine jet blast.
- The blast pad's asphalt thickness was significantly less than the levels recommended by ICAO and FAA guidelines.
- The lack of clear markings on the blast pad meant the crew was not aware the surface was not designed for taxiing loads.
- The use of a standing takeoff technique, involving holding the brakes while increasing thrust to maximum, likely contributed to the surface failure.
- A cross-wind during the final alignment turn may have generated a suction force capable of initiating the lifting of the asphalt surface.