History of the Flight
The pilot intended to fly from Brimpton Airfield to Thruxton Airfield. The weather forecast was good, but prior to departure he observed gusty wind conditions at approximately 8 to 10 kt with an estimated mean direction of 180°, varying by approximately 30° in each direction. He considered that this favored a departure from Runway 25, which had a dry and well-cut grass surface. However, the grass on the 50 m runway starter extension had not been cut, and due to a significant upslope, he elected not to use it, commencing the takeoff from the Runway 25 threshold.
The initial takeoff roll appeared normal, but the aircraft did not achieve expected acceleration. The pilot noted a hump at the runway midpoint, by which point he normally expects a speed of 50 kt; on this occasion the speed was 45 kt. He expected the aircraft to achieve 55 to 60 kt before the runway end, but there was no further acceleration. The aircraft overran the runway and came to rest nose-down in a crop field approximately 10 to 15 m beyond the runway end. The pilot was uninjured and exited without assistance, attributing his lack of injury to wearing a full harness.
A witness, another pilot who had elected not to fly due to local weather conditions, observed the takeoff. He noted from the airfield anemometer and windsock that wind direction varied between 220° and 050°, with strength approximately 2 to 4 kt, occasionally increasing to 8 kt. Temperature was 24 °C and QNH 1016 hPa. Concerned about a possible tailwind and its implications on takeoff performance, he was closing up his aircraft when he saw G-BMVM commence its takeoff roll. By the runway midpoint, it was evident the aircraft lacked sufficient speed, and he expected it to stop but it continued and overran.
Discussion
The aircraft was based at Brimpton and regularly operated from there without takeoff performance issues. The pilot retrospectively performed an approximate takeoff performance calculation indicating that, at maximum weight, the required takeoff distance on a grass runway would be 519 m, and a 5 kt tailwind component would increase that to 621 m. Although the aircraft weight at the time was 190 lb less than maximum takeoff weight, which would somewhat reduce the distance required, Runway 25 is 520 m long. Given the ambient conditions, a tailwind would have meant adequate takeoff performance could not be assured.
The pilot considered that the wind may have changed direction during the latter part of the takeoff roll, giving a tailwind component that required a greater takeoff distance than available.