What happened
On 3 April 2010, a Grob G109B, registration G-UILD, was preparing for a private flight from Wing Farm, Wiltshire, to Compton Abbas. The airfield features a 500 m grass runway with a 2.2% downslope. Upon arrival, the pilot observed that the grass was short and damp, with some soft patches, and noted a light 5 kt southeasterly wind. To evaluate acceleration, the pilot performed a trial run on Runway 09.
During the takeoff attempt, the pilot noted a sluggish initial response despite normal engine parameters. Although the aircraft reached 40 kt just before the 350 m marker and became airborne at that point, it subsequently settled back onto the surface. The pilot attempted to abort the takeoff by reducing power to idle and applying brakes; however, the brakes were ineffective on the damp grass. The pilot then switched off the magnetos to stop the engine, but the aircraft continued down the runway, eventually colliding with trees and a hangar located just beyond the runway end.
The investigation
The AAIB examined the aircraft's performance capabilities and the airfield conditions. The investigation established that the pilot had performed a prior check of the engine and confirmed the aircraft's weight and balance were within limits. The investigation also reviewed the pilot's familiarity with the specific performance limitations of the Grob G109B when operating from short, grass strips under marginal conditions.
Findings
- The aircraft's takeoff performance was marginal due to the damp and soft nature of the grass runway.
- The pilot's use of brakes was ineffective because of the low friction provided by the damp surface.
- The pilot had become less appreciative of the narrow performance margins required when operating the aircraft from this specific strip over time.