What happened
On 31 March 2016, a Cessna 172S Skyhawk, registration G-ENNK, was attempting a takeoff from the grass Runway 15 at Sherlowe Airstrip in Shropshire. During the takeoff roll, the pilot observed that the aircraft was failing to accelerate at the expected rate due to the soft nature of the ground. In an attempt to prevent an overrun, the pilot decided to abort the takeoff. However, the aircraft could not be stopped within the available distance, running off the end of the approximately 240 m runway and into a soft, cultivated field. The impact caused the aircraft to flip onto its back. The pilot escaped the wreckage without injury.
The investigation
The investigation focused on the performance calculations used for the departure. The pilot, an experienced flyer with 8,000 total hours, had calculated a 40% safety margin for takeoff based on the Pilot’s Operating Handbook (POH) performance figures for short-field operations.
According to the POH, a short-field takeoff on a flat, dry, paved surface requires a minimum ground roll of 186 m under specific conditions. The manual also mandates adding 15% to this distance when operating from dry grass, resulting in a calculated minimum roll of 214 m. This left a calculated margin of only 12% to account for the damp and soft conditions present on the day. The investigation also reviewed guidance from the CAA’s Safety Sense leaflet regarding the necessity of establishing a clear decision point to abandon a takeoff if engine or aircraft performance is unsatisfactory.
Findings
- The aircraft sustained damage that likely rendered it beyond economic repair.
- The primary cause of the excursion was the insufficient acceleration caused by the soft ground conditions.
- The pilot's calculated safety margin of 40% was mathematically inaccurate when compared to the POH requirements, leaving only a 12% margin for the actual runway conditions.