Synopsis
The aircraft had landed at a remote field site on the Isle of Mull. During preparations for departure, the aircraft unexpectedly left the ground. Despite the application of full power and right aileron, the aircraft rolled left, struck the ground, and rolled inverted. Both occupants were uninjured.
History of the Flight
The aircraft was operating from Glenforsa Airfield on the Isle of Mull to remote field sites. The site where the accident occurred was west of Ben More, at an elevation of approximately 1,500 ft amsl, and had been visited earlier in the day. Each occupant held a pilot’s licence, and the passenger flew the aircraft into the landing site. No local meteorological information was available, but the commander had obtained a forecast wind for the area of 170° at 9 kt. He assessed the wind at the sites as 190° at 14 kt. The pilot lined up the aircraft for takeoff and assessed a slight crosswind from the right. While preparing for takeoff, the aircraft suddenly lifted into the air and rolled left. The pilot instinctively applied full power and attempted to correct the left roll. The left wingtip struck the ground, followed by the nose. The aircraft then overturned about the wingtip and nose, coming to rest inverted. Both pilots vacated the aircraft unhurt. The pilot reported his belief that a strong, unexpected gust of wind affected the aircraft. The aircraft was badly damaged by the accident and damaged further during its recovery by helicopter. The damage was subsequently assessed as beyond economic repair.
Aircraft Information
The aircraft was a modified PA-18-150. Its short takeoff and landing characteristics gave it a stalling speed below the minimum speed of 40 mph that could be displayed by the airspeed indicator. The aircraft was fitted with large-diameter tyres to allow operation from unprepared sites.
Information from the Pilot
The pilot assessed that a gust of 25 kt would be sufficient to lift the aircraft into the air but that “in a pre-take-off scenario there is not the necessary airspeed to the ailerons to counteract a rolling tendency once the wind has lifted the aircraft into the air.”
Meteorology
A summary of findings from a Met Office review of the area meteorology stated: “From the information available, the most likely weather conditions across the Island of Mull at around 1415 UTC on Thursday 25th July 2019 were generally fine, good or very good visibility with few or scattered amount of cloud with bases above 3000 ft. The wind direction would have been from a south-east or south‑south-east direction, with mean surface speed most likely to be 15‑20 Knots, increasing to 20-30 Knots at 2000 ft. The air would have been stable hence gusts would have been unlikely.”
Analysis
The aircraft had a very low stalling speed and would have been capable of flight at very low airspeeds, and the pilot believed that a gust of 25 kt could lift it into the air. Although the Met Office considered gusts unlikely, its report suggested that the freestream wind at 1,500 ft amsl might have been about 20 kt. It is possible that local wind effects, influenced by the terrain, caused the strong gust of wind near the ground reported by the pilot.
Conclusion
The aircraft rolled out of control at low airspeed, struck the ground, and inverted.