What happened
On 29 May 2005, a Kolb Twinstar Mk 3, registration G-MYMI, was conducting a private cross-country flight from Staffordshire to Netherthorpe Airfield. During the arrival sequence, the pilot initially performed an overhead join but aborted the first attempt after observing another aircraft on long finals.
While performing a second circuit, the pilot was on final approach for Runway 24 when the aircraft encountered intense turbulence, causing a left bank of approximately 45 degrees. In an attempt to correct the bank and compensate for a low altitude, the pilot applied power. As the aircraft crossed the runway threshold, further turbulence caused the aircraft to sink. Despite applying full power, the pilot could not arrest the high rate of descent, leading to a heavy landing that caused the left main landing gear leg to collapse. The pilot sustained one serious injury, while the passenger remained uninjured.
The investigation
The investigation examined the environmental conditions and flight data to determine the cause of the sink rate. While the pilot considered the possibility of wind rotor from nearby trees or houses, GPS data indicated light winds of approximately 52 mph with lower speeds than the reported 10 mph south-westerly. The investigation also assessed whether wake turbulence from the preceding aircraft was a factor, but concluded that the preceding aircraft's size and weight made such an encounter unlikely.
Analysis of the onboard GPS revealed that the aircraft's average groundspeed during the final approach was 51 mph. The pilot had maintained a consistent indicated airspeed of approximately 55 mph throughout the circuit without using flaps. The investigation noted that the aircraft's engine configuration resulted in a nose-down pitching tendency when power was applied.
Findings
- The sudden roll and subsequent sink were likely caused by thermally induced updrafts or downdrafts.
- The heavy landing and high rate of descent were likely caused by a loss of airspeed, potentially exacerbated by the turbulent air.
- The aircraft was flying at its normal approach speed without the use of flaps.