Recorded data showed that take-off from the runway went as expected, but the pilot was subsequently unable to maintain a safe climbing speed or trajectory.
The movement seen by witnesses was suggestive of an aircraft at or close to stalling, the report said.
Concluding the report, investigators said they were unable to determine why the aircraft did not climb normally, or what factors contributed to the pilot not being able to carry out a successful landing when the climb performance was not as expected.
The investigation did not find evidence of any technical issue that might have limited engine performance, and a study of the plane’s controls found an accidental reduction in throttle by the pilot was “not likely”.
The AAIB added a study found the “potential for fouling of the controls” meant “an inadvertent throttle reduction was a possibility”.
The report said: “When the participants were seated in a relaxed posture, all controls on the lower centre console were fully or partially visually obscured and the controls on the left side of the centre console were partially physically obstructed by the right leg of the person in the left seat.
“Moving their right leg to the left to avoid obstructing these controls, resulted in the fouling of the centre column.
“In particular, the throttle control was positioned in the crook of the left seat occupant’s knee and inadvertent movement was possible, for example if moving legs to avoid obstructing the rudder pedals.”
Investigators also found that the aircraft’s payload at take-off was approximately 206kg (454lb), more than 35kg (66lb) over the advised maximum weight.
Inspectors did not find a record of weight and balance calculations made by the pilot on the day of the crash and, although the pilot had been observed preparing the aircraft for flight, the daily inspection book for the plane had not been signed.
However, investigators said while the aircraft took off above its maximum approved weight, they found this to be a contributory – and not a causal – factor.