An unsecured buckle interfered with a pilot's controls and caused a plane to crash, according to an accident report.
The Yak-52 aircraft crashed as it returned to the airport to land
The pilot and passenger of Yak-52 died when the plane came down close to the runway at Bournemouth Airport on 22 July last year.
The Air Accidents Investigation Branch (AAIB) said a loose strap and buckle had probably become jammed in the controls hindering the pilot.
The AAIB has recommended new safety procedures for Yak-52 aircraft.
In the report released on Thursday, the AAIB concluded: "The most likely cause of any restriction of the controls was that a buckle on the unsecured crotch strap may have become jammed in the flight controls.
"The fact that the passenger's shoulder and crotch straps were unsecured was considered a contributory factor, which could had lead to the passenger feeling insecure and affecting the controls in the final part of the flight."
The 40-year-old pilot and 60-year-old passenger who died had been on a pleasure flight and were returning from Sandbanks.
The crash happened in the north-east sector of Bournemouth Airport, where the business park and flying clubs are located.
No other buildings or people were affected and the Dorset airport was not closed.
The AAIB has now made new safety recommendations to the Civil Aviation Authority.