A company whose plane crashed last year, seriously injuring two people, had used an unauthorised fuel, a report has found.
A trainee pilot and instructor were taken to hospital after the plane crashed during a training exercise at Cranfield Airport, Bedfordshire.
The Air Accidents Investigation Branch (AAIB) said Hinton Pilot Flight Training also had incomplete maintenance records.
The company is yet to comment.
The trainee pilot had been practising a touch-and-go landing, where the plane lands and takes off again without stopping, last June.
"During the climb following a touch-and-go landing, the engine began to run roughly," the report said.
"A turn was commenced during which the engine failed and control of the aircraft was lost."
The Piper Tomahawk plane was destroyed in the crash.
Investigators said they could not be sure why the engine failed, but believed a combination of carburettor icing and the instructor's decision to make a tight turn back to the airfield, instead of carrying on straight, had probably caused the crash.
The report said the aircraft stalled "at a height that was insufficient to allow recovery".
Investigators also found traces of unleaded motor vehicle petrol, which is not authorised for planes used for training, in the fuel tank.
Maintenance logs including fuel levels and flight times had not been completed and unqualified people had been working on the aircraft, the AAIB report said.
Hinton Pilot Flight Training is no longer in business, having had its operating licence suspended by the Civil Aviation Authority.
Its owner, Christopher Alexander, has yet to be reached for comment.