Air accident investigators have found part of a common housefly in the fuel system of the Robin HR200 which lost power and plunged into Cromarty Firth on October 29.

Pilot Peter Brooks, 49, and trainee commercial pilot Gavin Keen, 24, scrambled from the aircraft and congratulated themselves on a textbook escape as they swam ashore.

As they reached a jetty, Mr Keen was unable to scale it and was swept back out to sea. His body was found in the firth almost one month later.

After its recovery from the seabed, the single-engine plane was transported to the Farnborough-based Air Accidents Investigation (AAI) branch of the Department of Transport.

A detailed examination of the year-old #70,000 aircraft has since been carried out - and the thorax of a fly was found in part of the fuel system.

The AAI will not comment on the investigation or its findings while the cause of the accident is still under investigation. However, it is understood the discovery of the fly led to investigators calling in an entomological expert to try to determine the geographical origin of the insect.

This could have allowed investigators to establish at what stage the fly entered the fuel system but it emerged the type of fly is found throughout the world.

Although the aircraft is French made, a source close to the investigation said the fly was found in a sealed unit manifactured elsewhere.

He added: ''Whether the introduction of a foreign body caused the accident or not, it had the potential to do so.''

Shortly after the accident, close to the Nigg oil fabrication yard, pilot Peter Brooks spoke of how his young co-pilot came within inches of safety before being swept to his death.

Both men swam 200 metres to within a few feet of the shore. Mr Brooks hauled himself on to the jetty but when he turned to look for his trainee - who had attempted to scale a different part of the jetty - he had vanished.

Mr Brooks, who works for Highland Airways at Dalcross Airport, Inverness, was not at work yesterday and could not be contacted.

Shortly after the tragedy, he said: ''We were flying at 2000ft when we suffered power failure. It was a textbook ditching and Gavin's contribution was vital.

''We were almost congratulating ourselves for getting out despite the plane flipping over in the water.

''Then it all went wrong. Gavin went to a different part of the 15ft high jetty wall and couldn't find anywhere to climb it.

''As I got up, there was no sign of Gavin.''

Mr Keen, a commercial helicopter company worker based in Aberdeen, was on a training course with Highland Airways, a charter taxi service and flight training school.

His father, Roy, 62, of Dingwall, spoke of his son's love of flying and determination to become a fully qualified commercial pilot.

Yesterday, the company refused to comment on the latest development and directed all inquiries to the Department of Transport.

A spokesman for the air accident branch of the DoT said accident reports were published within four to six months after the event.

He said the only time comment was made on investigations was when a defect had to be brought to the immediate attention of the aviation industry.