He had taken off from New Jersey on Friday night for the 90-minute flight to Massachusetts, but disappeared from radar screens at about 9.40pm.

Clues gleaned from the plane's final radar traces show the final descent into the ocean of Martha's Vineyard may have been terrifyingly rapid - at 5,000ft per minute.

Radar records show the Piper Saratoga was descending normally when it levelled out at 2,300ft and began climbing slowly.

But at 2,600ft the plane turned to the right and began its steep dive - it would have crashed into the water less than three minutes later.

A massive rescue operation followed.

In the days after the crash much was been made of the facts that with an injured foot and less than 100 hours experience as a pilot, Mr Kennedy should not have flown in the adverse weather conditions that prevailed.

A pilot at Essex Country Airport in Fairfield, New Jersey, from where Mr Kennedy took off on Friday night, said he had decided against flying the route to Martha's Vineyard because of the hazy skies.

Mr Kennedy was not licensed to fly on instruments and did not file a flight plan for the trip - but this is not unusual for general aviation pilots flying under visual flight rules.

However, flying at night, in a hazy sky and under visual flight rules is challenging.

Manufacturer joined search

Mr Kennedy acquired the plane secondhand last year, having obtained his private pilot's license - restricting him to operating single-engine craft - in April 1998.

His aircraft, number N9253N, was built in June 1995, by Florida-based manufacturers The New Piper Aircraft, Inc.

Company president Chuck Suma said the company had sent a representative to join the investigation into the disappearance of Mr Kennedy's flight.

It is customary for the plane or engine manufacturer to assist the National Transport Safety Board during an investigation.

A company spokeswoman said the emergency locater transmitter could be triggered manually but was also designed to transmit if subjected to forces four times greater than gravity, such as occur in a crash. However, its signal does not penetrate water well.

'Reliable and safe'

The Piper Saratoga is widely regarded as reliable and safe, according to David Learmount, safety editor at Flight International magazine.

"But anything which has only one engine has the problem that if the engine does fail, then you have a glider on your hands," he added.

The last fatal crash of a Piper Saratoga occurred in California, on 3 April. Four people died when the plane hit the ground on a personal flight under instrument flight rules.

"This is a very safe airplane that is equipped very well," said Mr Suma, adding that there were over 7,500 Piper Saratogas in service around the world, with over a half a million hours flown every year.

It has been in production since the 1960s, with 1999 models of the Saratoga II HP - as well as the Saratoga II TC - fitted with a new cockpit aimed at reducing pilot workload "because there are fewer buttons to push and fewer individual instruments to monitor".

Both models have had the S-TEC System 55 autopilot fitted as standard equipment. But like most general aviation aircraft, it does not carry a flight data recorder.

The Piper Saratoga - which seats five or six passengers - has a maximum speed of between 160 and 190 knots (sea miles per hour), with a range of around 800 nautical miles.

The plane tends towards the more expensive end of the light aircraft market, with a suggested retail price of around $380,000.