A statement by the world's No. 1 airline said the crew member was a relief pilot on the Boeing 767-300 with 198 passengers, meaning he was a backup to the captain and first officer. The crew member was not identified.

He was arrested on suspicion of being under the influence of alcohol after reporting for duty at Manchester airport, the carrier said. A court appearance was scheduled for Monday.

American said in a statement it was investigating and would not provide additional details.

"Our primary concern is for the safety and comfort of our passengers and crews," the airline said. "American Airlines has strict policies on alcohol and substance abuse and holds its employees to the highest standards."

Flight 55 was due to arrive in Chicago shortly after 4 p.m. local time, three hours late.

Timber company D&R Henderson executive Robert Henderson, his daughter Jackie, friends Alan Stark, Belinda Andrews and Geoff Brockie were killed when the twin-engined Piper Cheyenne crashed into a tree-covered ridge near Benalla in Victoria's northeast on July 28, 2004.

Also killed was pilot Kerry Endicott, who had flown D&R Henderson staff from Sydney to the company's Benalla factory at least once a week since 1988.

The Australian Transport Safety Bureau (ATSB) today released its final report into the accident and said they would probably never know why Mr Endicott veered off course moments before the crash.

ATSB deputy director Alan Stray said the crash was so severe, most of the plane's instruments were destroyed, hampering the investigation.

"Unfortunately, we will never know why the pilot descended and entered the pattern that he did in making the approach," he said.

Mr Endicott was supposed to fly towards one of three points from where the Piper would fly into Benalla airport.

Air traffic controllers were not concerned when the aircraft did not fly in on the agreed approach but appeared to be heading to another further south.

But investigators could not establish why the pilot then went even further south, dropped off radar and crashed about 30km west of the airport.

Mr Stray said the pilot might have thought he had reached the approach and had mistakenly begun to turn the plane.

"Why he was way off course, having flown that flight so many times over so many years is a mystery, and will more than likely remain a mystery.

"We have spent considerable time, energy, efforts and resources, not only of our own people but also our counterparts overseas," he said.

"We still can't come up with a definitive (answer)."

The ATSB has recommended all pilots cross reference their position on more than one piece of navigational equipment.

A spokesman for budget airline Nok Air said the Boeing 737-400 bound for the southern resort island of Phuket was forced to land shortly after takeoff due to engine trouble.

A spokeswoman for the Airports of Thailand, which operates the country's airports, said the aircraft skidded off the runway.

"The plane did go off the end of the runway, but it wasn't a skid," the Nok Air spokesman said, asking not to be named. "The incident is being investigated."

He said one passenger suffered a twisted ankle and the other a grazed knee as they evacuated the aircraft on inflatable slides.

About 110 passengers and five crew were aboard flight DD7506 which left Bangkok in the early afternoon before it was forced to turn around a few minutes later.

The aircraft was climbing when the crew heard a noise in the right engine, which then lost power, the airline spokesman said.

Nok Air is a unit of national carrier Thai Airways, and leases aircraft from its parent.

After the plane's engine cut out, the pilot alerted air safety authorities of his forced emergency landing into the southern end of Lake Burbury.

On board were were eight people from Victoria and two French passengers, as well as the pilot.

A Tasmanian police spokesman said the emergency landing, at 6.18pm (AEDT), was successful, with everyone on board escaping injury.

Power to the single-engine aircraft was lost at 5.52pm (AEDT) and the aircraft took 26 minutes to descend and land safely on the lake.

The passengers had chartered a scenic flight in Strahan.

The amphibious aircraft, known as a Cessna Caravan, is operated by Wilderness Air in Strahan.

A police spokesman said local ambulance, fire services and SES crew attended and the Westpac rescue helicopter was called in, with a second helicopter on standby.

"Police were at the scene (at the time of the landing) and report that the pilot and passengers are not injured and there is no damage to property," the spokesman said.

"AUSSAR and air safety will be investigating the circumstances surrounding the emergency landing."

Wilderness Air and Queenstown police are transporting the passengers out of Lake Burbury.

A police spokesman said the passengers praised their brave pilot, who landed the plane after a "serious" warning light had been activated.