A Northwest Airlines A320 plane taxis at Minneapolis-St. Paul International Airport. There are suspicions that the pilots of a Northwest A320 that lost radio contact for more than an hour on Wednesday had fallen asleep. (Jim Mone/Associated Press)

U.S. federal investigators are studying the flight data recorders on a Northwest Airlines jet to determine what caused its pilots to fly 240 kilometres past the plane's destination as air traffic controllers, other pilots and a flight attendant in the cabin tried to get their attention.

Investigators don't know whether the pilots fell asleep, but National Transportation Safety Board spokesman Keith Holloway said Friday that fatigue and cockpit distraction will be looked into.

The plane's flight recorders were taken to Washington, D.C., on Friday, but the cockpit voice recorder is an older model that contains only the last 30 minutes of conversation.

That makes the investigation more difficult since those minutes were taken up by the flight back to Minneapolis — the intended destination — and the landing there Wednesday night.

Northwest Flight 188 was en route from San Diego to Minneapolis when air traffic controllers lost contact with the plane for more than an hour.

A police report released Friday identified the pilots as Timothy Cheney and Richard Cole. The report offers few new details about why they flew past the airport.

The three-page report says the pilots "were co-operative, apologetic and appreciative" and both voluntarily submitted to alcohol-breath tests. The tests found no evidence that the pilots had been drinking.

Four military fighter jets were readied to chase down the plane before contact with the Airbus 320 was re-established. By that time, the plane had overshot its destination and flown across the state line into Wisconsin airspace.

When the plane landed in Minneapolis, the crew told airline and security officials that they lost "situational awareness" during a spirited discussion about airline policy. The pilots have been suspended from flying by their airline while it, too, investigates.

Suspicions pilots were asleep

Aviation experts say the pilots should have had numerous warnings as they approached and passed Minneapolis: cockpit displays, controllers trying repeatedly to reach them by radio, data messages and the city lights twinkling below.

Yet the pilots didn't discover their mistake until a flight attendant in the cabin contacted them by intercom, a source close to the investigation who spoke on condition of anonymity said.

"It just doesn't make any sense," said Bill Voss, president of the Flight Safety Foundation in Alexandria, Va. "The pilots are saying they were involved in a heated conversation. Well, that was a very long conversation."

Ben Berman, an airline pilot and former chief of major accident investigations at the NTSB, said it becomes second nature for pilots to know when they need to begin landing preparations.

'[It's] unbelievable to me that they weren't paying attention'—Passenger Andrea Allmon

Those preparations should have begun when the flight was still 160 kilometres or more away from Minneapolis, he said. It would require a fairly dramatic event to lose track of that kind of awareness, so shop talk "pretty clearly wasn't all that was going on," Berman said.

While the passengers were apparently unaware what was happening as they passed their destination, police on the ground were preparing for the worst and the Air National Guard had put fighter jets on alert at two locations.

Passenger Andrea Allmon didn't know anything was amiss.

"Everybody got up to get their luggage, and the plane was swarmed by police as we were getting our bags down from the overhead bins," she said.

She said they were kept on the plane briefly while police talked to the crew. Allmon said she was "horrified" to learn what had happened and it was "unbelievable to me that they weren't paying attention. Just not paying attention."

The FAA is updating rules governing how many hours commercial pilots may fly and remain on duty. The NTSB also cautioned government agencies this week about the risks of sleep apnea contributing to transportation accidents.

In January 2008, two Go! Airlines pilots fell asleep for at least 18 minutes during a midmorning flight from Honolulu to Hilo, Hawaii. The plane passed its destination and was heading out over open ocean before controllers raised the pilots. The captain was later diagnosed with sleep apnea.

With files from The Associated Press