The first officer of the Northwest Airlines jet that missed its destination by 150 miles says there was no fight in the cockpit, neither he nor the captain had fallen asleep and the passengers were never in any danger.

But in an interview two days after he and a colleague flew past their destination as air traffic controlled tried frantically to reach them, pilot Richard Cole would not say just what it was that led to them to forget to land Flight 188.

"It was not a serious event, from a safety issue," Cole said in front of his Salem, Oregan, home. "I would tell you more, but I've already told you way too much."

Air traffic controllers and pilots tried for more than an hour Wednesday night to contact Cole and the flight's captain, Timothy B Cheney, using radio, cell phone and data messages. On the ground, concerned officials alerted National Guard jets to prepare to chase the airliner from two locations, though none of the military planes left the runway.

Cole would not discuss why it took so long for the pilots to respond to radio calls, "but I can tell you that airplanes lose contact with the ground people all the time. It happens. Sometimes they get together right away; sometimes it takes awhile before one or the other notices that they are not in contact."

A police report released on Friday said the pilots passed breathalyzer tests and were apologetic after the flight. The report also said that the crew indicated they had been having a heated discussion about airline policy.

But aviation safety experts and other pilots were deeply skeptical they could have become so distracted by shop talk that they forgot to land an airplane carrying 144 passengers. The most likely possibility, they said, is that the pilots simply fell asleep somewhere along their route from San Diego.

"It certainly is a plausible explanation," said Bill Voss, president of the Flight Safety Foundation.

Unfortunately, the cockpit voice recorder may not tell the tale. New recorders retain as much as two hours of cockpit conversation and other noise, but the older model aboard Northwest's Flight 188 includes just the last 30 minutes – only the very end of Wednesday night's flight after the pilots realised their error over Wisconsin and were heading back to Minneapolis.

Cheney and Cole have been suspended and are to be interviewed by National Transportation Safety Board. FAA spokesman Tony Molinaro said in general, an unsafe condition created by a pilot could lead to the suspension of the person's pilot license and possibly a civil penalty.