This is your pilot sleeping ...

Could that be why a Northwest Airlines plane flew 150 miles beyond its destination?

In an incident to send a chill down the spine of any frequent flyer, an A320 Airbus belonging to Northwest Airlines was en route from San Diego to Minneapolis- St Paul when the cockpit crew for some reason stopped paying attention. Instead of descending as it should have to the twin cities' airport, the plane, which was carrying 144 passengers, kept on going for another 150 miles. It was flying over Wisconsin before anyone noticed the mistake.

The explanation of the pilots, who have both been suspended, is that they "lost situational awareness" because they were having a furious argument. The reason regarded as more probable, and scary, is that they both fell asleep at the wheel. Whatever the truth, investigators intend to get to the bottom of Wednesday evening's incident which seems to belong more to the realm of spoof disaster movies such as Airplane!

But it was not funny at the time. By the time cabin staff, sensing that something was amiss, had called the pilots on the internal phone, the Air National Guard had been alerted and jet fighters were on the runways of two air force bases in the region ready to take off and intercept the errant airliner. The situation was all the more fraught because, for more than an hour and a quarter, the pilot and co-pilot had ceased all communication with air traffic control. On the ground, fears were growing for the plane and its 144 passengers.

In the end, no one was harmed and no damage was done. The aircraft turned around, began its belated descent and made its way back safely to Minneapolis. Upon landing, the two pilots told police investigators that they had "lost situational awareness" because they had got into a "heated discussion over airline policy".

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

Similarly sceptical is Ben Berman, a former chief airline accident investigator for the National Transportation Safety Board. Noting that it is second nature for pilots to begin preparations for descent in good time, he said that debate about their employer's policies "pretty clearly wasn't all that was going on". Other experts pointed out that, at the very least, the pilots should have noticed the bright lights of Minneapolis below as they passed above the city.

The investigation is only just beginning. More interviews with the pilots are pending, and the black boxes have been removed from the aircraft for analysis. Expects speculated yesterday that the two men may have been covering up for having fallen asleep. The US is in the midst of re-writing the rules on working practices for pilots amid concern that dozing off is the biggest threat to aviation safety.

It is the second embarrassing incident in a week for Delta Air Lines, which purchased Northwest Airlines last November. On Monday, one of its wide-bodied jets landed on a taxiway at Atlanta airport at the end of a flight from Brazil. That incident, however, also ended without damage or injury.