A Delta Northwest flight -- with 147 passengers on board -- last week overshot its destination in the US by 150 miles before the cockpit crew realized the had to turn around.

The question everyone is asking is: 'how did this happen?'

The crew told investigators they were in a heated conversation about airline policy -- and lost track of the radio and the flight.

Air traffic controllers lost contact with the flight -- en route from San Diego to Minneapolis -- just before 7pm and could not reach the crew for the next hour and 20 minutes. Not until 8:15pm -- when the crew re-established contact and requested permission to turn around from its location in Wisconsin.

After landing -- the crew explained that they'd been arguing and lost situational awareness. Some airline expert say it's likely the pilots fell asleep.