Aviation authorities have launched an investigation in the US after a pilot overshot his destination by 150 miles and had to turn back.
An alert was sparked when a Northwest Airlines flight from San Diego to Minneapolis lost radio contact with air-traffic controllers for more than an hour.
The Airbus A320, with 144 passengers on board, was 150 miles off course by the time the pilots realised the error, re-established communications and requested permission to turn around.
By this time, the US military had been alerted and had put fighter jets on stand-by, reports Sky News.
The pilots reportedly told the Federal Aviation Administration that they lost track of where they were during a "heated discussion" over airline policy.
However, a spokesman for the National Transportation Safety Board (NTSB) has said that it will investigate the possibility that the pilots fell asleep on the job.
Delta Airlines, which owns Northwest, has suspended the two pilots while the cockpit voice recorder and flight data recorder are being analysed.
However, the airline has refused to give any further information on either the pilots or events during the flight.
In January 2008 a Go! Airlines flight in Hawaii passed its destination when two pilots fell asleep in the cockpit for at least 18 minutes. They were eventually roused by controllers and landed the plane safely.
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