A significant breakdown in cockpit management in an off-course Ryanair plane was the most likely cause of a serious incident when the Irish aircraft and a British aircraft came too close.
A report by the UK's Air Accidents Investigation Branch (AAIB), which investigates civil aircraft accidents and serious incidents, said it could not establish why the Ryanair flight was off-course.
A Ryanair Boeing 737-200 and an Air UK plane passed each other significantly closer than the accepted distance set by the UK Civil Aviation Authority. A spokesman for the authority said such a "loss of separation" between two aircraft constituted a "near miss".
The incident occurred when the Ryanair plane was taking off on a scheduled flight from Stansted Airport in London to Dublin on September 30th, 1997, while the Air UK plane was descending to London City Airport on a flight from Edinburgh.
The AAIB's report, published this month, found the Ryanair plane had deviated from the standard route for departures from Stansted, known as the Standard Instrument Departure, "for reasons that the investigation has been unable to determine".
When air traffic control noticed the aircraft was off-course, it immediately instructed both planes to change their courses. The aircraft passed each other with a horizontal distance between them of 0.91 nautical miles. They were 200 feet apart vertically. The accepted distance between aircraft approaching an airport is three nautical miles horizontally and 1,000 feet vertically, according to a spokesman for the Civil Aviation Authority.
Ryanair said in a statement that it had fully co-operated with the investigation and had advised the Irish Aviation Authority after receiving the AAIB's report. The company had also started its own internal investigation.
"Ryanair operates to the highest international standards of flight operations, maintenance and safety and is an approved Joint Airworthiness Requirement Operator," the statement said.
The AAIB investigation found it was "most likely that there was a significant breakdown in the management of the cockpit resources on the [Ryanair] flight deck during this departure, particularly with regard to the requirement for the pilot non-flying to monitor the performance of the pilot flying.".
Its report said the Ryanair pilots believed they had complied with the tracking requirements for the departure and could not understand how the deviation had occurred.
The weather was "benign", the ground and airborne navigational aids were serviceable and there were no distractions for the flight crew, which was well rested and familiar with the route, the report says.
The commander of the Ryanair flight had considerable experience and was familiar with the route and the departure from Stansted. His first officer had a commercial pilot's licence and 1,400 hours' experience and was also familiar with the departure route.