Japanese investigators have detailed the extraordinary in-flight upset involving an All Nippon Airways Boeing 737-700 which resulted in the aircraft banking to a near-inverted attitude.

Flight NH140 from Naha had been cruising at 41,000ft, en route to Tokyo on 6 September, and had been some 43km south of Hamamatsu when the incident occurred.

Japan's Transport Safety Board said the captain had stepped briefly out of the cockpit, but when he returned the co-pilot inadvertently operated the rudder trim control while attempting to let the captain back in.

The 737 banked slightly right, then twice rolled sharply to the left, achieving a bank angle of nearly 132° and "rapidly" pitching 35° nose-down, the inquiry board added.

It descended at high speed, losing 6,300ft before recovering at 34,700ft and climbing.

During the incident the aircraft, which had been on a heading of 052° north-east towards Tokyo, eventually stabilised on a south-west course of 257°.

In its preliminary report into the event the safety board said the aircraft had been on autopilot, but a left turn on the rudder trim control was recorded.

While individual configurations differ, both the rudder trim dial and the cockpit door lock switch on the 737-700 tend to be located towards the rear of the centre pedestal between the two pilots. The door switch requires a turn to the left to unlock the cockpit entry.

The aircraft involved, a four-year old twinjet registered JA16AN, achieved an excessive speed of Mach 0.828 during the event and experienced forces of 2.68g.

Inspectors assessed the aircraft's condition following the event but found nothing abnormal, said the safety board.

There were 117 occupants on board the aircraft, five of them crew members, but only minor injuries were sustained.

Further investigation by the Transport Safety Board will involve verification of its initial simulations, examination of the position of the trim control and cockpit door switch, and analysis of the radar track and radio communications.

Source: Flight International