What happened
On September 6, 2011, at approximately 22:49 JST, an Air Nippon Co., Ltd. Boeing 737-70 and registration JA16AN was operating a scheduled flight from Naha Airport to Tokyo International Airport. While cruising at 41,000 feet, approximately 69 nm east of Kushimoto, the aircraft entered an unusual attitude. This upset progressed into a nosedive, resulting in a situation where the aircraft lost lifting force. The flight crew, which included the captain, a first officer, and three cabin attendants, managed to stabilize the aircraft, though two cabin attendants sustained slight injuries. There was no damage to the aircraft.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight data, cockpit voice recordings, and the physical characteristics of the aircraft's controls. The investigation focused on the sequence of events leading to the unusual attitude, the first officer's manual inputs, and the effectiveness of the crew's recovery maneuvers. Investigators specifically analyzed the similarities between the rudder trim control and the door lock selector, as well as the first officer's previous training and experience on the Boeing 737-500 series.
Findings
- The incident was triggered when the first officer erroneously operated the rudder trim control while intending to operate the door lock control to allow the captain to re-enter the cockpit.
- The first officer's previous experience on the Boeing 737-500 contributed to the error, as the placement, shape, and size of the door lock control on the 737-700 were similar to the rudder trim switch on the older model.
- The first officer's recognition of the developing upset was delayed due to an excessive dependence on autopilot and a lack of focus on monitoring flight conditions.
- Recovery efforts were insufficient because the first officer was startled by the activation of the stick shaker and lacked specific training for high-altitude upset recovery involving stall warnings.
- The physical environment of the night flight, including limited visibility of the horizon and the movement of the control column, hindered the crew's ability to identify the deviation immediately.