What happened
On June 4, 2011, a SAAB 340B, registered JA03HC, operated by Hokkaido Air System Co., Ltd., was performing a scheduled flight from Hakodate Airport to Okushiri Airport. During the approach to Runway 31, the flight crew initiated a go-around procedure. While the aircraft initially began to climb, it soon entered an unplanned descent.
At approximately 11:38 JST, the crew realized the aircraft was losing altitude rapidly and executed emergency maneuvers to prevent a crash. The aircraft's radio altitude dropped to as low as 92 feet during the incident. Following the emergency recovery, the crew returned the aircraft to Hakodately Airport after performing holding patterns over Okushiri. There were 13 people on board, including 10 passengers and 3 crew members; no injuries or aircraft damage were reported.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data from the digital flight data recorder (DFDR) and the terrain awareness and warning system (TAWS), alongside cockpit voice recorder data and interviews with the crew. The investigation focused on the behavior of the Autopilot/Flight Director (AP/FD) system during the transition from approach to go-around.
Investigators found that during the go-around, the autopilot was disengaged, but the vertical mode remained in the 'ALTS' (altitude select) state because the altitude preset on the autopilot panel had not been updated to the new missed approach altitude. Consequently, the Flight Director command bar directed the pilot to descend to the previous altitude setting. Furthermore, the investigation looked into the crew's monitoring of flight instruments and the company's training standards regarding automated systems.
Findings
- The primary cause of the unintended descent was that the Pilot-in-Command followed Flight Director instructions that commanded a descent, because the altitude preset was not updated to the initial go-around altitude.
- The flight crew failed to notice the aircraft's descending trend in a timely manner, which delayed necessary recovery maneuvers.
- There was an excessive reliance on the autoflight system by both the pilot and the first officer.
- The first officer was unable to provide close monitoring of flight instruments due to other simultaneous cockpit duties.
- The company lacked standard procedures for confirming and calling out mode changes when using the AP/FD system and had not provided adequate training regarding these specific operational risks.