What happened
On July 9, 2016, a Jetstar Japan Airbus A320-232, registered JA04JJ, was operating a scheduled flight from Fukuoka Airport to Narita International Airport. While cruising at 37,000 feet, approximately 96 km south-southeast of Chubu Centrair International Airport, the aircraft encountered severe airspeed fluctuations. At 09:37 JST, the airspeed indicators on both the captain's and co-pilot's sides began to fluctuate wildly, eventually leading to a failure of the indication and the appearance of error flags on the primary flight displays.
In response to the unreliable airspeed, the crew manually disconnected the autopilot and autothrust. To maintain flight stability and prevent a stall, the captain maintained a pitch attitude between 2.5 and 5.0 degrees. The aircraft descended to 25,000 feet to exit the problematic area. The airspeed indications eventually recovered, and the aircraft landed safely at Narita International Airport at 10:26 JST. There were no injuries among the 156 persons on board, and no damage was sustained by the aircraft.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data recorder (FDR) and cockpit voice recorder (CVR) data, alongside maintenance records and meteorological information. The investigation focused on the cause of the simultaneous failure of the Air Data Reference (ADR) systems. Investigators analyzed the behavior of the computed airspeed (CAS) and the angle of attack (AOA) data, noting that the aircraft had been deviating from its original flight plan to avoid adverse weather. The investigation also reviewed the presence of ice crystals in the vicinity of high-altitude cumulonimbus clouds and evaluated the crew's adherence to the Quick Reference Handbook (QRH) procedures during the emergency.
Findings
- The primary cause of the incident was icing within the Pitot tubes, which resulted in the temporary loss of reliable airspeed indications on both sides of the cockpit.
- It is likely that the icing was caused by the aircraft flying through an area containing ice crystals near a high-altitude cumulonimbus cloud.
- The flight crew performed appropriate emergency procedures, specifically maintaining a stable pitch attitude and engine thrust to prevent a stall during the period of unreliable airspeed.
- The small size of the ice crystals made them difficult to detect using the aircraft's onboard weather radar.