What happened
On 30 December 2009, two separate scheduled flights operated by Mount Cook Airline encountered extreme weather conditions while flying from Christchurch to Wellington. The first incident involved an Aerospatiale-Alenias ATR 72-212A, registration ZK-MCP, which encountered severe turbulence approximately 50 nautical miles north of Christchurch. During the encounter, the aircraft experienced a rapid drop in airspeed and a sudden increase in the angle of attack, triggering the stick-shaker and causing the autopilot to disengage. The aircraft's rate of descent briefly exceeded 5500 feet per minute, and loose items were tossed around the flight deck.
Roughly 35 minutes later, a second Aerospatiale-Alenia ATR 72-212A, registration ZK-MCJ, encountered severe turbulence in the same general vicinity. While no injuries were reported and no structural damage occurred to either aircraft, the intensity of the weather was significant enough to disrupt flight stability.
The investigation
The Transport Accident Investigation Commission (TAIC) examined the meteorological data, air traffic control (ATC) communications, and the operational procedures of the airline. The investigation looked into the adequacy of the weather information provided to pilots and the effectiveness of the communication between air traffic controllers and the Meteorological Service of New Zealand. Investigators also reviewed the flight crew's responses to the turbulence and the airline's dispatch processes.
Findings
- The investigation identified that the first crew attempted to manage the situation by deploying flaps to increase stall margins, but this coincided with a pitch-down maneuver that caused the airspeed to exceed the structural limits for that flap setting.
- The air traffic controller failed to pass a specific pilot report regarding turbulence to the meteorological service, which could have influenced the availability of updated weather warnings.
- The airline's flight dispatch system lacked the robustness required to better mitigate the risks of such severe weather encounters.
- There was a lack of clarity in the communication of SIGMETs (significant meteorological warnings) regarding specific geographic regions.
Safety action
The Commission issued a recommendation to the Director of Civil Aviation to improve the clarity of SIGMETs by using more recognizable geographic references to assist pilots in interpreting hazardous weather warnings.