What happened
On the night of 3 May 2005, a Fairchild-Swearingen SA227-AC Metro III, registration ZK-POA, was operating a scheduled night freight service from Auckland to Blenheim. The aircraft, operated by Airwork (NZ) Limited, was carrying approximately 1790 kilograms of cargo.
During the cruise phase of the flight, the crew decided to perform in-flight fuel balancing by opening the fuel crossflow valve to redistribute weight between the wing tanks. To manage the resulting imbalance, the captain instructed the first officer to use rudder trim to maintain flight attitude while the autopilot remained engaged. This maneuver placed the aircraft in a significant sideslip. Shortly after the procedure began, the autopilot disengaged, likely because a servo reached its torque limit. This triggered an immediate bank angle and a sudden roll into a spiral dive. The crew was unable to recover the aircraft from the descent, and the resulting aerodynamic loads caused the aircraft to suffer structural failure and break up over rural farmland near Stratford. Both crew members were killed in the accident.
The investigation
The investigation examined the flight data recorder, cockpit voice recorder, and the wreckage of the aircraft. Investigators analyzed the sequence of events leading to the loss of control, specifically focusing on the interaction between the fuel balancing procedure and the aircraft's automated flight systems. The examination of the wreckage confirmed that the aircraft's disintegration was caused by extreme overstressing during the dive. The investigation also reviewed the crew's training, the operator's standard operating procedures, and the limitations provided in the aircraft flight manual.
Findings
- The crew attempted to balance fuel between the wing tanks while in level flight using the crossflow valve.
- The aircraft was being flown at a large sideslip angle through the use of rudder trim while the autopilot was active.
- The autopilot disengaged because a servo reached its torque limit, which precipitated the sudden roll and dive.
- The crew was unable to arrest the spiral dive, leading to the aircraft exceeding its structural load limits.
- The first officer's reluctance to challenge the captain's instructions may have been influenced by his relative inexperience and developing crew resource management skills.
- The aircraft flight manual lacked a specific warning or limitation regarding the necessity of disconnecting the autopilot during in-flight fuel balancing.
Safety action
Following the accident, the operator implemented several changes, including a new policy requiring fuel to be balanced prior to engine start to avoid the need for in-flight transfers. The operator also updated its training manuals to mandate that the autopilot and yaw damper must be disconnected before using the fuel crossflow switch. Additionally, a safety recommendation was made to the Director of Civil Aviation to ensure that aircraft flight manuals for the Metro family include explicit instructions and warnings regarding autopilot use during fuel balancing operations.