On 3 May 2005, Fairchild-Swearingen SA227 (Metro III), being operated by Airwork (NZ) Limited, was on a night air transport freight flight when there was a loss of control which developed into a spiral dive. The crew did not recover control and the aircraft became overstressed which resulted in an in-flight break up and terrain impact, killing both crewmembers.
The following is an extract from the official accident Report published by Transport Accident Investigation Commission (TAIC), New Zealand:
"On the night of Tuesday 3 May 2005, Fairchild-Swearingen SA227-AC Metro III aeroplane ZK-POA, operated by Airwork (NZ) Limited, was on a night air transport freight flight with 2 crew and 1790 kilograms of cargo when it suffered an in-flight upset which developed into a spiral dive. The crew did not recover control and the aircraft became overstressed and broke up, to fall in pieces about rural farmland near Stratford. Both crew were killed and the aircraft and cargo destroyed. The crew was balancing fuel between tanks, flying the aircraft at an excessive sideslip angle with the rudder input trimmed, while on autopilot. The autopilot capability was exceeded and it disengaged, precipitating the upset."
The Report explains how the crew got into this situation:
The flight was scheduled for 2100 and to compensate for accumulated delay“…the crew ordered 570 litres (about 1000 pounds (lb) or 450 kg) of additional fuel and instructed the refueller to put it all into the left wing tank, rather than put half of the ordered amount into each tank, as was company practice. Refuelling was completed at 2130, with 2100 lb (950 kg) of fuel on board. The company Pre-Start checklist required that the fuel tanks be balanced within 200 lb (90 kg) before starting engines, and for take-off and landing.”
The report further uncovers:
“The dark night conditions and probable cloud cover below would have prevented the crew seeing any external visual cues such as ground lights or terrain features to assist in orientation, or in early perception of the aircraft’s departure from its normal attitude. There may have been a few stars visible. The crew’s principal attitude reference would have been the flight instruments, and close attention to these would have been less likely while flying on autopilot than when flying manually.
The necessary intervention did not occur in time to prevent the aircraft wing structure failing in overload from excess g force and airspeed. […] Opportunity for the crew to make a safe recovery from the spiral dive probably ended as the airspeed rose significantly above VMO [maximum operating airspeed]. This occurred quickly, about 12 seconds after the captain’s instruction to the FO [first officer] to take over manual control. By that stage it was unlikely, even if they had then applied optimum control inputs, including reducing power and using right rudder pedal pressure to remove the left rudder input from the left rudder trim, that a normal attitude could have been regained without seriously overstressing the aircraft structure.”
The following is an extract from the Report's Fingings (for the complete list see Further Reading):
- The captain’s instructions to the FO while carrying out fuel balancing resulted in the aircraft being flown at a large sideslip angle by the use of the rudder trim control while the autopilot was engaged.
- The FO’s reluctance to challenge the captain’s instruction may have been due to his inexperience and underdeveloped CRM skills.
- The autopilot probably disengaged automatically because a servo reached its torque limit, allowing the aircraft to roll and dive abruptly as a result of the applied rudder trim.
- The applied rudder trim probably contributed to the crew’s inability to recover control of the aircraft.
- The crew’s other control inputs to recover from the spiral dive were not optimal, and contributed to the structural failure of the aircraft.
- The flying conditions of a dark night with cloud cover below probably hindered the crew’s early perception of the developing upset.
- The AFM [Aircraft Flight Manual] limitation on use of the autopilot above 20 000 ft should have led to the crew disconnecting it when climbing the aircraft above that altitude.
- If the aircraft had been manually flown during the fuel-balancing manoeuvre, the upset would probably have not occurred.”
Following the accident, the operator initiated safety actions including
- Notice to Pilots advising that forthwith the SOP was to give the refueller the volume of fuel to be put into each wing tank to achieve a balanced load prior to engine start, in accordance with the Pre-Start checklist, Metro Training Manual and AFM. The Notice emphasised that it was not acceptable to put the required fuel uplift into one tank only with the intention of balancing the fuel later. The importance of not compromising safety by rushing checks was also mentioned in the Notice.
- Operator amended the Metro checklist to add to the Line-up and Approach checklists the item “crossflow closed”.
- Operator amended the autopilot Standard Operating Procedures section of the company Metro Training Manual
The Report's recommendations beginning on page 41 focus on institutional issues (see Further Reading).
For further information see the full accident report published by TAIC New Zealand.