Background
The flight, designated Post 23, departed from runway 23R at approximately 2136. The first officer was the pilot flying, with the captain supervising. The planned cruise level was FL180, but due to turbulence, the crew requested and received clearance to climb to FL220. The autopilot was engaged for climb and cruise. The crew used the de-icing system to remove trace or light icing from the wings.
Flight Sequence
At 2159, air traffic control cleared Post 23 to track from near New Plymouth VOR direct to Tory VOR. At 2206, control was transferred to Christchurch Control. Climb power remained set for about 15 minutes after reaching cruise level to make up for departure delay. At about 2212:28, after reducing power to cruise setting and completing cruise checks, the captain instructed the first officer to open the fuel crossflow valve and trim the aircraft with left rudder pedal input.
The captain repeated the instruction five times over 19 seconds, telling the first officer to step on the left pedal and trim to take pressure off, and to get the ball out to the right as far as possible. The first officer sought confirmation, saying he was being cautious; the captain replied, "Don't be cautious mate, it'll do it good." Nine seconds later, the first officer asked "How's that?" and the captain responded positively.
Accident Sequence
While the fuel transfer was being attempted, the aural alert of automatic horizontal stabiliser movement sounded for 27 seconds as the aircraft slowed. Forty-seven seconds after opening the crossflow, the captain said, "Doesn't like that one mate… you'd better grab it." Within one second, the aural alert "Bank angle" sounded, followed by a chime tone (likely altitude deviation warning). Both pilots expressed surprise. After 23 seconds, the captain asked if the autopilot was off; the first officer confirmed it was off just before the recording ended at 2213:41. The bank angle alert was heard seven times total.
During the last 25 seconds of radar data, Post 23 lost 2000 feet of altitude and the track turned left more than 180°. Radar data ceased at 2213:45 when the aircraft was descending through FL199, about 1700 meters southeast of the accident area. Air traffic control attempted contact three times without response and initiated search and rescue.
Witness Observations
Multiple witnesses reported a very loud, unusual noise described as "high-revving" or "roaring." Witness A, about 3 km south of the radar track, saw an orange-yellow light descending through broken clouds at high speed, followed by a big burst and three or four separate fireballs. No explosion was heard. Witness B, nearly 7 km northeast, observed a fireball below cloud at about 6° elevation, described as a "big bright circle" followed by two smaller fireballs. Witness C, less than 1000 m from the diving flight path, saw the nose section falling after an explosion, then wings falling after a smaller fireball. Witness D also saw parts falling illuminated by fireballs. All witnesses agreed the first fireball was round and orange, then shrank. Smaller fireballs had more persistent, streaming flame falling steeply.
Wreckage
Emergency services and onlookers converged on the area; those within 1000 m reported a strong fuel smell. The first wreckage was located at about 2315. The main wreckage field was on hilly farmland 7 km northeast of Stratford at an elevation of approximately 700 ft.
Probable Cause
Official findings: The flight crew was appropriately licensed and qualified. The aircraft had a valid Certificate of Airworthiness and was maintained properly. Some fuel imbalance led the captain to decide on fuel balancing during cruise. The captain's instructions resulted in a large sideslip angle while the autopilot was engaged. The first officer's reluctance to challenge may have been due to inexperience. The autopilot probably disengaged automatically because a servo reached its torque limit, causing abrupt roll and dive. The crew was unable to recover from the ensuing spiral dive before airspeed and g limits were grossly exceeded, resulting in structural failure and in-flight break-up of the aircraft.