Wellington

The following causal factors were reported:
- The decision by the pilot to descend beneath the cloud layer.
- Illusions, created by the conditions, producing misleading visual references.
- The pilot's misidentification of distant terrain features.
- An undetected heading error, and loss of "positional awareness".
- Insufficient forward visibility, during the later stages of the descent.
- The high speed of the aircraft.
- The decision making by the pilot: the delayed decision to descend beneath the cloud layer; continued flight into deteriorating conditions.
The following contributing factors were reported:
- The pilot's familiarity with the area, creating a false sense of security.
- Improved general weather conditions, causing the pilot to relax.
- A possible false horizon, created by a sloping cloud base.
- The absence of documented procedures to ensure pilots crossed the coast south of Rarangi.

The failure of the left main undercarriage to fully extend was caused by the seizure of the piston rod within the undercarriage jack assembly. The piston rod and the spring housing through which it passed had become scored, eventually resulting in the seizure. The piston rod had been installed, new, in the jack assembly some seven months earlier and had completed 787 landings. Further investigation found that the synthetic wiper ring had been installed incorrectly and would allow foreign material to find its way into the spring housing when the undercarriage was retracted. Additionally, hydraulic components on the undercarriage may have become contaminated during cleaning in an inadequately filtered cleaning booth, which was also used for general cleaning involving the use of abrasive pads.

The following findings were reported:
- Icing conditions,
- The aircraft was not equipped with deicing systems,
- The pilot was tired due to a duty period more than 18 hours,
- The pilot loaded and unloaded more than 10 tons of cargo during the day,
- The pilot was soaked by rain while doing so because no foul weather gear was provided,
- The pilot did not have sufficient rest time,
- The pilot did not have a proper brake time and meal,
- The pilot was not properly trained concerning indoctrination course and suffered hypoxia in flight.

Due to lack of evidences, the exact cause of the accident could not be determined. However, the following causes have not been ruled out: a loss of control after the pilot suffered a spatial disorientation while trying to deal with an emergency situation in limited visibility (clouds), a loss of control because of airframe ice build-up, a structural failure with a severe fire in the engine bay, an explosion in hazardous cargo, a failure of the flying control system or a possible criminal mischief.

The cause of the accident was the detachment of the starboard mainplane in flight. A contributory cause was the decision of the pilot to fly close to the summit of the mountain in an aircraft in which, unknown to him, the starboard wing structure had been appreciably weakened by a combination of spar cap fractures and fatigue cracking derived from a past incident. Severe turbulence or some pilot manoeuvre caused the starboard propeller to strike a part of the mountain and the resultant vibrational loads, together with the effects of violent turbulence encountered thereafter, imposed stresses which the weakened wing structure was incapable of withstanding.

Deviation from the prescribed flight path for unknown reasons. Low visibility due to bad weather conditions was considered as a contributory factor.

A Board of Inquiry set up to investigate the accident found William Smillie guilty of 'culpable carelessness and neglect' and recommended to the Minister in Charge of the Air Dept that his 'A' licence be suspended for 3 months. The Board also disallowed the witness travelling expenses due to Mr. Smillie for attending the Inquiry and ordered that he pay the sum of 10 Pounds towards the Board's costs and expenses.