Private Australian

The extensive fire damage hampered the investigation of the accident. The surviving passenger believed that the aircraft was on fire before the collision with the tree. No other evidence of an in-flight fire could be obtained, and it was considered possible that the survivor's recall of the accident sequence had been affected by the impact and the fire. Such discrepancies in recall are not uncommon among accident survivors. The elevator trim control jack was found to be in the full nose-down position, but it was not possible to establish whether the trim was in this position prior to impact. Such a pre-impact position could indicate either a runaway electric trim situation or that, in his hurry to depart, the pilot had not correctly set the trim for takeoff. The aircraft was known to have had an intermittent fault in the engine fire warning system. The fault apparently caused the fire warning light to illuminate, and the fire bell to sound, usually just after the aircraft became airborne. The pilot was aware of this fault. It was considered possible that, if the fault occurred on this occasion as the aircraft entered the fog shortly after liftoff, the pilot's attention may have been focussed temporarily on the task of cancelling the warnings. During this time he would not have been monitoring the primary flight attitude indicator, and would have had no external visual references. It was also possible that, if for some reason the pilot was not monitoring his flight instruments as the aircraft entered the fog, he suffered a form of spatial disorientation known as the somatogravic illusion. This illusion has been identified as a major factor in many similar accidents following night takeoffs. As an aircraft accelerates, the combination of the forces of acceleration and gravity induce a sensation that the aircraft is pitching nose-up. The typical reaction of the pilot is to counter this apparent pitch by gently applying forward elevator control, which can result in the aircraft descending into the ground. In this particular case, the pilot would probably have been more susceptible to disorientating effects, because he was suffering from a bronchial or influenzal infection. Although all of the above were possible explanations for the accident, there was insufficient evidence available to form a firm conclusion. The precise cause of the accident remains undetermined.
It is considered that some of the following factors may have been relevant to the development of the accident
1. The pilot was making a hurried DEPARTURE. It is possible that he did not correctly set the elevator trim and/or the engines may not have reached normal operating temperatures before the takeoff was commenced.
2. Shortly after liftoff the aircraft entered a fog bank, which would have deprived the pilot of external visual references.
3. The aircraft had a defective engine fire warning system. Had the system activated it may have distracted the pilot at a critical stage of flight.
4. The aircraft might have suffered an electric elevator trim malfunction, or an internal fire, leading to loss of control of the aircraft.
5. The pilot may have experienced the somatogravic illusion and inadvertently flown the aircraft into the ground. The chances of such an illusion occurring would have been increased because the pilot was evidently suffering from an infection.

Inspection of the wreckage indicated that the aircraft struck the the top of a ridge line, 250 metres south-west of the highest point of the Mt Williams area. At the time, the aircraft was on a west-north-westerly heading, flying wings level and climbing at a angle of about five degrees. No fault was found with the aircraft that could have contributed to the occurrence. At the time the aircraft entered cloud, the pilot should have reverted to Instrument Flight Rules procedures. To comply with these procedures a pilot is required, inter alia, to ensure that adequate terrain clearance is achieved during climb to the lowest safe altitude. The relevant altitude for the route segment Cairns to Biboohra is 4500 feet above mean sea level (amsl). As the aircraft was apparently under control at the time of impact with the ground at about 3250 feet amsl, it was likely that the pilot had overlooked the lowest safe altitude requirements.

Although wind conditions were light and variable when the engines were started, shortly after the accident the wind was moderate from the west/south-west. A detailed analysis conducted by the Bureau of Meteorology indicated that while the pilot was preparing for take-off, a cold front with winds in excess of 20 knots had probably passed over the aerodrome. As the pilot had conducted the take-off on runway 08, there was probably a substantial tailwind component. Conditions were also assessed as suitable for the development of microbursts, but the lack of recording instruments in the area prevented confirmation that this type of phenomenon had in fact occurred. The pilot had been deprived of the opportunity to observe changing wind conditions at the aerodrome. The wind direction indicator adjacent to the threshold of runway 08 was not lit, and the illuminated wind direction indicator was not visible from the point where the aircraft was lined up for take-off.

The subsequent investigation did not reveal any defect or malfunction which might have affected the operation of the aircraft. The pilot was conducting an operation known as a "bank run", and there is pressure on pilots performing such runs to adhere to the prescribed schedules. The pilot's decision to perform a low level circuit and land downwind was considered to be related to his desire to arrive at the terminal as close as possible to the scheduled time. While conducting the circuit, the aircraft stalled during a turn at a height which was too low to allow the pilot to recover control before impact with the ground.

Failure of the left engine in flight due to defective manufacture of camshaft. The following contributing factors were reported:
- Fatigue failure of camshaft,
- Complete loss of power from left engine,
- Aircraft not reconfigured for optimum single engine performance,
- Airspeed fell below minimum for effective control,
- Insufficient height for recovery.

The engine had failed from fuel exhaustion. The pilot was not aware of the amount of fuel added to the tanks at the previous refuelling and he had subsequently relied on the fuel gauge readings to assess the remaining endurance.

Investigation did not reveal any defect or malfunction of the aircraft which might have contributed to the development of the accident. Both engines were operating at high power settings and the gear and flaps were up. The aircraft had been operating under the Instrument Flight Rules when the pilot reported his intention to descend. Conditions at the destination were suitable for visual flight, and the reason the pilot elected to proceed at a low height above the ground was not determined. It was likely that while cruising below the cloud, the pilot was suddenly confronted by localised adverse weather conditions in the vicinity of the accident site. The maintenance of control of the aircraft under these conditions should have presented little problem to the pilot, who was suitably qualified to operate in instrument conditions. In these circumstances, the precise sequence of events leading to the evident loss of control of the aircraft could not be established.

Due to lack of evidences, the exact cause of the accident could not be determined.