Australia

No aircraft defects were found which may have been factors in the accident. The investigation indicates that the pilot attempted to fly under the low cloud base, in order to reach the Port Welshpool destination where weather conditions were earlier reported to have been partially sunny. Port Welshpool is not serviced by an approved navigation aid. The pilot attempted to descend below the cloud base, hoping to achieve visual flight conditions to continue to his destination.

Two main hypothesis were proposed. One was that the pilot was distracted from a less than demanding task by business worries. The other was that the pilot had perceived that the SWER line was on the right of the windmill and outside the fence and therefore did not present an obstacle to his procedure turn. Neither hypothesis could be substantiated. An additional concern was the fact that the pilot had died from impact injuries in an accident that, prima facie, was survivable. Concern focused on whether the pilot had secured his harness properly and/or whether the inertia reel had failed. Detailed engineering inspection of the inertia reel by the Bureau and the manufacturer could not positively determine the mode of operation of the inertia reel. However, the post mortem report showed that the nature of injuries to the pilot, while sufficient to cause death prior to the fire, were such as to indicate that the inertia reel had probably not failed. The pilot did not see the powerline in time to avoid a collision.

The landing gear problem arose when the left main landing gear would not lower. Examination of the aircraft revealed that both hinges fitted to the inboard landing gear door had fractured. The forward hinge had fractured as a result of fatigue and the rear hinge as a result of overload. The fatigue crack initiation had occurred at a sharp edged, prominent forging flash on the inner radius of the hinge and had grown over approximately 4000 load cycles. A similar fatigue problem had been identified on an earlier version of the hinge (part number 46653-00), however, regular inspections for fatigue cracking were discontinued when hinges with part number 47529-32 (as fitted to VH-DEG) were introduced in 1980. Similar fatigue cracking was found in the forward door hinge of another PA31 during the investigation. The fractured hinges jammed the left main landing gear mechanism and neither the normal or emergency extension systems could extend the gear. The pilot was apprehensive about wheels up landings. Much of his decision making was aimed at reducing the risk of fire and minimising the damage the aircraft would sustain during the landing. eg. Selection of a dirt runway instead of the sealed strip, landing with flaps retracted etc. During the pilot's attempts to rectify the landing gear problem, and up until the time of his touchdown, he was subjected to considerable radio transmission traffic involving questions, directions and suggestions which distracted him from his primary tasks. The pilot indicated on at least two occasions that he was ready to land, however, each time advice and questions from the ground personnel involved overrode his intentions. When the pilot was asked if he wanted a flare path on runway 27 there was still some natural light available and he was intending to land. However, by the time he was able to make his final approach it was dark and he was unable to see the ground. Studies have shown that aircrew subjected to high levels of stress can suffer skill fatigue and cognitive task saturation, which in turn can lead to a breakdown in the decision making process. It was apparent from the pilot's radio transmissions and the quality of the decisions made in the latter part of the flight that his information processing and decision making abilities had been degraded by the stress of continuous radio transmissions and continuous, and sometimes conflicting, instructions. As a result, what should have been a relatively simple wheels up landing in daylight was turned into an extremely difficult wheels up landing at night. With the landing gear retracted the aircraft's taxi and landing lights were not available to the pilot.
The following factors were considered relevant to the development of the accident:
1. Manufacturing defect. A forging flash created a stress concentration which led to fatigue cracking.
2. Inadequate inspection procedures. Previous inspection procedures introduced to disclose similar cracking were withdrawn on the introduction of later part numbered hinges.
3. Apprehension of the pilot. The pilot was apprehensive about apparently significant dangers of landing an aircraft, wheels up, on a sealed runway.
4. Inordinate interference in aircraft operations by ground based advisors. The ground advisors input overrode the pilot's decision on a number of occasions with the result that a simple exercise became very complicated.
5. Cognitive task saturation and skill fatigue. The amount of information, advice and suggestions being passed via the radio communications system overloaded the pilot decision making abilities.
6. Improper in-flight decisions. As a result of task saturation the final decision made by the pilot to attempt a night landing on an unlighted strip was incorrect.
7. The pilot did not see and therefore was unable to avoid the levy bank.

The exact cause of the accident could not be determined.

The following factors were considered relevant to the development of the accident:
1. The pilot selected the incorrect takeoff direction for the wind conditions prevailing.
2. The pilot continued the takeoff into an area of unsuitable swell. This accident was not the subject of an on-scene investigation.

The pilot declined to provide any information which might have clarified the circumstances of the accident, however, available information indicates that he carried out an unauthorized low pass over the boat. During the turn following the low pass, he misjudged the aircraft's height and the right wing tip struck the water.
The following factors were considered relevant to the development of the accident:
- The pilot was neither trained nor authorized to conduct operations at low level,
- The pilot exercised poor judgement by operating at an unnecessarily low height,
- The pilot misjudged his height above the water.

The reason why the aircraft flew into the sea could not be determined.

It is probable that the pilot did not have an adequate understanding of the operations of the MU-2B-60 aircraft at high altitude. The meteorological conditions were conducive to the formation of ice on aircraft flying in cloud above the freezing level. It is probable that loss of control occurred above the freezing level on climb to an amended altitude of FL 210.

No fault was found with the aircraft or its systems which might have contributed to the accident. It could not be determined who was manipulating the controls of the aircraft at the time of the accident. Evidence was obtained that it was the habit of the check pilot to have pilots undergoing endorsement or check to fly two circuits landing into wind and then to carry out crosswind landings. The check pilot and the pilot under check had previously operated at the dam and alighted on to both the east/west and the north/south channels. Having been observed to fly two into wind circuits and then head north and not be sighted again, it is possible that the aircraft then commenced crosswind operations onto the north/south arm of the lake, landing in a southerly direction with a crosswind from the left. Information from the Bureau of Meteorology indicated that the surface wind in the area at the time of the accident was 090` magnetic at 15 knots. This information was confirmed by witnesses at the dam wall who observed white caps on the surface of the dam. The north/south channel of the lake was bounded on its east side by steep hills rising to 70 metres above water level. The effect of this high ground was to partially blanket the north/south channel from the easterly wind. The position of the wreckage was in the area where the wind shadow effect would have ended and where the wind would have blown at full strength along the main east/west channel of the lake. The crosswind limitation for the aircraft as stated in the flight manual was 8.7 knots. Commenting in early 1988 on an enquiry regarding the raising of this limit, the aircraft manufacturer emphasised the 8.7 knot limit and advised that any test work to raise the limit should proceed cautiously starting at or below the current (8.7 knot) limit. If the aircraft was conducting crosswind operations in the north/south channel, and suddenly encountered a 15 knot crosswind on exiting the wind shadow area, the control difficulties confronting the pilot could have been significant. The aircraft wreckage was intact except for the floats which had been torn off by water impact forces. The right float was severely damaged while the left was intact. The forward tip of the right float had been severed by the propeller. The remaining forward section had then been forced upwards and outboard and had broken off. This weakened the float support structure, causing it to fail, and allowing the remaining section of the right float to strike the right side of the fuselage just aft of the cabin. Damage of this type an magnitude was most probably caused by the nose of the right float digging into the surface of the lake at relatively high speed. For this to occur, the aircraft was banked to the right at float impact - a possible consequence of encountering a strong crosswind from the left. There was no evidence that the aircraft had hit a submerged object. The factors associated with the development of this accident could not be determined.

The subsequent investigation established that the flight crew were properly qualified to conduct the flight, and that the aircraft was appropriately certified and maintained. The flight was conducted in accordance with the conditions of the operating contract. At the time of the occurrence the aircraft had not deployed the sleeve target and no firing was being carried out. No evidence was found to suggest an in-flight structural failure or fire. The installation of the target towing equipment was not considered to have been a factor in the development of the accident. There was a loss of control of the aircraft following an apparent engine malfunction. The precise reasons for the accident have not been established.
The following factors were considered relevant to the development of the accident:
1. Apparent engine failure or malfunction.
2. Control of the aircraft was lost for reasons which have not been determined.