The pilot had been operating the aircraft on gypsum spreading operations in mountainous terrain during the morning. The task involved flying along a valley, crossing a ridge at the head of the valley and spreading the gypsum on the western slopes of the ridge. Shortly after the aircraft departed on the accident flight, smoke was observed rising from the valley. It was discovered that the aircraft had impacted the side of the valley with a high rate of descent and very low forward speed. The hopper load had not been jettisoned prior to impact and the hopper doors were closed. The aircraft was destroyed by impact forces and the subsequent fire. The investigation did not reveal the presence of any pre-existing mechanical defect which might have contributed to the accident. The engine was operating at high power at the time of impact. Because of the damage to the aircraft, the hopper and fuel loads were not able to be precisely determined. Evidence suggested that the aircraft was probably heavier than the maximum all up weight approved for agricultural operations. On previous flights the aircraft had been observed to orbit over the valley in order to gain sufficient height to safely clear the ridge. It is considered possible that on this occasion the pilot commenced the orbit from a position and height which did not allow sufficient room for the manoeuvre to be completed at moderate bank angles. It was evident that during the orbit the aircraft stalled and entered an incipient spin. Insufficient height remained to permit recovery before impact with the ground. The weather conditions at the time were conducive to the formation of downdrafts, but whether this phenomenon had any bearing on the loss of control leading to the accident was not established. In addition, the reason that the hopper load was not jettisoned could not be determined.

The pilot had not previously landed at the particular area, but had carried out a detailed inspection to ensure no debris was present in the water, which was about one metre in depth. Almost immediately after touchdown the nose yawed some 20 degrees to the left and the aircraft pitched forward and overturned. A subsequent inspection revealed considerable damage to the hull below the cabin floor. The investigation discovered that the right hand nosegear door had previously been damaged. The repairs carried out had not corrected weaknesses in the door resulting from this damage. The area selected for landing on this occasion was too short for normal operations, and discolouring of the water prevented the pilot from detecting any obstructions below the surface. The aircraft adopted a nose-low attitude shortly after touchdown, but whether this was pilot induced or the result of striking a sankbank was not established. The resulting pressure of water on the nose area distorted the already weakened gear door and forced the left door sideways into about the normal open position. During the subsequent yaw and overturn, the right wing and the nose of the aircraft struck the bottom of the river.

The pilot had flown the aircraft, without incident, a number of times prior to the accident flight. He made a full flap, normal approach to land after checking that the landing gear was down and locked. The wind at the time was a light crosswind from the left. Touchdown was normal, but immediately afterwards the right gear leg collapsed. The aircraft veered sharply to the right and the other gear legs failed. The aircraft slid to a halt on its lower fuselage. The strip had a transverse gradient of three percent down to the right. Witness evidence and ground markings confirmed that the touchdown was normal. Inspection of the damaged components of the right gear indicated that the initial failure had been from overload forces. No significant pre-existing damage was evident.

The pilot was engaged in the chemical spraying of several properties in the general vicinity of his own property near Gatton. After completing the spraying of two properties, the pilot began positioning the aircraft in order to spray his own crop. A witness observed the aircraft as it turned to the right and flew at low level beneath power lines across a highway. The aircraft was then seen to bank slightly to the right and strike another set of power lines on the opposite side of the highway. Two of the wires were broken and the third remained entangled with the aircraft. The aircraft struck the ground in an inverted attitude. The power lines struck by the aircraft had been erected about three weeks before the accident. They had been erected at the request of the pilot to service a portion of his property. He had flown under the wires on several occasions in the preceding three weeks.

The aircraft is not frequently used except for short, concentrated periods of activity. At other times it is left parked at Coolangatta Aerodrome and is subject to the harsh coastal environment. During an eight day business trip around Queensland in the aircraft, some gear problems were encountered. At Mackay, several days before the accident, the pilot discovered that a left main gear door linkage had sheared because lack of lubrication had seized the ball joint. Upon inspection, the right gear door linkage was similarly seized. The damaged items were serviced by a licenced aircraft maintenance engineer before the flight continued to Hamilton Island. Later, departing Hamilton Island, the pilot noticed that the GEAR UNSAFE light remained illuminated after gear retraction. Recycling the gear gave a correct gear down indication each time but the GEAR UNSAFE light remained lit after each retraction. A visual inspection by the Tower Controller convinced the pilot that the gear was indeed stowed and he assumed that the problem was of an electrical nature. The unsafe indication continued for the duration of the round trip. When the pilot selected gear down upon his arrival at Archerfield, the right maingear remained in the wheelwell. Attempts were made to shake the gear down but to no avail. With last light approaching, the pilot decided to commence an approach for a wheels-up landing on Runway 04R. During late final approach the pilot shut the engine down. The resultant high rate of descent surprised the pilot who realised that the aircraft would now land short of the threshold. The aircraft landed heavily on cross runway and slid onto the threshold. Investigation has revealed that the right maingear torque link bolt sheared, causing the oleo to over-extend and jam the wheel in the wheelwell. The failed bolt and bushes had corroded and seized in the torque links causing the bolt to shear with movement of the torque links. The torque link bolts are not normally removed and lubricated during routine servicing.

The pilot was conducting aerial spraying on a banana plantation in hilly country. While flying a clean-up run along one side of a section of the plantation, the aircraft struck a set of powerlines which ran diagonally across its flight path. The wires became entangled in the main landing gear and were dragged 160 metres before the aircraft impacted the ground and caught fire. The cockpit area was burnt out and the engine compartment severely damaged by the fire. The engine and remaining aircraft structure and components were examined and, not withstanding the severe heat damage to some engine components/accessories, no abnormality was found. The pilot, who suffered serious burns, told a friend a short time after the accident that he was distracted by a sprinkler pole and had forgotten about the wires. In hospital two days later, the pilot reportedly spoke of experiencing elevator control problems shortly before hitting the wires. When he was able to be interviewed, some seven weeks after the accident, the pilot maintained that he felt the engine lose power as he was aligning the aircraft for the run and that the aircraft descended into the wires as a result. The nature and extent of the pilot's injuries indicated that trauma induced amnesia was unlikely to have caused his conflicting accounts of the accident. There are, however, unconscious pyschological mechanisms which can explain these conflictions. The banana plants were served by a large number of sprinkler poles of narrow gauge water pipes which protruded above the plant tops. Seeing these would have required considerable attention from the pilot. The power poles supporting the wires the aircraft struck were well outside the pilot's forward field of view and the wires themselves would have been difficult to see against the background of dark vegetation. However, the weight of human behavioural evidence leads to the conclusion that the pilot was probably distracted and forgot about the wires. The pilot reported that he was unable to open the left cockpit door by the normal or emergency method and had to egress through the right door where the fire was more intense. Both cockpit doors were open when the wreckage was examined. No reason was found which would have prevented the emergency exit from operating normally.

The glider was undergoing a test flight following an annual inspection. After being released the aircraft was climbed to 4300 feet prior to the commencement of a high speed descent. At an indicated airspeed of 110 knots the pilot reported that there was a slight buffet from the tailplane area. The descent was continued and at an indicated airspeed between 118 and 122 knots severe vibrations from the tailplane area were encountered. The airspeed was reduced and after completing a control check the aircraft was landed. Subsequent inspection revealed damage to the rear fuselage near the fin root. This accident was investigated by the Gliding Federation of Australia and was not the subject of an on-site investigation by the Bureau. The Federation's investigation indicated that the amount of freeplay in the horizontal tailplane of the aircraft was just within the limits specified in the manufacturer's manual the pilot was inexperienced for the task he may have actually reached VNE and his reaction to the vibration may have overstressed the aircraft.

Circumstances:

The accident occurred on the first gliding flight of the day. The air temperature was approximately two degrees Celsius, and a shallow low-level layer of moist air was present. Shortly after take-off on a dual tow, and at approximately 300 ft above the ground, what appeared to be ice formed on the outside of the canopies of both gliders causing a complete loss of forward vision. Although the pilots were able to see the ground through their clear view panels (left hand side) visibility was severely restricted. The pilots of both gliders released their tow ropes (short tow first) and attempted emergency landings. The long tow glider turned left through 90`, sighted a suitable paddock, turned left through 270` and completed a successful landing. The short tow glider (VH-IXL) turned left through 240`, sighted the airfield and had commenced a right turn to line up for a landing when it struck the top branches of a tree, crashing near the base of the tree. The Bureau of Meteorology reported that the most likely cause of the loss of visibility was the rapid deposition of hoar frost on the canopy after the cold aircraft (having radiated energy all of the previous night) ascended from a 200 to 300 ft thick, cold but relatively dry band of air into an inversion layer of warm and relatively moist air. In the Bureau's opinion, it would be virtually impossible to routinely give prior warnings of such an unusual phenomenon, or for aircrew to be able to determine the state of the atmosphere from a purely visual observation. It was considered that the time of the occurrence was significant. The two layers would have mixed together as the sun continued to rise thus diminishing the potential for the occurrence. The air temperature had risen to six degrees Celsius within 75 min of the accident. The pilots were aware of the standard procedure to attempt to land straight ahead following a low-level tow release. However, this procedure assumes forward visibility, and, on this occasion, visibility was severely restricted. Both pilots were forced to manoeuvre in an attempt to sight a clear landing area through their clear vision panels. The procedure worked for one aircraft, and it almost worked for the second. Consideration was given, by at least one of the pilots, to jettisoning the canopy however, this was not done because of the possibility of tail damage and complete loss of control.

Significant Factors:

The following factors were considered relevant to the development of the accident

1. There were unusual unobserved meteorological conditions which created a situation where frost could form suddenly on the aircraft's windscreen.

2. Inflight visibility was suddenly restricted due to frost deposits.

3. The pilot was unable to see a large tree to the right and below the aircraft during his final turn towards the landing area.

4. The aircraft became uncontrollable following the collision with the tree.

Recommendations:

The Gliding Federation of Australia (GFA) recommended three specific procedures for operations at the Stirling Range strip.

1. That early morning launches be delayed until the rising sun has caused some mixing of the air, possibly as late as 0800 hours.

2. If ice is present on the parked gliders, it will be allowed to melt off rather than be scraped off thus allowing the airframe to warm up before the first DEPARTURE.

3. The tug check flight will occur as near as practical to the first glider launch, will follow the same flight path as is intended for the gliders and will climb to at least 800 ft before returning. On its return the tug will be checked for signs of leading edge ice. If ice is present launching will be delayed until further warming has occurred. A further GFA recommendation related to the jettisoning of the canopy when complete icing occurs. BASI recommends that the GFA consult with the particular aircraft manufacturer before this course of action.

VH-EBQ departed Cairns for Narita at 1202 local time. At 1221, a 'PAN' call (urgency message) was made, and the pilot requested an Air Traffic Control (ATC) clearance to return to Cairns. The captain stated that as the aircraft was climbing through Flight Level (FL) 200, moderate airframe vibration was felt. Checks revealed no abnormal cockpit indications. The second officer carried out a visual inspection from the cabin and noticed a damaged wing panel on the upper inboard trailing edge of the right-hand wing.

A return to Cairns was initiated and a clearance to dump fuel was obtained. Cairns ATC advised the aircraft that they had diverted an Australian Airlines Boeing 737 (B737) to escort the aircraft during the return to Cairns. A fuel dump of approximately 25,000 kg was carried out, followed by a descent and circling approach to runway 33. The captain stated that with the flaps extended, there was a slight tendency for the aircraft to roll to the right. The aircraft was landed at 1259 without further incident.

The escorting B737 landed approximately 5 min ahead of VH-EBQ. A readout of the digital flight data recorder was made to determine the data recorded on climb and during the subsequent landing at Cairns. Examination of this data showed that acceleration traces had minor inconsistencies or scatter as the aircraft was climbing through a recorded pressure altitude of FL 205 from 1214 07 to 1214 13. It is possible that the wing panel failure occurred at this time. No other anomalies were observed in the recorded data to indicate that the loss of a wing panel had occurred.

Data recorded during the landing at Cairns was consistent with a crosswind landing and no evidence of airframe vibration or other anomalies was observed in the recorded data. The failure of the right-hand inboard trailing edge fibre reinforced plastic panel was accompanied by the loss of a 2.7 m by 18 cm section of the fore flap trailing edge, and some damage to the mid-flap. The operator had previously had some 20 incidents involving this panel on Boeing 747 aircraft.

On 19 January 1991, the same panel failed on VH-EBP in flight between Sydney and Narita. The manufacturer indicated that there had been several incidents in the previous 18 months resulting from cracking, delamination and separation of this panel.

The possible related causes for failure of the panel were

(a) Damage caused by personnel stepping on the panel.

(b) Damage caused by tyre burst.

(c) Manufacturing anomalies.

(d) Misrigging. (The panel is an aerodynamic surface and is fixed in position with screw jacks to preload it with flaps retracted.)

The panel is subject to a coin 'tap test' to check for delamination, and a push test on both upper and lower surfaces at each 'A' check (400 hrs). VH-EBQ was last inspected on 8 December 1990. When the trailing edge flaps are fully retracted and the fuel tanks are full, the trailing edge of the fixed panel is deflected upwards by as much as 4.5 cm by contact with the flaps. The operator has re-rigged the trailing edge panels so that the maximum deflection is 3.5 cm, thus reducing pre-load stress.

In addition, 'NO STEP' signs have been painted on the panels to reduce possible damage during maintenance. The failed panel from VH-EBQ was not available for examination; however, the subsequent failed panel from VH-EBP was available, and a specialist examination was carried out. Specialist opinion was that both failures were of a similar type, whereby trailing edges had peeled off as a result of debonding and bending due to pre-load rather than aerodynamic forces.

The manufacturer has incorporated design changes to overcome this type of failure. The operator has increased the frequency of periodic inspections of the panel, and it is considered that sufficient corrective action has been taken to overcome the problem. A comprehensive investigation of Air Traffic Services (ATS) aspects was carried out. The investigation concluded that the decisions and actions taken by the ATS personnel involved in the handling of the situation were prompt and effective.

The provision of an escort aircraft was justified in view of the information received initially from VH-EBQ and is consistent with the relevant operational practices and techniques documented in the Airways Operations Instructions (AOI). The captain of VH-EBQ initially felt some concern about the provision of an escort aircraft because of a lack of a clear understanding as to the function of the escort aircraft in relationship to his aircraft.

Selection of the aircraft to perform the intercept/escort was in accordance with published AOI guidelines, and implementation of the escort action was efficient and effective. Consideration of weather conditions was not included as a factor in determining the provision of an escort aircraft in the current AOI.

During a spray run, the pilot heard a loud bang, followed by a power loss and smoke in the cabin. He elected to land the aircraft in a paddock ahead. The mainwheels struck a log during the landing role and the aircraft nosed over. Inspection revealed that the top of the number three cylinder head had separated at the second barrel attachment thread. This failure caused a drastic loss of power and allowed combustion products into the engine cowling and cabin. Specialist examination indicated that the cylinder head separated due to metal fatigue cracking. The age of the cylinder could not be established.