The pilot was conducting his first commercial agricultural spraying operation and had been operating on the task for several hours in the afternoon. He was asked by the farmer who owned the property being sprayed, to avoid spraying in the dam on the paddock. Whilst the pilot was attempting to identify the dam, the aircraft's left wing clipped the top of a tree, and the aircraft descended steeply into the ground. The pilot was able to escape from the wreckage, having sustain soft tissue damage, fractures to lower vertebra and both ankles dislocated.

The pilot of a Brasilia aircraft had an estimated time of arrival (ETA) over Bundaberg non-directional beacon (NDB) of 0920 UTC. On descent, he was notified by Brisbane Centre of a Shorts 3-60 inbound to Bundaberg from Gladstone, ETA 0917 and a Beech A36 inbound to Bundaberg from Hervey Bay, ETA 0927.

When the Shorts pilot changed to the Bundaberg mandatory broadcast zone (MBZ) frequency 126.9 MHz, he advised that he was on the inbound leg of the runway 14 NDB approach. As the Beech 36 was several minutes behind the Brasilia, it was not in conflict at that time.

When the Brasilia was approaching the NDB, the Shorts pilot advised he was at the minima, not visual, and was executing a missed approach. The Brasilia pilot maintained 3,600 ft and tracked over the aid.

When the Shorts pilot advised that he was 5 NM by GPS south of the NDB and maintaining 2,600 ft, the Brasilia pilot commenced an NDB approach. When the Brasilia was turning inbound, the Shorts pilot advised he was over the aid and would commence an approach and follow the Brasilia in the procedure.

At the minima for the NDB approach procedure, the Brasilia crew were not visual with the runway, and commenced a missed approach in heavy rain at 0927. The Brasilia pilot was aware that the Beech 36 was now in conflict (ETA 0927), and tried to contact the pilot on both the mandatory broadcast zone (MBZ) and area frequencies without any response. Brisbane Centre was then contacted to ascertain the whereabouts of the Beech 36. Brisbane Centre said that the pilot had last reported when he changed to the MBZ frequency. Brisbane Centre then passed traffic on a DHC8 inbound from Brisbane to Bundaberg ETA 0935. Communications were established between the Brasilia and the DHC8 on the MBZ frequency.

The Brasilia pilot then executed a sector entry for a further attempt at the NDB approach. As the Brasilia approached the aid, the Shorts pilot advised that he was going around at the minima, and he also tried to contact the pilot of the Beech 36. As the Brasilia pilot commenced the second approach, the Shorts pilot advised that he was diverting to Brisbane. When the Brasilia pilot was turning inbound, the DHC8 pilot advised that he was over the aid and would follow the Brasilia in the procedure.

The Brasilia pilot said that approaching 800 ft they noticed the weather was improving, and they became visual at approximately 700 ft. After landing on runway 14, the Brasilia pilot executed a 180-degree turn to backtrack and saw the landing lights of an aircraft on short final for runway 14. The Brasilia pilots turned on all landing lights to illuminate themselves to the oncoming aircraft, then expedited to the taxiway as the other aircraft executed a low-level go-around and joined left downwind. The Brasilia pilot said he assumed the aircraft they had seen was the Beech 36, and he made a broadcast to the pilot of the inbound DHC-8 to alert him of the circuit traffic. The pilot of the Beech 36 was still unable to be contacted on the radio, and he landed in front of the approaching DHC-8, taxied off the runway, and parked. Prior to shutdown, the Brasilia pilot heard Brisbane Centre ask the DHC-8 pilot to attempt to contact the pilot of the Beech 36 on the superseded MBZ frequency 119.1 MHz. The pilot of the Beech 36 was located on this frequency and the pilot of the DHC-8 advised him of the correct frequency.

The investigation revealed that the pilot of the Beech 36 had been referring to an out-of-date Bundaberg runway 14 NDB instrument approach procedure. The page showed the superseded MBZ frequency. The pilot had written the correct frequency, which he had obtained from the en-route supplement, on his flight plan. However, he had not referred to the flight plan again. Due to the adverse weather conditions and high workload, he had only referred to the out-of-date approach chart, and this had led him to select the incorrect frequency. During his arrival, because he had not heard any radio transmissions from the other aircraft that were inbound, he assumed that they had landed. He had then conducted a holding pattern, a sector entry, and the NDB approach procedure with the crews of the other aircraft being unaware of his presence.

SAFETY ACTION

As a result of this occurrence, representatives of the Bureau of Air Safety Investigation held a meeting with officers from Airservices Australia and the Civil Aviation Safety Authority to discuss the safety deficiencies identified during the investigation. Agreement was reached that as a matter of priority, the Civil Aviation Safety Authority and Airservices Australia would:

1. amend pilot operating procedures so that when a frequency change is made, notification will be provided to other traffic of the location and frequency of the MBZ/CTAF to which the pilot is changing; and

2. institute radio checks for pilots in IMC entering a MBZ/CTAF who have been previously advised of traffic information and are subsequently unable to make radio contact with the other pilot(s).

The meeting considered a number of other issues in relation to MBZ/CTAF operations and the consequent effect on safety. In this context, the Bureau representatives agreed to review these issues with a view to framing appropriate recommendations. Subsequently, the Bureau issued Interim Recommendation IR970110 to Airservices Australia and the Civil Aviation Safety Authority on 4 July 1997. The interim recommendation stated the following:

"The Bureau of Air Safety Investigation recommends that Airservices Australia and the Civil Aviation Safety Authority:

1. implement methods for the timely dissemination of the MBZ or CTAF frequency to pilots;

2. implement methods of providing to pilots confirmation of the correct selection and operation of an MBZ or CTAF frequency;

3. examine the requirement for the establishment and operation of traffic alerting services at all aerodromes during RPT operations;

4. examine the provision of additional radar coverage in the Bundaberg area; and

5. examine the provision of surveillance for other locations serviced by RPT operations".

The following response was received from Airservices Australia on 1 September 1997:

"Reference is made to the Bureau's Air Safety Interim Recommendation No IR970110 which relate to communications procedures for MBZ and CTAF. With regard to Interim Recommendation 1, Airservices have issued a NOTAM instructing pilots to report the frequency to which they are changing as part of the "Changing To" call. The frequency quoted is, whenever practicable, recorded by ATS for the information of other pilots. Airservices do not intend providing the MBZ or CTAF frequency to pilots on an individual basis as a matter of routine. Other methods of disseminating the MBZ or CTAF frequency, e.g. via AWIB broadcast will be taken into consideration.

It should be noted however, that the longevity of this procedure is not great, given the likely directions of Airspace 2000 and introduction of the National Advisory Frequency (NAF) in Class G airspace.

Interim Recommendations 2 and 3 fall within the CASA areas of responsibility for a response.

Interim Recommendations 4 and 5 relating to the provision of additional surveillance in the Bundaberg area and for other locations serviced by RPT will be considered by Airservices."

The following response was received from the Civil Aviation Safety Authority on the 13 November 1997:

"I refer to your letter of 4 July in relation to BASI Occurrence 9701646 generating Air Safety Interim Recommendation IR 970110. The occurrence resulted from incorrect frequency usage in the Bundaberg MBZ.

The following are the CASA responses to the individual parts of the Interim Recommendation:

IR part 1

The normal AIS process is designed to ensure that pilots have the correct information pre-flight. This is supported by CAR 233(1)(h) which requires pilots, before commencing a flight, to have the latest editions of maps, charts and other aeronautical information.

IR Part 2

A NOTAM was issued requiring pilots to advise the frequency changing to; this was complemented with an instruction to ATS staff to record this frequency for advice to subsequent aircraft as required. CASA is awaiting a response from BASI as to the reduction in the number of incidents since this procedure was implemented.

[The Bureau forwarded the incident information to the Civil Aviation Safety Authority on 25 November 1997.]

The CASA Board has determined to initiate a legislative process to require the provision of third party communications at uncontrolled airports served by RPT; industry and Airservices will be advised in the near future. CASA is also seeking results of the initial trials of the "beep back" facility which, in responding to radio calls, confirms selection of the correct frequency.

IR part 3

As part of the "third party communications" legislative initiative mentioned above, CASA will examine the requirement for the operation of traffic alerting services at aerodromes during RPT operations. This would need to integrated with Airspace 2000 initiatives and policy issues such as classification of operations.

IR parts 4 and 5

CASA does not have specific standards for the provision of radar services, but has encouraged use of existing radar coverage to enhance safety. While it may prove to be a significant safety enhancement to provide additional radar coverage in the Bundeberg [sic] area, it would not seem practical or cost efficient to require radar surveillance of all terminal areas serviced by RPT operations."

The flight was planned as a familiarisation flight for the passenger. The pilot did not detect any fault with the engine during the pre-flight inspection, and he reported that the compression on all cylinders appeared normal. The aircraft was close to its maximum weight at take-off.

During the take-off roll engine RPM was 2800. Maximum RPM permitted was 3300 however, the manufacturer reported that the engine/propeller combination fitted to the aircraft was only expected to produce between 2900 and 2950 RPM. Although the take-off performance appeared below normal, the pilot believed it was caused by the aircraft being close to its maximum weight. Witnesses reported that the aircraft used almost all of the 900 m strip for its take-off roll. The manufacturer reported that the take-off roll in a Jabiru 1600 at maximum weight should be between 250 and 300 m.

After becoming airborne the aircraft would not climb, and engine RPM had fallen to 2700. The pilot turned to follow the road in case the engine lost more power. He completed trouble checks but was unable to identify the reason for the loss of performance. Shortly after, the engine regained some power, and the pilot decided to attempt a return to the airstrip.

During the subsequent turn the engine stopped completely. The pilot then attempted a forced landing in a tree covered area. Whilst attempting to avoid a medium sized tree, at a height of approximately 20 ft above the ground, one wing collided with the tree and the aircraft stalled and crashed. The fuselage turned through 180 degrees and slid backwards into another tree before coming to a stop.

Although the aircraft was extensively damaged the cockpit area was relatively intact. A fuel filter had fractured allowing fuel into the cockpit however, there was no fire. The pilot turned the fuel off and secured the electrical system before he and the passenger exited the wreckage.

An inspection of the engine disclosed that the exhaust valve, seat and spring on the No. 3 cylinder had been subjected to excessive heating. The valve guide was loose in the cylinder head and the valve spring had lost its tension. There was an accumulation of grease and metal shavings in the rocker cover which had been there for sufficient time to congeal indicating that the valve, seat and spring had been in this condition for some time. The faults meant that the valve might or might not seat correctly during engine operation. The variable nature of the valve operation could lead to an intermittent power loss at those times when the valve was not seated correctly. It is possible that the valve was seated correctly when the pilot checked the compression during the pre-flight inspection; however, the post-accident investigation was unable to reproduce normal cylinder compression. It is possible that the low RPM and excessive take-off distance, reported by the pilot, was a symptom of the valve problem.

The engine was fitted with a reconditioned cylinder head and the manufacturer reported that there had been other cases of valve guides coming loose. To manage this type of problem the manufacturer had included a compression check and, if this proved irregular, a tappet check in the daily inspection requirements. Although the precise reason for the valve guide failure could not be determined the guide is held in place by a using a temperature-dependent shrink-fit. As there was an indication of excessive heating, it is possible this caused the fit to come loose.

Although the valve position could lead to a loss of power, experience indicates it would not normally be sufficient to stop the engine. It could not be determined why the engine stopped. The manufacturer reported that any reduction in power, during three cylinder operation, might stop the engine. The pilot reported that he did not reduce power.

At the low speeds, normally used during a forced landing, any abrupt manoeuvres made to avoid trees would probably cause the aircraft to stall.

SAFETY ACTION

The manufacturer has discontinued production of the 1600 cc engine and replaced it with a 2200 cc engine which is manufactured using a different process.

This will alleviate the problem with the valve guides. The manufacturer is also working to relocate the fuel filter to prevent fuel from entering the cockpit in the event of a failure.

The owner reported that the tail rotor of the helicopter came into contact with a small tree as the pilot was pushing some cattle into a yard. The helicopter then spun out of control to the ground, landing heavily on the left skid before rolling over. The pilot was able exit the helicopter without injury. The owner advised the helicopter was damaged beyond economical repair.

Post flight examination showed that one of the tail rotor blades had separated in flight when it came into contact with the tree.

Weather at the time of departure from Emerald was reported to be 6 to 7 OKTAS of low cloud with patches of rain in the area. Light rain was falling at the time the aircraft taxied. The wind was light with a slight crosswind from the right. Visibility was in excess of 10 KM. The runway was damp and there were several patches of water on the runway.

The pilot reported that the take-off proceeded normally until approximately 50 knots when he disengaged the nose-wheel steering. At this point, the aircraft started to diverge to the right of the centreline. In an attempt to return the aircraft to the centreline the pilot used rudder, but this was ineffective. The pilot then rejected the take-off by moving the power levers to idle, but the aircraft continued to diverge to the right and departed the sealed surface of the runway.

The pilot used reverse thrust in an attempt to slow the aircraft, but this appeared ineffective, and the aircraft continued to turn to the right and slide on the wet grass surface. The aircraft by this time was heading for a small levee bank beside a drainage ditch. The pilot assessed that the aircraft would probably climb over the levee bank, so he elected to introduce power on the left engine in an attempt to turn the aircraft before the levee bank. However, this was only partially successful, and the aircraft came to rest adjacent to the levee bank facing approximately 180 degrees from the take-off heading. The left propeller contacted the bank as the aircraft stopped.

The aircraft had travelled 121 M after leaving the runway. The distance at which the aircraft left the centreline of the runway could not be accurately determined due to the runway surface. The nose-wheel track was approximately 1 metre to the left of the right-hand wheel track and continued at this distance until approximately 5 metres from the end of the tracks, at which point it crossed over the top of the right wheel track.

The aircraft suffered damage to the left propeller and engine. Examination of the engine when removed from the aircraft confirmed that it had suffered considerable internal damage consistent with a propeller strike at high power.

The operator reported that there had been several minor problems with the nose-wheel steering system in the 5 weeks prior to the accident. These problems, however, caused the system to fail in the off mode.

The nose-wheel steering system was examined and a series of tests conducted. It was found that with the nose-wheel steering armed, any movement of the speed levers caused an uncommanded movement of the nose-wheel to maximum right deflection. The fault was traced to the steering amplifier and this was subsequently removed for further testing. Once the amplifier was removed from the aircraft and replaced with a serviceable unit, the fault did not re-occur. Extensive testing of the amplifier has been unable to reproduce the fault.

The patches of water on the runway were not considered to be a factor in this accident.

A significant proportion of this report is based on information provided by the operator.

Whilst approaching Bindook at FL180, on a scheduled air transport flight from Dubbo to Sydney, the crew of a SAAB 340B observed the right engine air intake caution light illuminate. The abnormal procedures checklist for this condition was carried out and the caution light extinguished.

A short time later a strong burning smell became evident on the flight deck. However, there were no associated warning indications. Discussions with the flight attendant (FA) revealed that the smell was isolated to the cockpit, indicating that the burning smell may have been associated with the right air conditioning system, which supplied conditioned air to the cockpit. The 'Air Conditioning Smoke' emergency checklist was not utilised. The right engine bleed air valve was closed in an attempt to further diagnose the problem. Several minutes later the bleed air valve was re-opened. The burning smell in the cockpit intensified. Reports from the FA continued to indicate no noticeable smell in the cabin.

The co-pilot then looked out the right, side window and reported to the pilot in command (PIC) that there was a fire in the right engine. There were no fire warning indications in the cockpit. The engine fire checklist was carried out, the right engine fire bottle was discharged, and the engine shut down. An inflight emergency was declared to Air Traffic Control, who initiated a distress phase and expedited the entry of the aircraft to Sydney.

All three crewmembers then prepared for an emergency landing at Sydney. The PIC advised the FA that the landing would be normal, and would be followed by a normal disembarkation on an adjacent taxiway. With local emergency services in attendance, the aircraft subsequently landed safely on runway 07 and was stopped on taxiway G3. All occupants disembarked normally through the main cabin door and were transported to the terminal.

An engineering examination revealed that the right engine lower intake anti-ice duct had short-circuited and was excessively burnt around the inner-upper lip at the particle separator mounting flange. The damage was consistent with the duct material smouldering for some time. Based on the recollection of the co-pilot, there had been no visible flames but rather a very bright red/orange glow inside the intake. The lower duct, and the particle separator, were replaced with serviceable items and the aircraft returned to service.

The engine intake anti-ice system consists of electrically heated intake ducts. The heater elements are made of a copper alloy material sprayed onto a glass fibre mat and covered with an outer glass mat. The mat is bonded to the metal inlet, making them an integral unit. Should the duct intake surface suffer damage which allows moisture to penetrate to the heater elements, short-circuits can occur. Although required as part of the engine fire emergency checklist, the activation of the fire extinguisher has no effect on the engine intake duct. The intake duct anti-ice remote circuit breaker is designed to trip as a result of a short-circuit. In this case the circuit-breaker failed to trip.

The aircraft manufacturer has been aware of short-circuiting intake duct heaters in aircraft equipped with this particular type of intake, and had developed a specific checklist titled 'Intake Sparks' to cover this particular situation. At the time of this occurrence the checklist was being distributed as an amendment to the Aircraft Operations Manual, but had not been available to the flight crew.

Local safety action

As a result of this occurrence the company has introduced improvements to flight operations training and procedures, designed to assist flight crews in the recognition and handling of intake duct problems. In addition, both flight and cabin crew emergency disembarkation procedures are being reviewed.

Other safety actions include increasing the frequency of engineering inspections and resurfacing of duct heater elements, as well as streamlining procedures to ensure new information from the aircraft manufacturer is distributed to all crews without delay.

The recently rated agricultural pilot was conducting circuit and landing practice in a Cessna Ag wagon. Whilst landing on runway 04 the right brake lost effectiveness, resulting in the pilot losing directional control. The aircraft ground looped to the right, damaging the right landing gear and right wing, before coming to rest adjacent to the runway. The weather was CAVOK with light winds.

FACTUAL INFORMATION

A Reims F406 aircraft was conducting a coast watch surveillance flight from Darwin to Gove. The aircraft was approximately 100 NM north-west of Gove operating below 2,000 ft in non-controlled airspace. The crew heard a broadcast on the marine very high frequency - frequency modulation (VHF-FM) radio. This broadcast was on the nominated emergency frequency of 156.8 MHz (which was Channel 16 on marine transceivers) and was from a foreign warship notifying an intention to commence live weapon firing in the area from the surface to 45,000 ft.

The F406 crew attempted to contact the warship crew on the marine frequency to advise them of the location of their aircraft, which was within the firing area. Reception with the foreign warship was unreadable, but eventually the F406 crew were able to talk to the radio operator of a Royal Australian Navy (RAN) warship that was also in the area. The F406 crew sought advice of a safe distance to avoid the ships. The radio operator in the RAN warship advised the crew of the ship's position and to remain outside a circle of 20 NM radius centred on that position. The intended track of the F406 was within 10 NM of the ships.

The F406 crew altered the aircraft's course to avoid the firing area. At the same time the crew reported their position to Adelaide Flight Service (FS) and also advised that they were aware of the live firing being conducted in the area. The FS operator checked the aeronautical information service for a notice to airmen (NOTAM) regarding the firing activity. There was no NOTAM for the activity. The operator requested the crew of the F406 to relay to the foreign warship to contact FS on high frequency (HF) radio. The foreign warship contacted FS and it was established that the firing was an unnotified live firing on the high seas. Shortly after, the operator on the warship advised FS on HF and broadcast on the marine VHF frequency, that firing was completed. The firing had been conducted for approximately 40 minutes.

The F406 crew were monitoring six radios which were selected to both aviation and marine, emergency and normal operating frequencies (HF, VHF and ultra high frequency (UHF)). The only notification of the firing activity that they heard was on the marine VHF emergency channel. No broadcasts were heard on the aviation emergency or the normal operational very high frequency - amplitude modulation (VHF-AM) or HF frequencies.

Royal Australian Navy procedures

Royal Australian Navy personnel had briefed the crew of the foreign warship on the RAN procedures for weapons firings.

Warships planning to conduct firing operations were required to notify the appropriate local area command 4 days prior to the event. This was to enable the command to arrange for the issue of a NOTAM notifying the location, times and vertical limits of the operation. The investigation was unable to establish the reason for the short notice of the firing operation in this instance.

Operations could be conducted without this notification subject to a number of criteria being satisfied. In this situation the warship intending to conduct the firing had to either visually or electronically scan the area to be used. This scanning was to ensure that no vessels or aircraft were within the firing area. The firing was to be conducted no closer than 10 NM to controlled airspace or a published air route, and outside of 20 NM from land and 10 NM from offshore man-made structures.

The warship was also required to broadcast intentions prior to commencement and on completion of each operation on the marine VHF-FM and HF (2828 KHz) emergency frequencies. The actual frequencies on which the foreign warship did broadcast were unable to be confirmed by the investigation. There was no requirement to broadcast on aviation emergency or normal operational frequencies.

Civil aircraft radio requirements

Civil aircraft are only required to fit and use VHF-AM and/or HF radios. Civil aircraft would normally not be capable of receiving or transmitting on the VHF-FM band. Because of the nature of the coast watch task undertaken by the F406, that aircraft was fitted with and used a number of non-aviation radios.

Air routes

Part of air route B472 between Gove and Ambon, Indonesia and air route J199, the published route between Kupang, Indonesia and Port Moresby, Papua New Guinea, traversed the area in which the firing was conducted.

Adelaide FS advised Brisbane air traffic control (ATC) of the firing once they were aware of the details. Subsequently, Brisbane ATC routed aircraft clear of the area until the activity was completed.

Airspace

Non-controlled airspace extended from sea level to flight level (FL) 245 in the firing area. FS was responsible for communications in non-controlled airspace. Above FL245 and up to FL450 the airspace was controlled airspace. Brisbane ATC was responsible for the management of this airspace.

ANALYSIS

The crew of the foreign warship would appear to have not conducted an adequate scan of the firing area prior to the firing operation and was unaware of the proximity of the coast watch aircraft.

The warship's notification of firing was not broadcast on the aviation, emergency or normal operating frequencies. Consequently, civil aircraft, which may have been in the area, would not have received warning of the firing operation unless they had special equipment to monitor the marine VHF-FM emergency frequency.

Air traffic services (ATS) used VHF-AM and HF radios to communicate with crews of civil aircraft. The foreign warship crew could have used either of these radios to establish communications with ATS to notify the firing activity. FS would have then been able to notify aircraft in non-controlled airspace by broadcasting a hazard alert to known aircraft in the area. Also, crews of aircraft unknown to FS in the area, would probably have also been alerted if they were monitoring the appropriate area frequency. In controlled airspace ATC would have had prior notice to enable them to route aircraft clear of the firing area.

SIGNIFICANT FACTORS

1. A NOTAM was not issued for the firing activity.

2. The foreign warship conducted a firing activity at short notice.

3. The foreign warship crew was unaware of the approaching coast watch aircraft.

4. Royal Australian Navy procedures did not require broadcasts to be made on either the aviation emergency or normal operational frequencies

5. Brisbane air traffic control was unaware of the firing activity that impinged on controlled airspace.

6. The firing activity was conducted in close proximity to two air routes in controlled airspace.

SAFETY ACTION

Following investigation of aspects of the notification of weapons training areas the Bureau of Air Safety Investigation issued the following recommendations to the Australian Defence Force and to Airservices Australia on 29 June 1998:

"IR980086

The Bureau of Air Safety Investigation recommends that the Australian Defence Force, with the assistance of Airservices Australia, review airspace activation procedures to ensure that appropriate and fail-safe notification actions have been completed prior to the commencement of military activities.

IR980087

The Bureau of Air Safety Investigation recommends that Airservices Australia assist the Australian Defence Force to review airspace activation procedures to ensure that appropriate and fail-safe notification actions have been completed prior to the commencement of military activities."

The pilot reported his aircraft had been hangared since April 1994. He had become seriously ill in November 1994 but, in the past 12 months, he had made a good recovery. He decided that he would go with a friend, who was a qualified pilot, to pick up his aircraft from Roma where it was hangared. The aircraft was flown from Roma to his friend's property.

On the afternoon of the accident, the pilot was using a 220 m runway for take-off, but this distance was well within the performance capability of the aircraft. The friction nut for the throttle control in the aircraft was not functioning correctly, and it was possible for the throttle control to slide back from the full throttle position by about 2.5cm, if the throttle was not held in position manually.

The pilot stated that after becoming airborne in about 70 m he went to apply full flap after building up some speed to clear some low trees at the end of the runway. He said that as he let go of the throttle to apply extra flap, the throttle control slid back with a corresponding reduction in engine power. By the time he realised what had happened and reapplied full throttle, it was evident that the aircraft would not clear trees at the end of the strip. He said he was then left with no alternative but to pull the nose up and 'belly' the aircraft through the trees.

The Boeing 737 departed Darwin for Adelaide at 1250 CST. During cruise at FL330, with Autopilot A selected in Command mode, the crew reported that the aircraft exhibited Dutch roll tendencies and that an Air Data Computer malfunction had occurred. Autopilot A was disengaged, and the aircraft was flown manually for approximately 2 minutes before Autopilot B was selected in Command Mode. Sometime later Autopilot A was re-engaged with no problems observed. (Each Autopilot receives inputs from a different Air Data Computer.)

Engineering staff removed the digital flight data recorder from the aircraft in Adelaide and sent a copy tape to the Bureau for analysis. The data showed that at 1357 CST five cycles of left then right roll of increasing amplitude occurred. The total duration of the five cycles was 40 seconds. Approximately 1 1/2 minutes later a similar disturbance occurred followed quickly by a third disturbance of lesser magnitude. The maximum roll experienced during these disturbances was 3 degrees. Only very small heading changes were associated with this roll. Autopilot A provided aileron control inputs to counteract the roll.

Occasional large fluctuations in recorded Air Data Computer parameters (Pressure Altitude, Computed Airspeed, Mach Number, Total Air Temperature and Static Air Temperature) were observed in the recorded data commencing at 1401 CST. These indications are consistent with a malfunctioning Air Data Computer.

Neither Rudder Position nor Yaw Damper status are directly recorded so the cause of the Dutch roll could not be positively determined however the yaw damper does have airspeed as an input. While rudder pedal position is recorded there is no mechanical feedback from the yaw damper to move the rudder pedals.

At 1600 CST shortly before top-of-descent Autopilot A was re-engaged in Command mode. No anomalies were observed in the recorded data at this time.

The Air Data Computer suffering the malfunction was removed from the aircraft in Adelaide and quarantined for fault-finding. During subsequent examination, a fault was found with this unit. The aircraft's in-service performance has been closely monitored by the operator since this occurrence and no anomalous behaviour has been reported to date (2 July 1997).

There was no safety action as a result of this occurrence.