VH-CZC, a Boeing 737-300, had taxied at Cairns for departure from runway 15 bound for Sydney.

VH-TJW, a Boeing 737-400, had taxied after CZC, also for a departure from runway 15 bound for Brisbane.

Both crews had been cleared via the runway 15 SWIFT 2 standard instrument departure. That procedure required crews to turn their aircraft at the earlier of 400 ft or the departure end of the runway and then track to 030 degrees M until climbing through 4,000 ft. At that point the aircraft should be turned right onto a track of 170 degrees M to intercept the 139 degrees radial of the Cairns VOR (VHF navigation aid). The SWIFT 2 standard instrument departure was designed to counter the limitations of high terrain surrounding Cairns aerodrome and the tracking requirements of inbound aircraft from the south and east.

At 0604:15 Eastern Standard Time, the aerodrome controller coordinated the two planned departures with the approach/departures controller using the phrase "next CZC followed by TJW". The reply was "CZC unrestricted, TJW unrestricted". That instruction authorised the aerodrome controller to allow the takeoffs in accordance with Cairns Local Instruction TOWER-20, which stated that the aerodrome controller must ensure that 3 NM separation between aircraft would exist prior to the second aircraft passing the upwind end of the runway.

At 0604:48, the crew of CZC was cleared for take-off and at 0606:43, the crew of TJW was cleared for take-off. Radar analysis showed that both aircraft commenced a left turn at approximately 400 ft at, or about, the departure end of the runway and that the required 3 NM spacing was achieved.

The approach/departures controller had approved a request for a change of level from a pilot of an aircraft that had departed Cairns approximately 7 minutes earlier. After issuing the departure clearances, the controller commenced the process of making the change in the air traffic computer; an action that required nine clicks of the mouse. In order to make this change, the controller looked away from the air situation display (which was on the main screen) and used the auxiliary screen to observe the flight plan window while using the keyboard to input the data.

While the controller was performing the information change task, the crews of the departing aircraft contacted him as required. The controller acknowledged the radio broadcasts then returned to the data input task. He did not continue to check the positional information on the air situation display. A few moments later, he glanced at the display and realised that TJW had turned earlier than CZC and was also out-climbing that aircraft. As he was about to take corrective action, the controller saw that TJW had climbed through 4,000 ft and had commenced the right turn onto a heading of 170 degrees M. The manoeuvre had placed TJW on a track inside that of CZC and led to a rapid reduction of horizontal separation to less than the required standard of 3 NM. The vertical separation standard of 1,000 ft had not been achieved at the time. Radar analysis showed that the aircraft passed 1.7 NM horizontally and approximately 400 ft vertically apart.

The controller immediately cancelled the standard instrument departure for TJW and issued the crew with a radar heading of 100 degrees M to ensure that, although separation standards would be breached, TJW would pass behind CZC. The crew of TJW reported that they had the other aircraft in sight and monitored their flight path to ensure that they remained clear of that aircraft. Both crews subsequently reported that they received a traffic alert and collision avoidance system (TCAS) advice.

CZC, the B737-300 series aircraft, had taken 1 minute and 56 seconds to reach 4,000 ft whereas TJW, the B737-400 series aircraft, took only 1 minute and 27 seconds to pass the same altitude. In addition, the initial opening speed created by the departure spacing off the runway had reduced to zero within 1 minute and 30 seconds of the departure of TJW.

The weather at Cairns was visual meteorological conditions but with passing showers, especially over the western escarpment. The cloud base and visibility associated with the showers restricted visual flight in or near the showers. Tower control had advised the terminal area controllers that conditions were marginal for visual approaches from the north-west.

The base of controlled airspace in the north-western sector was 3,500 ft reducing to 2,500 ft at 12 NM from the Cairns airport, and ground level at about 6 NM west of the airport. In the south-western sector, the base remained at 3,500 ft until reducing to ground level at about 5 NM west. The terrain was mountainous, with average heights of more than 2,000 ft to within 5 NM west of the airport. The higher peaks in this general sector of airspace were over 3,000 ft, with some over 4,000 ft to the south-west.

An Embraer E110 (Bandeirante) was operating under the instrument flight rules and on descent to 5,500 ft on an arrival to Cairns from the north-west. Air traffic control approved the crew to divert right and left of track to avoid showers and to facilitate visual contact with the ground. On first contact, approach control gave the crew a radar vector of 060 degrees and descent to 4,000 ft, the lowest altitude the controller could assign with reference to the published radar lowest safe altitudes. Shortly after, the crew reported "visual" and were cleared to descend to 2,000 ft visually and to track direct to Cairns under their own navigation. This clearance required the crew to maintain their current track and continuous visual reference with the ground until within 5 NM of the aerodrome. It allowed flight below the lowest safe altitude and transferred the responsibility for terrain clearance to the crew.

A Piper PA24 (Comanche) was operating under the visual flight rules and maintaining 5,500 ft on approach to Cairns from the south-west. The pilot encountered showers in the area and requested descent to remain visual. The controller issued the pilot with a clearance limit of Stoney Creek (a visual reporting point approximately 6 NM due west of Cairns airport where the terrain contours form a "saddle" feature) and a maximum altitude of 5,500 ft. This clearance allowed the pilot to proceed both inside and outside controlled airspace and to track as required to Stoney Creek. The position of Stoney Creek was at the common boundary of the 3,500 ft, 2,500 ft and ground level control area steps. About 1 minute later, the controller observed that the Bandeirante had deviated left of track and, when this was confirmed by the crew, issued instructions for a visual approach to a right base to runway 15 at maximum speed. The instructions included a frequency change to aerodrome control at 5 NM. The crew were still required to maintain their track direct to the airport until within 5 NM.

The pilot of the Comanche reported at 3,000 ft and 5 NM south-west of Stoney Creek. Although this position was outside controlled airspace, the controller cleared the pilot to descend to 1,000 ft and asked him to report if any diversions were required to remain in visual conditions. No amendment to the clearance limit of Stoney Creek was issued.

Shortly after, the controller realised that the Bandeirante had diverted right of track and was converging with the Comanche to the extent that radar separation standards would not be maintained. The controller attempted to again radar vector the Bandeirante but the crew had radio difficulties and did not hear the instruction. After a brief check of the radio, they transferred to the aerodrome control frquency when about 7 NM from Cairns. The radar vector was for the crew to turn onto a heading of 060 degrees. The instruction was issued as the aircraft was descending through the base of controlled airspace and was below the radar lowest safe altitude.

Radar analysis subsequently indicated that, at that point, the Bandeirante was about 2 NM north of the Comanche, on a converging track and with both aircraft at approximately 2,300 ft. These positions placed both aircraft outside controlled airspace and below surrounding terrain.

The controller observed the altitude of the Bandeirante on the radar display screen but did not advise the crew that they had descended outside controlled airspace. MATS required a radar controller to advise a pilot when the aircraft was observed to deviate significantly from its cleared route or if the controller believed it would deviate from controlled airspace.

The controller then asked the pilot of the Comanche if he could see the Bandeirante. When the pilot answered "negative", the controller instructed him to make a right orbit. The pilot began the turn as directed but realised that the turn would place the aircraft close to terrain that was above his altitude. He requested a left turn and, although this direction initially placed the aircraft closer to the Bandeirante, he then saw that aircraft. The controller then formally instructed the pilot of the Comanche to maintain separation with the Bandeirante. No traffic information on the Comanche was passed to the crew of the Bandeirante.

As both aircraft were outside controlled airspace, there was no technical infringement of separation standards. Radar analysis indicated that the closest point was 1.3 NM when the aircraft were approximately 100 ft vertically apart.

The descent procedure used by the controller after the crew of the Bandeirante reported "visual", was allowed under the terms of visual approach procedures in both the Australian Manual of Air Traffic Services (MATS) and the Australian Aeronautical Information Publication (AIP). However, the AIP also required crews to maintain an altitude of not less than 500 ft above the lower limit of controlled airspace. MATS stated that a controller must provide this same buffer except at pilot request or when visual approach procedures applied.

Although cleared to track direct to the airport, the crew of the Bandeirante diverted initially 1 - 2 NM left of track. About 1 minute later, they diverted right of track and entered into conflict with the Comanche. Both turns were to avoid showers and to maintain visual contact with the ground but no request for track diversion was made to air traffic control. The AIP stated that a pilot must maintain track while on a visual approach and should not report visual until certain that such tracking can be achieved. If a pilot on a visual approach finds that such tracking is no longer appropriate, air traffic control must be informed immediately.

When the controller issued the second radar vector, the crew did not hear the instruction but thought the transmission was for their aircraft. They made an attempt to acknowledge but when they again could not hear the approach controller, and considering that they were only 2 - 3 NM from the nominated frequency change point, they elected to make an early change to the airport control frequency. The crew remained on that frequency and had no further communication problems. Why there was a period of poor air-ground communications could not be determined. The attempt to radar vector the aircraft onto a heading of 060 degrees occurred when the aircraft was descending through 2,500 ft (the base of controlled airspace) and in an area where the terrain rose above 2,000 ft.

During the descent from 2,500 ft to about 2,200 ft to track through the Stoney Creek gorge, the Bandeirante passed within 1,000 m of terrain spot heights of 2,139 ft and 2,119 ft.

As the controller realised that the aircraft were coming into conflict and that the crew of the Bandeirante were not receiving radio broadcasts, the pilot of the Comanche was instructed to make a right orbit. The intention of this instruction was to provide adequate separation between the aircraft, however, it would have resulted in the pilot flying in close proximity to terrain that was higher than his current altitude. Spot heights on the Cairns Visual Terminal Chart indicated heights of between 3,070 ft and 3,477 ft within 3 NM to the right of the track, with general terrain above 2,000 ft within 1 NM. Had the pilot not countered the instruction by changing direction to a left turn, the aircraft would have been placed in a potentially unsafe situation.

The pilot of the Boeing 747 requested descent from FL370. The air traffic controller denied the request because an adequate separation standard did not exist with following traffic at a lower level. The pilot then requested an immediate descent due to icing. The controller advised the pilot of the traffic and issued a clearance for the aircraft to descend. Consequently, there was a breakdown of separation although the pilot of the following aircraft advised that he had the Boeing 747 on radar.

The pilot of the Boeing 747 subsequently reported that the aircraft had been operating in light icing conditions for 20 minutes before it entered an area of warmer air. The aircraft consequently required an increase in thrust to compensate for the increased temperature, however the number four engine throttle lever did not respond to auto-throttle commands. Soon afterwards, the crew found that they were unable to move any of the throttle levers and had to descend the aircraft. After about 15 minutes in the descent, the crew regained control of the throttle levers and levelled the aircraft at FL290.

After the aircraft landed, a maintenance inspection was unable to identify any problems that might have resulted in restricted throttle lever movement. The cables were lubricated and the aircraft was returned to service.

The investigation established that there was a known problem associated with restricted movement in throttle levers, believed to be a result of moisture on throttle cables freezing to seals and fairleads. The aircraft manufacturer had issued Service Bulletin 747-76-2060, which strongly suggested that aircraft operators replace rigid throttle vapour seals and fairleads with new flexible seals and fairleads. The manufacturer further indicated that implementing the service bulletin would prevent restricted throttle lever movement due to ice accumulation on throttle cables. However, this service bulletin only applied to aircraft with line numbers from 001 to 584. The aircraft involved in this occurrence was line number 713, which had the modifications to the throttle cable system installed during manufacture. In order to establish why a modified aircraft had apparently experienced freezing of the throttle cable system, the manufacturer requested that the operator carry out further inspections of the throttle cable seals, fairleads and drain holes. The operator advised that these inspections did not identify any anomalies that could have contributed to the restricted throttle lever movement.

The aircraft was cruising at flight level (FL) 350 with the autopilot engaged and 35,000 ft pre-selected in the altitude display on the autopilot mode control panel (MCP). The crew observed the aircraft make an uncommanded climb from FL350 to FL351, where it remained for approximately 20 seconds until the autopilot returned the aircraft to FL350. The crew then noticed that the `Mach Trim Fail' warning light had illuminated. The `Mach Trim Fail' light extinguished for a short time, re-illuminated, then extinguished again. During this sequence of events, the crew reduced the aircraft speed from Mach 0.76 to Mach 0.73 in accordance with the appropriate Quick Reference Handbook (QRH) procedure. Shortly after, the `Stab Out Of Trim' warning light illuminated. The `Stab Out Of Trim' warning extinguished after descent was commenced.

The descent was initiated using the autopilot and was normal until approximately FL250 when the aircraft pitched down steeply and the airspeed increased rapidly. The autopilot was disconnected, and the crew flew the aircraft manually, noticing that a considerable amount of back trim was required to stabilise the descent at 300 kts. The aircraft was flown manually for the remainder of the descent, and the aircraft was landed safely without further incident.

After landing, the pilot in command notified maintenance support of the in-flight problems that had been encountered. However, he did not lodge an incident report with the operator until 2 days after the occurrence. Upon receipt of the incident report, the operator took action to recover the aircraft's digital flight data recorder (DFDR). However, the DFDR was found to be faulty, and information from the flight was not available. The operator tested the DFDR and reported that the recorder would intermittently return the recording head to a random section of tape, resulting in existing data being overwritten. In order to obtain data about the aircraft flight path, air traffic control radar information was examined during the investigation.

The operator also reported that during the subsequent maintenance investigation, the K11 relay in the autopilot accessory unit was found to have an intermittent fault when it was subjected to initial testing. However, further testing was unable to reproduce the fault that was experienced in flight, and both the K11 and K6 relays were replaced as a precaution. The "Mach Trim Actuator" was also tested by the operator. The tests were conducted under various temperature conditions but they revealed no indication of defect. The manufacturer was also unable to fault the actuator. The aircraft manufacturer suggested that the operator verify sections of the aircraft wiring and perform the Mach trim and stabiliser rigging checks specified in the Boeing 737-300 Maintenance Manual.

The automatic flight system (AFS) of Boeing 737-376 aircraft comprises the autopilot flight director system (AFDS) and the autothrottle (A/T). Management of the vertical and lateral navigation of the aircraft is controlled by pilot selection of appropriate AFDS MCP settings in conjunction with the flight management computer (FMC). Normally, once an aircraft is airborne and the AFS engaged by the crew, the AFDS and A/T are controlled automatically by the FMC to fly an optimised lateral and vertical flight path throughout the climb, cruise and descent phases of a flight.

Data entry into the FMC is made by the crew on the flight management system control display unit (CDU) fitted to the cockpit centre stand. The CDU displays various "pages" of information that are required for the management of the flight. The crew will enter relevant data into these pages, including the intended cruise level.

Once an aircraft reaches the planned cruise level, it accelerates from the climb speed to the cruise speed. During acceleration, rearward movement of the wing aerodynamic centre of pressure results in a tendency for the aircraft to pitch nose down in a phenomenon known as "Mach Tuck". The Boeing 737-376 is equipped with a Mach Trim System (MTS) to provide improved stability at the high altitudes and airspeeds typically encountered during the cruise phase of flight. The MTS functions automatically at speeds in excess of Mach 0.615 and causes elevator adjustment as speed increases. Failure of the MTS is indicated by the illumination (amber) of the Mach Trim Fail light, and the crew response is to limit the aircraft speed to below Mach 0.74.

During cruise and with the autopilot engaged, the AFS causes the horizontal stabiliser to be trimmed to a position which will ensure that the pitch of the aircraft maintains the required cruise level. The Stabiliser Out Of Trim (Stab Out Of Trim) light illuminates amber to alert the crew if the autopilot is not trimming the stabiliser properly and that the elevator position exceeds a certain value in relation to the stabiliser and/or that the elevator is positioned too far away from the neutral position. The crew response for a "Stab Out Of Trim" amber warning is to disengage the autopilot and re-trim the stabiliser. The autopilot may then be re-engaged as required.

The Cessna 310 (C310) was the first aircraft in a busy arrival sequence and had been programmed by the flow controller for a visual approach to right base for runway 15. The weather had been fluctuating around marginal visual meteorological conditions for some hours and although the cloud base was generally 2,500 ft, it was lower in passing, heavier showers. The visibility was generally 8 km but reduced to 3,000 - 4,000 m in the showers.

When the pilot of the C310 reported that he was unable to establish visual contact with the ground, the approach controller had to change the arrival plan and vector the aircraft to the east of the aerodrome to facilitate further descent. This amendment to the traffic management plan was intended to assist the pilot to become clear of cloud but it necessitated extra track miles for the aircraft. Consequently, an adjustment to the arrival sequence was required and the C310 became number three in the landing order. This change also meant that the timing of sequential landings had fallen behind that which the flow controller had planned and following aircraft would need to be delayed.

The Aero Commander 500 S (AC50) was the fourth aircraft in the sequence and had been held outside controlled airspace by the approach controller until a separation standard could be guaranteed with the other aircraft.

During the course of these events, several verbal exchanges took place between various tower and terminal control area staff regarding the availability of visual approaches in the deteriorating weather conditions. It was agreed that all jet aircraft, and other aircraft arriving from the north and west, would be processed for instrument landing system (ILS) approaches. While these conversations were taking place, the weather had deteriorated such that the cloud base was 1,500 ft and the visibility was generally 3,000 - 4,000 m.

The terminology of what was "north and west" was never positively determined. The flow controller thought that the track of the C310 (via Copperlode Dam - approximately 210 degrees from Cairns) was south-west and not west. Other controllers considered anything west of 180 degrees was "west" and would not have used this track for a visual approach under the prevailing weather conditions.

When the pilot of the C310 reported that he was "visual", the approach controller authorised a visual approach and transferred the pilot to the tower control frequency. Shortly after, he issued a clearance for the AC50 to enter controlled airspace at 1,000 ft. He judged that, as their observed ground speeds were similar, he would be able to maintain the required radar standard of 3 NM while keeping the aircraft comparatively close to each other in order to minimise delays in the landing sequence.

As he was monitoring the progress of the aircraft, the approach controller noticed that the groundspeed of the C310 had reduced on final approach and that the 3 NM separation with the AC50 was unlikely to be maintained. He asked the aerodrome controller if he could use the "sight and follow" procedure, which would allow the pilot of the AC50 to establish and maintain visual separation with the C310 and, consequently, allow the distance between the aircraft to reduce to below 3 NM. However, due to the deteriorating weather and his resultant inability to sight the aircraft, the aerodrome controller declined the request.

As the AC50 closed on the C310, the approach controller realised that the 3 NM standard was not going to be maintained and asked the aerodrome controller to visually separate the two aircraft, a procedure that would again allow the aircraft to proceed with less than the required radar standard. Once again, the aerodrome controller declined the request as he could not see the aircraft in the prevailing weather conditions.

The approach controller then decided to re-sequence the AC50 and issued instructions for the pilot to turn away from the approach. As the aircraft turned, the horizontal separation reduced to 2.8 NM. An infringement of separation standards had occurred.

The flight attendant was preparing the cabin for landing when an area of severe air turbulence was encountered, causing the flight attendant to strike his head on the cabin roof. He received minor injuries and was relieved from further duty that day. The flight attendant has since returned to flight duties.

During the investigation, it was revealed that after striking his head, the flight attendant was unable to occupy a nearby spare aisle seat during the turbulence encounter, as another passenger's hand luggage had been secured to the seat with the seat belt. The flight attendant returned to his seat at the front of the aircraft. It was further revealed that it was common company practice to secure passenger cabin baggage on spare passenger seats due to the problem of stowage on smaller aircraft types.

Civil Aviation Safety Authority (CASA) regulations required cabin baggage to be stowed securely so that movement of cabin baggage would not cause injury to any person or damage to the aircraft, during turbulence or unusual aircraft accelerations or manoeuvres. The regulations also required that cabin baggage be stowed whenever the seat belt sign was on and specified that luggage be stowed under a passenger seat with approved forward-movement restraint, or in an approved overhead locker or rack (Civil Aviation Order 20.16.3, subsection 9).

Following the occurrence, the operator sought clarification of the regulation regarding the stowage of cabin baggage from CASA. CASA had initially agreed with the operator that soft bags could be stowed with a seat belt through the straps, on a window seat that was not an emergency exit.

The Bureau conveyed its concerns to CASA that seat belts were designed primarily to restrain the human form and that bag handles were not stressed to take loads that could occur under turbulent conditions or impact forces. Bags so secured on a passenger seat by a seat belt might become loose during turbulence or impact, or could swing about the seat belt, presenting injury or evacuation obstruction hazards.

History of the flight

During approach, the crew of the Boeing 737 selected "gear down". A loud "bang" was heard and the "gear safe" green light for the right main landing gear illuminated immediately. The left main and nose landing gear lights illuminated in a time consistent with a normal extension sequence. As the cockpit indications showed that the landing gear was locked down, the approach was continued, and a normal landing was completed.

Inspection of the aircraft revealed that the right main landing gear (MLG) trunnion pin attach lugs had failed. As a result, the right main gear hydraulic actuator disconnected from the MLG assembly, resulting in free-fall of the right MLG during extension to the "down and locked" position. The trunnion pin failure resulted in little damage to the actuator and surrounding structure. No damage to control cables or hydraulic lines occurred.

Diagrams of the B737 landing gear assembly and photographs of the damaged trunnion pin are available on the ATSB website linked to this occurrence number.

The Sydney approach radar controller was operating a combined departures/approach service during the early morning shift when staffing was minimal, and had been on duty since 0245.

For noise abatement reasons, runway 34 was the preferred runway for arrivals prior to 0600, but was not utilised due to an excessive downwind component. As a result, a number of inbound aircraft were required to hold, in order to land on runway 16 after 0600. The approach controller was required to nominate to the adjacent sector controllers the minimum longitudinal spacing required between successive arriving aircraft. Local procedures recommended a 15 NM spacing. The approach controller requested and was provided with 10 NM longitudinal spacing between aircraft, including a Boeing 747 (B747) approaching from the south-west, which was sequenced to land ahead of a Boeing 767 (B767) arriving from the north. The controller was also managing a number of other arriving aircraft.

Independent visual approaches (IVAs) to runways 16L and 16R were in use. The approach controller subsequently amended the initial arrival sequence when it became apparent that the B767 would arrive earlier than the B747. This placed those aircraft as number two (runway 16L) and three (runway 16R) respectively in the arrival sequence.

The crew of the B767 were vectored to intercept the runway 16L localiser at approximately 30 NM, and instructed to report when they had that runway in sight. The B747 crew had been instructed to turn right onto a heading of 120 degrees in order to intercept the runway 16R localiser. They were subsequently cleared to make a visual approach after reporting that they had runway 16R in sight. However, as the B747 turned onto final, the aircraft drifted to the left through the centreline of the 16R approach path, triggering a resolution advisory, from its traffic alert and collision avoidance system (TCAS), for the crew to descend. The approach controller observed the close proximity of the aircraft and issued instructions to both crews to turn their respective aircraft from final using a "breakout" procedure. The B767 was sighted by the B747 crew as their aircraft passed through the final approach path. The lateral distance between the aircraft was reduced to 0.3 NM at a time when the vertical separation was 500 ft.

Normally, IVAs are conducted by a director controller using specific procedures, which included the use of a 20 NM scale on the radar display, and a map for intercept guidance. In this instance the approach controller had his display set to a scale greater than 20 NM, and did not use the IVA map. Controllers are also required to provide a radar vector not exceeding 30 degrees for intercept of the localiser. The vector issued to the B747 crew provided a 36-degree intercept of the localiser. Moreover, crews subject to IVA procedures are responsible for a number of actions detailed in the Aeronautical Information Publication (ENR 1.1 - 48, paragraph 36.3.1) including, "ensuring that the runway centreline is not crossed during intercept".

Just prior to the occurrence, two controllers arrived to commence the morning shift from 0600. One of these would have normally replaced the overnight controller; however, due to the number of arriving aircraft, these controllers were instructed to staff the director and flow control positions. A third controller arrived and was waiting to take over from the approach controller when the incident occurred. However, the approach controller's workload prevented him from handing over responsibility for the position at that time.

The approach controller limited his options by coordinating the provision of a 10 NM spacing between successive arriving aircraft. As a result, when the crew of the B747 allowed their aircraft to pass through the extended centreline there was limited margin for error, resulting in an immediate traffic confliction. The performance of the controller was probably degraded by the effects of fatigue and workload. The performance of the B747 crew was likely to have been affected by similar factors.

The provision of additional staff prior to 0600 to provide either a flow or director controller to assist the approach controller, or to relieve the approach controller earlier, would have reduced the approach controller's workload during a critical period.