Loss of separation

An infringement of the 3NM radar separation standard occurred at 0606 western standard time approximately 5NM to the northeast of Perth. The aircraft involved were a British Aerospace 146 (BAe146) that departed Perth on a RWY 03 BIU2 standard instrument departure (SID) followed shortly after by a Boeing 737 (B737) that departed on a RWY 06 PEPPA3 SID.

The 03 BIU2 SID is designed to provide system separation with military airspace to the north of Perth and, as a consequence joins the 06 PEPPA3 SID at position REDIL, 8NM to the northeast of Perth. Military airspace was not active at the time of the incident.

The aircraft were under the control of the Perth Departures (DEP) controller at the time. The responsibility for the provision of separation of aircraft on these two SIDs was defined in Perth Local Instructions as the responsibility of the DEP controller.

The DEP controller was monitoring a trainee controller on DEP who was approaching the final stages of rating training. The controller did not use an "override" box that would have allowed him to override the trainee and transmit instructions to the crews.

The aerodrome controller (ADC) advised the trainee DEP controller that the B737 was next for take-off. The trainee DEP controller issued the ADC an unrestricted clearance for the B737 to depart.

The crew of the BAe146 heard the B737 being cleared for take-off and realised that the two aircraft were on conflicting departure routes. The crew asked the trainee DEP controller for a clearance direct to Ballidu in an attempt to resolve the situation. The controller advised the crew that he "would advise" when the direct clearance was available.

The B737 departed and the crew contacted DEP control and advised that they were leaving 1,800 ft. The crew of the BAe146 monitored the proximity of the B737 visually and on the Traffic Alert and Collision Avoidance System (TCAS). The TCAS equipment did not provide a traffic or resolution advice.

The DEP controller did not intervene directly and prompted the trainee to turn the BAe146 away from the B737. The trainee then issued an instruction to the crew of the Bae146 to cancel the SID and track direct to Ballidu. The DEP controller then instructed the trainee to give the crew of the BAe146 a heading in order to resolve the situation. The trainee responded by instructing the crew of the BAe146 to turn left heading 360 degrees for separation. The trainee then instructed the crew of the B737 to turn right heading 090 degrees for separation.

Traffic information was not passed to either crew, nor was the word "immediate" used in passing heading instructions. There was no relay of the urgency of the situation to either crew by phraseologies used by the trainee. At that time the training officer utilised the APP console handset to contact the ADC on the hotline. The ADC was asked by the DEP controller "can you just monitor", to which the response from the ADC was "Yes I'm monitoring". The DEP controller later reported that in his mind it was a request for the ADC to provide visual separation between the aircraft. He also reported that the response received from the ADC indicated to him that the ADC had been providing separation throughout the event by visual monitoring of the aircraft.

The ADC was monitoring a trainee controller by use of a headset. The "B", or training system handset, was not being utilised. The ADC was unable to override the trainee without actually taking over the ADC handset from the trainee.

The DEP controller was not aware of the training on the ADC position and the ADC controller was not aware of the training on the DEP position.

Recorded radar data indicated that separation reduced to 1.4NM with a vertical separation of 200ft. The aircraft were separated by 1.5NM while at the same level. Minimum required radar separation for these aircraft while not 1,000ft vertically separated was 3NM.

The procedures in use were the standard procedures applicable for the use of runways 03/06 for departing aircraft prior to first light. These procedures required the issue of departure instructions for aircraft departing at night or in IMC to be in the form of a SID. It was not permissible for a controller to cancel a SID and issue a radar departure instruction while the aircraft is on the ground prior to first light. It was only permissible in these circumstances to cancel the SID after the aircraft was airborne and had reached the Minimum Vectoring Altitude (MVA). In the case of the 03 BIU2 SID, this situation provided a very short window of opportunity between the aircraft reaching the MVA and commencing a right turn off runway heading. Reliance on controller intervention to cancel this SID and issue a maintain runway heading instruction in order to maintain separation with SID departures from RWY 06 provided no separation assurance.

The aerodrome controller cleared VH-CZO for a night-time take-off from the Brisbane runway 01, believing that VH-OGG, after landing, had vacated the runway onto the high-speed taxiway A4S. At the time, CZO was at the A7 holding point. The crew of OGG alerted the controller (and the crew of CZO) that they were on the active runway. Shortly after, the controller asked the crew to confirm that they were on taxiway A4S and was advised that they were not. Aware that CZO had not lined up on the runway, the controller did not cancel the take-off clearance, but monitored the situation until OGG vacated the runway at taxiway A4.

The high-speed taxiway A4S was 1,900 m from the runway 01 threshold, and taxiway A4 was 2,310 m from the 01 threshold. Taxiway A4 required that the crew turn the aircraft through 90 degrees to exit the runway.

Taxiway A4S was equipped with uni-directional centreline lighting, which was not visible from the control tower. This made it difficult in conditions of reduced visibility for controllers to determine that an aircraft had vacated the runway and was on the high-speed taxiway. However, in visual meteorological conditions at night, it was not normal practice for controllers to ask crews to report when their aircraft was clear of the runway.

The Manual of Air Traffic Services (MATS) 6-3-1 paragraph 2 stated:

"When take-off or landing separation is based on the position of the preceding landing or taxiing aircraft and visual determination, particularly at night or in reduced visibility, is limited by poor azimuth resolution or other factors, the pilot of that aircraft shall be instructed to report when the aircraft has:

1. crossed and is clear of a runway intersection; or
2. stopped short of a runway strip; or
3. vacated the runway."

The MATS 6-2-3, paragraph 31 stated:

"Before clearing an aircraft for take-off, and immediately before take-off is commenced, the tower controller shall make a visual check from the control tower to determine as far as practicable, that the take-off path is not obstructed. If the take-off path is obstructed, take-off clearance shall be withheld or cancelled as appropriate, until the obstruction no longer exists."

The prescribed separation standard was detailed in MATS 6-3-4 paragraph 24. The standard required that a departing aircraft shall not be permitted to commence take-off until the landing aircraft has vacated and is taxiing away from the runway.

The controller cleared CZO for take-off using the provisions of MATS 6-2-3 paragraph 32. This paragraph stated:

"Take-off clearance need not be withheld until prescribed separation exists if, in the opinion of the controller, no collision risk exists and there is reasonable assurance that separation will exist when the aircraft commences take-off roll."

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 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 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.

Sequence of events

The Short Bros SD 3-60-300 (Shorts) was being radar vectored by air traffic control for an instrument landing system (ILS) approach to runway 15 at Cairns.

The weather conditions were fluctuating about the landing minima with low cloud and rain passing through the local area in general "stream" conditions. The automatic terminal information service was quoting a cloud base of 1,000 ft with lower patches and visibility reduced to 5,000 m in passing showers. The conditions were observed to deteriorate during periods of heavy rain showers and the air traffic control tower staff updated each crew as appropriate.

As the Shorts intercepted the final approach path at 14 NM from touchdown, the aerodrome controller informed the approach controller that the weather had deteriorated at the aerodrome and that there was a likelihood that the Shorts would carry out a missed approach. The approach controller informed the crew and transferred them to the tower frequency so that the aerodrome controller could provide timely updates of the changing weather situation.

At 0744:05 Eastern Standard Time, the pilot of a Cessna 208 (Cessna) reported ready to depart on the aerodrome control frequency and was instructed to line-up. At that time, the Shorts was approximately 9 NM from touchdown with a ground speed of 120 kts.

The aerodrome controller received a departure instruction of "Left 360 unrestricted" from the approach controller (who was also providing the departure service). The pilot of the Cessna was then given a take-off clearance in accordance with that instruction but with an additional instruction to remain on the aerodrome controller's frequency when airborne. The aircraft commenced take off roll at approximately 0745:00; when the Shorts was approximately 7.5 NM from the runway 15 threshold.

At 0745:05, the crew of the Shorts was informed that the rain was increasing at the field and that the runway lights were on stage 6; the maximum intensity.

At 0746:04, the pilot of the Cessna was asked to expedite his climb through 2,000 ft and, at 0746:30, the crew of the Shorts was cleared to land.

At 0748:29, the pilot of the C208 apologised for the slow rate of climb and commented that he was "... just doing his best". At about the same time, the crew of the Shorts commenced a missed approach and, at 0748:39, they were instructed to maintain 1,500 ft and informed that there was "... traffic abeam you now at the 9 o'clock position". That transmission by the aerodrome controller was followed, at 0748:56, by a broadcast to the pilot of the Cessna saying "... caution traffic in the missed approach".

At 0749:07, the aerodrome controller updated the traffic information to the crew of the Shorts with "... caution, the traffic is in your 12 o'clock position at half a mile". The reply was garbled and included the statement "... we are maintaining one thousand at the moment..." which was followed by words that were not completely discernible but included "... one thousand five hundred..."

At 0749:27, the pilot of the Cessna was instructed to report leaving 1,500 ft and replied that he had left that altitude. A subsequent radar analysis indicated that the aircraft was climbing through 1,550 ft at that time.

At 0749:40, the approach controller checked with the aerodrome controller to ascertain his plan for separation. The reply was that there was "... not much to do". However, at 0749:51, the aerodrome controller broadcast to the crew of the Shorts that radar indicated traffic in the same position at the same level. The reply was that the crew had sighted the Cessna and were passing that aircraft.

Radar analysis indicated that the aircraft had passed at approximately 0749:50 with a minimum horizontal distance of about 70m and a vertical displacement of between 100 - 200 ft. The required separation standard was either 3 NM horizontally or 1,000ft vertically. There had been an infringement of separation standards.

Missed approach and departure procedures

Due to the constraints of terrain surrounding the Cairns aerodrome, both the missed approach and departure procedures required tracking in a 40 degree sector to the north-east. The missed approach required an initial climb straight ahead to the Middle Marker, then a climbing left turn onto a heading of 030 degrees to intercept the 045 radial of the Cairns VOR (Very High Frequency navigation aid), with a climb to 4,000 ft or a level assigned by air traffic control.

The standard instrument departure required a left turn at the earlier of 400 ft or the departure end of the runway, onto an assigned heading between 350 - 030 degrees. The lowest altitude for radar vectoring in this sector was between 1,000ft and 3,300ft depending on the precise position of the aircraft at the time.

This combination of tracking requirements resulted in a guaranteed tracking conflict whenever a missed approach was commenced when a departing aircraft was within 3NM of the aerodrome. Furthermore, an infringement of separation standards would occur whenever a missed approach was carried out in instrument meteorological conditions without vertical separation being established.

Separation standards

Air traffic controllers were limited to a minimum altitude of 1,500 ft for terrain clearance in the case of a missed approach. To establish the vertical separation standard of 1,000 ft (or 500 ft in an emergency), a departing aircraft would have to be at or above 2,500 ft (or 2,000 ft in an emergency). When the Shorts was established in the missed approach turn, it was climbing through 700 ft while the Cessna had left 1,300 ft.

The radar standard was not achieved as the tracks crossed each other approximately 2NM north-east of the runway. As the aircraft had to track in the same narrow sector of airspace, it would have been some minutes before a radar standard could be established.

Visual separation was not an option due to the prevailing weather conditions. The aerodrome controller could not see either aircraft as they came into conflict.

Lateral separation - the Manual of Air Traffic Services section 6-4-3 stated "lateral separation is considered to exist between an arriving aircraft that subsequently commenced final approach, and a departing aircraft that has been cleared on a segregated flight path". That is, a situation where the departing aircraft will not be manoeuvring within 45 degrees of the reciprocal of the final approach path while an aircraft is on the final approach path. The assigned heading of 360 degrees for the Cessna did not comply with that standard.

Longitudinal separation standards did not apply because they required distances greater than those required for radar standards.

Aircraft performance

The Shorts, a 36 passenger aircraft, had seven persons on board and very little freight. Consequently, when the missed approach was commenced, the crew attained a rate of climb of 700 - 800 ft/min. They stated that 300 - 600 ft/min was their expected rate of climb.

The Cessna was at maximum take-off weight and, because of the heavy rain, the pilot had selected the Inertial Separation Handle to bypass mode. That operation helped to divert heavy rain droplets around the engine so that the risk of flame-out was reduced.

When airborne, the pilot of the Cessna experienced severe turbulence and downdrafts, along with buffeting from the gusty wind and heavy rain. The combination of the prevailing weather conditions and the selection of bypass mode resulted in a degraded climb performance from that normally expected. Radar analysis indicated that the Cessna had an average rate of climb of 400 ft/min from take-off to the point of closest proximity and, at times, a rate of climb near zero. The pilot stated that he expected a rate of climb between 800 - 1,200 ft/min.

Air traffic control procedures

The Manual of Air Traffic Services (MATS) section 6-4-3 specified the procedures to be applied in the case of an arriving aircraft and an aircraft taking off. It stated that "a departing aircraft may be permitted to take off during the period before an arriving aircraft will commence its final approach" but goes on to say that such take-off clearance "... is conditional upon the application of separation after take-off is commenced".

Final approach was defined as 8NM from the runway 15 threshold. The aerodrome controller may have complied with the distance requirement as the Cessna was cleared for take-off when the Shorts was approximately 8NM from the threshold.

Cairns Local Instructions (TWR - 29) reinforced the provisions of MATS in the specific instance of the conflicting missed approach and departure headings from runway 15. They stated that "... consideration must be given to increase the cut-off distance used between the landing aircraft and the aircraft commencing take-off when weather conditions are such that visual or radar separation of the overshooting aircraft and departing aircraft cannot occur". Local Instructions did not specify clearly who had the responsibility for separating the missed approach from the departing aircraft. Approach control was responsible for the airspace but the aerodrome controller had to advise the approach controller of the most appropriate heading consistent with the ability to provide separation with other airborne traffic.

Cairns controllers operated in such a way that the aerodrome controller separated an aircraft on the missed approach path with other traffic, using visual separation until an alternative standard could be achieved. As the weather conditions precluded such an option, a specific alternative was required. Tower controllers were not rated to provide radar separation and could only use the radar display "for information". However, Local Instructions specified that it was the aerodrome controller's responsibility to ensure that radar separation existed between an aircraft on short final and not yet in sight and an aircraft becoming airborne.

MATS 6-5-1 allowed a tower controller to provide an uncoordinated radar vector to initiate separation in cases such as a missed approach. However, MATS required Local Instructions to specify the details. Cairns Local Instructions did not specify any such details.

Aerodrome controller

The controller had considered extending the cut-off distance but believed that, as the Shorts had not commenced final approach, he could safely clear the Cessna for take-off. He had witnessed numerous departures by Cessna 208 type aircraft and had an expectancy that the aircraft would climb at a rate which would enable vertical separation to be easily achieved if the Shorts commenced a missed approach. He also expected the Cessna to proceed at a speed that would position that aircraft well ahead of the Shorts at the crossing point of the departure track and the missed approach path.

Although the approach controller had nominal responsibility for the airspace, the aerodrome controller had assumed separation responsibility when he retained the pilot of the Cessna on his radio frequency for departure. As the Cessna departed, the controller observed that it did not turn in accordance with the standard instrument departure instructions but continued for approximately half a mile before commencing the turn. He then noticed that the rate of climb was not as good as he had expected and, at 0746:11, asked the pilot to expedite through 2,000 ft and report leaving 2,000 ft. That instruction was to maintain his traffic management plan of achieving vertical separation with any missed approach procedure and would have provided a 500 ft emergency standard if he maintained the Shorts at 1,500 ft in the missed approach.

As the aircraft closed to within 1.5 NM, the controller gave traffic information to both crews. However, that information did not include the aircraft type, or height, or relative height. The information was only position and distance to the crew of the Shorts and included the words "... traffic abeam you now at the 9 o'clock position". That information was incorrect as the Shorts was in a left turn and the traffic was actually in the 12 o'clock position moving towards the 2 o'clock position. The pilot of the Cessna was advised "Caution, traffic on the missed approach". The controller asked the other controllers in the tower at the time for advice but they were unable, in the time available, to offer an alternative course of action.

When the crew of the Shorts made a broadcast that they were "... maintaining 1,000 (feet) at the moment..." in the middle of a transmission that was broken and partly unintelligible, the controller neither questioned the crew as to their mention of the words "... one thousand five hundred..." during that broadcast nor did he issue an altitude instruction. However, he did obtain an altitude report from the pilot of the Cessna which indicated that the aircraft had left 1,500 ft on climb. The controller's subsequent conversation with the approach controller indicated that he was satisfied with the separation standard saying "...(the Shorts) supposed to be maintaining 1,000 (ft) the other has left 1,500 (ft) ...".

The controller was of the opinion that the altitude read outs from the Shorts were varying so much that he did not know that the aircraft had not maintained 1,000 ft. Radar analysis of the readouts indicated a relatively steady increase in height over the 90 seconds from 0748:30 - 0750:00 but with two "spikes" at approximately 1,100 ft (at approximately time 0749:07) and 1,500 ft (at approximately time 0749:45).

Approach controller

The approach controller had issued the departure instruction for the Cessna believing that, as the Shorts was about 8NM from touchdown, the aerodrome controller would not depart that aircraft until after the arrival, or that the Shorts was in sight and reasonably assured of a landing. However, the terminology used by the aerodrome controller "next CYC" indicated that the Cessna would commence take off within 1 minute of the receipt of a departure instruction.

As the situation developed, the controller monitored the radio frequency of the aerodrome controller to observe the plan for separation. He had heard the requirement to remain with the tower given to the pilot of the Cessna and considered that the aerodrome controller had accepted the responsibility for separation with this action. As the radar display indicated that the aircraft were on conflicting tracks and at similar altitudes, other controllers asked what separation was being applied. The approach controller replied that the aerodrome controller was providing the separation but elected to prompt the tower with the question "you right?". This coordination occurred at 0749:40, approximately 10 seconds before the point of closest proximity, and was the first contact between the controllers since the departure instruction for the Cessna was issued.

Emergency training

The Cairns air traffic controllers had not received regular training in emergency or unusual situations. Tower controllers had last undertaken formal refresher training in October 1997. However, the aerodrome controller had been absent on recreational leave and did not attend the training. He had received formal tuition in dealing with similar circumstances during his aerodrome control training in December 1997 and January 1998.

The aerodrome controller stated that he had experienced only one missed approach due to poor weather in his 3 years at Cairns. As the incident developed, he asked for advice from his fellow tower controllers but as they were occupied with their own tasks, they were unable to give a properly considered response.

Pilot of the Cessna

The pilot was conducting a single pilot operation and had intended to comply with the requirements of the standard instrument departure by turning left at 400 ft or the departure end of the runway. However, when airborne, the initial climb did not proceed as well as expected, with severe turbulence and heavy rain buffeting the aircraft. The airspeed was not increasing as quickly as he would have liked and at the upwind end of the runway the aircraft had only reached an altitude of 200 - 250 ft. Consequently, the pilot elected to continue on runway heading until a more stable climb was achieved. The left turn was commenced at an altitude of 300 ft and an indicated airspeed of approximately 70 kts. There was no broadcast to air traffic control indicating the variation to the standard procedure.

Once the aircraft had turned onto the assigned heading of 360 degrees, the pilot made every effort to maximise the rate of climb but was limited by the aircraft performance in turbulent weather conditions. At 0748:29, he broadcast to the aerodrome controller that he was experiencing a slower than normal rate of climb.

When, at 0748:56, he was given conflicting traffic information, he attempted to sight the other aircraft but found visibility limited in cloud and heavy rain. Shortly after, the aircraft broke into a small clear patch and the pilot saw the Shorts just below and marginally to his left. He estimated that the aircraft would pass just behind his own and elected not to take any evasive action.

Crew of the Shorts

The co-pilot was the flying pilot and, as the aircraft approached the minima, the crew found themselves in cloud, heavy rain and subjected to severe turbulence culminating in their decision to commence a missed approach.

The crew's main preoccupation was to ensure the safe climb-out of their aircraft in the left turn required by the procedure and, as they were experiencing instrument meteorological conditions, their first priority was to fly the aircraft. Consequently, they did not immediately inform air traffic control of their commencing a missed approach. Before they could broadcast any details, the aerodrome controller issued an instruction for them to maintain 1,500 ft and passed traffic information. As they were still in cloud and rain, their lookout was both occasional and of limited effect.

A short time later, the controller issued an updated traffic alert indicating that the other aircraft was half a mile ahead. The response from the pilot in command was that they were maintaining 1,000 ft temporarily but still climbing to 1,500 ft as cleared by air traffic control. The pilot in command had intended to maintain 1,000 ft after the traffic information had been passed but the co-pilot did not hear the instruction as it was said during the radio broadcast to air traffic control and during a period of intense flying activity. As the pilot in command was about to reiterate the maintain 1,000 ft instruction to the co-pilot, the aircraft broke into the same clear patch as the Cessna and they saw that aircraft ahead and slightly above. The co-pilot levelled the aircraft momentarily to ensure that they would pass beneath the Cessna and then continued the climb to 1,500 ft when established clear of it.