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.
