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Two Fairchild Industries Inc S227 aircraft, VH-EEP and VH-UUQ, operating under instrument flight rules (IFR) were inbound to Mackay at approximately 0408 eastern standard time. EEP was from Rockhampton, maintaining FL140 and was being followed by UUQ on the Brisbane track at FL160. Near the descent point, approximately 55 NM south of Mackay, UUQ was above and abeam EEP when the Swampy sector controller issued instructions for the crew of EEP to leave control area on descent. The lower level of controlled airspace was 4,500 ft. About 1 minute later the crew of UUQ requested descent and advised the controller that they had EEP in sight. The controller instructed the crew to descend to FL140 and then to FL130. As EEP was descending through FL130, as indicated by the aircraft's Mode C altitude readout on the controllers' radar display, the lateral distance between the two aircraft reduced to 4.5 NM while there was less than the required vertical separation standard of 1,000 ft between them. The required radar separation standard was 5 NM. There was an infringement of separation standards.

The controller advised the crew of EEP that "traffic is UUQ". That was acknowledged by the crew who also advised that they had UUQ in sight. The air traffic system short term conflict alert activated and the controller queried both crews with respect to their ability to maintain their own separation on descent. Both crews acknowledged and advised that they could maintain their own separation. At this stage UUQ was ahead and above EEP. The controller instructed the crew of UUQ to leave control area on descent. Shortly after both crews reported transferring from the Swampy sector frequency to the Mackay mandatory broadcast zone frequency. As EEP was passing 7,000 ft it entered instrument meteorological conditions and the crew lost sight of UUQ. The crew of EEP contacted the crew of UUQ to establish the relative positions of the aircraft and found that the UUQ crew had descended their aircraft so that it was below EEP. The crew of EEP reduced power and manoeuvred their aircraft in an endeavour to increase the lateral spacing between them and UUQ. The aircraft subsequently landed at Mackay.

Both the area forecast and the Mackay terminal area forecast indicated the possibility of instrument meteorological conditions below 10,000 ft during the period when the aircraft were expected to be in the area.

The controller managing the Keppel sector, adjacent to the southern boundary of the Swampy sector, controlled the aircraft before the crews transferred to the Swampy sector. The Keppel controller noted the similar groundspeed readouts from the aircraft and queried both crews with respect to their respective indicated airspeeds. The crews both advised their indicated airspeeds as 205 kts. The controller advised the crew of UUQ that as they were about 2 NM behind EEP and that the next sector would probably make them second in the arrival sequence and, "if you would like you can start to reduce speed back to about 20 kt groundspeed reduction would probably fit you nicely behind". The crew acknowledged the transmission and advised that they had that aircraft in sight and were gaining on it.

The Keppel controller did not instruct the crew of UUQ to reduce speed and was not responsible for arranging the arrival sequence into Mackay. If the controller had issued such an instruction the crew of UUQ would have been required to read back and comply with the speed requirement. The Keppel controller informed the crew that he would advise the next sector that they had EEP in sight and subsequently told the Swampy controller. The Keppel controller did not advise the Swampy controller that he had pre-warned the crew of UUQ to possibly expect a speed requirement for sequencing. The investigation did not establish why the crew of UUQ did not reduce speed.

The crew of EEP was operating on the Keppel sector frequency and heard the advice passed by the controller. The crew later reported that they did not hear the response from the crew of UUQ but expected that aircraft to be following them on arrival into Mackay.

Due to the early hour, the Swampy sector was not busy and there was little other traffic in the area. The controller at the Swampy sector was endorsed and rostered for duty on the adjacent Daintree sector, but was not endorsed for the Swampy sector. The rostered and endorsed controller for the Swampy sector had left the position for a break. In that situation it was normal practice for the controller managing the adjacent position to monitor the radio frequencies and communication links while the position was vacant. If a radio or coordination call occurred, an appropriately endorsed controller would be recalled to operate the position. Immediately prior to and during the occurrence, the non-endorsed Swampy controller did not recall the other controller. Although the controller was not endorsed on the Swampy sector, the standards, procedures and techniques used were common to both Swampy and Daintree sectors. The controller should have been capable of maintaining separation using either radar, vertical, lateral, longitudinal or visual standards or a combination of these standards. The controller later reported that he believed that visual separation was being applied and consequently did not ensure that radar or vertical separation standards were maintained while the aircraft descended.

The Manual of Air Traffic Services (MATS 4-5-1) detailed how responsibility for separation may be assigned to a pilot using visual separation. For arriving aircraft above FL125 a controller was to instruct the pilot of one of the aircraft involved to follow and track behind the other aircraft, provided the pilot has reported sighting the aircraft and at least one of the aircraft is on descent. This was particularly so when the following aircraft was faster. In this case, a controller should have confirmed that the pilot was capable of following the slower aircraft. The Swampy controller did not instruct either crew to follow the other aircraft nor did he confirm whether the pilot of UUQ could follow EEP. Prior to a controller issuing any control instruction requiring a pilot to keep an aircraft in sight, the controller should consider a number of aspects that may limit a pilot's ability to comply. One of the aspects related to restrictions on atmospheric visibility that may not have been apparent to the pilot.

The Aeronautical Information Publication (AIP) contained a number of references about the application of visual separation. The references were different to what was in MATS. Also, the reference that related to controlled airspace provided little guidance to assist pilots in the application of visual separation. AIP ENR13-3 paragraph 3.2.1.d stated, "under certain conditions, the pilot of one aircraft may be given the responsibility for separation with other aircraft. In this circumstance, the pilot is also responsible for the provision of wake turbulence separation".

Controllers were responsible for assessing their fitness for an operational shift and if there were any doubts they were expected to notify a supervisor. The controller managing the Swampy sector had recently experienced some difficulties with obtaining satisfactory rest during his time off at home and had also been involved in a traffic incident the evening prior to the occurrence shift. The controller later reported that at the time he believed that he was capable of undertaking the shift despite his recent experiences.

Human performance varies during the day, tending to correspond with the body's circadian rhythm. Generally, the standard of human performance of some tasks decreases during the early morning hours. The reduction in performance is separate to that observed due to sleep deprivation. Additionally, an individual's ability to recognise the on-set of fatigue or a reduction in performance diminishes with fatigue and low points in the circadian rhythm.

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