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.
Although the Keppel controller intimated to the crew of UUQ that a minor reduction in airspeed would assist in the sequencing into Mackay, the crew either did not appreciate the need for a speed reduction or ignored the information. If the Keppel controller had advised the Swampy controller that the crew of UUQ had been forewarned of the likelihood of the imposition of a speed reduction, or had the crew reduced speed when advised, the loss of separation and subsequent situation may not have occurred.
The Swampy controller, despite not being endorsed for the sector, was only required to use standards and procedures that were commonly used on the Daintree sector. However, he did not issue the required instructions to either crew to establish separation using pilot visual separation procedures. The infringement of separation standards may not have occurred if the controller had issued instructions to either crew to follow the other aircraft and confirmed their ability to do so, or alternatively, issued control instructions to ensure that either vertical or radar separation was maintained until at least one aircraft exited controlled airspace.
The controller was probably more fatigued than he believed and he may have endeavoured to operate with minimal restrictions and/or control instructions in a situation which he thought was readily appreciated by both crews. The time of the day (early morning) and the resultant circadian disrhythmia may have also adversely affected his performance. As a consequence of the small amount of aircraft activity in the area at the time, the controller may have also been lulled into being less situationally aware than normal.
The lack of appreciation of the situation by the crew of UUG following the Keppel controller's suggestion to reduce speed and the acceptance by both crews of visual separation responsibility when the forecast indicated IMC in the area, were indicators that their performance may have also been sub-optimal. The limited guidance available in the AIP in relation to the assignment of separation responsibility to pilots does not clearly indicate pilot and controller responsibilities in such cases. Provision of additional information in the AIP would probably assist pilots in their decision making regarding the safe conduct of a flight.
The perceived benefit of transferring separation responsibility to pilots rather than imposing alternate air traffic control standards and procedures requires careful consideration by both controllers and pilots. This is more so the case for night operations when en route or terminal weather conditions may not be readily apparent. Additionally, any forecast that indicates that instrument meteorological conditions may be encountered should be an alert to crews and controllers to be wary of the use of visual separation procedures. The issued forecasts indicated potential instrument meteorological conditions for the Mackay terminal area and it would have been prudent for the pilots to request alternate separation procedures from the controller.
Local safety action
One of the operators involved in the occurrence issued a safety article to crews notifying the limitations in the use of pilot sight and follow procedure and reminding them to carefully consider the situation prior to participating in the application of visual separation.
Australian Transport Safety Bureau safety action
Following the review of a number of occurrences in which the use of visual separation criteria was investigated, there was insufficient evidence to support a recommendation. However, there were a number of concerns in relation to the guidance, for the use of visual separation procedures, provided to flight crew. Consequently, the following was sent to the Civil Aviation Safety Authority (CASA) on 27 October 1999:
"Subject: Aeronautical Information Publication Guidance for Operations within CTA and GAAP Aerodrome CTRs The AIP ENR 1.1 - 32 paragraph 2 1. 1.1 details a pilot's responsibilities in relation to operations in GAAP control zones including a requirement to:
"advise ATC if unable to sight, or if sight lost of, other aircraft notified as traffic." The AIP ENR 1.3 - 2 paragraph 3.2.1 details the requirements for the provision of separation in controlled airspace (excluding GAAP CTRs) and includes a special provision of:
"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."
This latter entry would appear to be inconsistent with the GAAP entry in that it does not provide any guidance or provisos which require air traffic control to be advised by the pilot when they have lost sight of a previously reported and sighted aircraft (or one that they were following).
Any pilot requirements in relation to the application of visual separation in the AIP should be consistent with the visual separation criteria in the Airservices Australia Manual of Air Traffic Services and should include:
- the requirement for pilots to advise air traffic control when they are unable to maintain sight of an aircraft, and
- the issue of traffic information to the pilot of an IFR aircraft that is subject to the application of visual separation.
I therefore request that these aspects be considered for future AIP amendments to ensure a consistent approach to the application of visual separation, both inside and outside controlled airspace, and between the AIP and the MATS."
CASA responded on 5 January 2000 and advised:
"I refer to your letter BS9710004 of 27 October 1999 in which you requested that aspects of visual separation criteria contained in MATS be considered for future amendment of AIP. In response to your request, the following amendments will be incorporated in AIP by the next amendment list. ENR 1.3 - 3 sub-paragraph 3.2.1 d. will be amended to include:
- the requirement for pilots to advise ATC when they are unable to maintain, or have lost, sight of an aircraft, and
- the advice that, where an aircraft has been instructed to maintain separation from, but not follow, an IFR aircraft, ATC will issue traffic information to the pilot of the IFR aircraft including advice that responsibility for separation has been assigned to the other aircraft."
|Date:||01 March 2000||Investigation status:||Completed|
|Time:||0400 hours EST|
|Location:||93 km SSE Mackay, (VOR)|
|State:||Queensland||Occurrence type:||Loss of separation|
|Release date:||07 September 2000||Occurrence class:||Airspace|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
Aircraft 1 details
|Aircraft manufacturer||Fairchild Industries Inc|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Nil|
|Departure point||Brisbane, QLD|
Aircraft 2 details
|Aircraft manufacturer||Fairchild Industries Inc|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Nil|
|Departure point||Rockhampton, QLD|