Boeing 737-476, VH-TJH

FACTUAL INFORMATION

On 17 July 2004, at about 1619 eastern standard time, a Boeing Company 737-476 (737), registered VH-TJH, was inbound to Hamilton Island from the south-east for a landing on runway 14. The Hamilton Island Aerodrome Controller (ADC) instructed the crew to descend to 4,000 ft above mean sea level (AMSL) due to the pending departure of a Boeing Company 717-200 (717), registered VH-VQB, from runway 14. The crew of the 737 requested and were approved by the ADC to track for a left downwind to runway 14. The ADC instructed the crew of the 717 to maintain 3,000 ft AMSL, to make a right turn to track to Mackay and that they were clear for takeoff. The weather was visual meteorological conditions (VMC) and the crew of the 737 reported to the ADC that they could see the 717. The ADC instructed the crew of the 737 to make a visual approach to left base that was amended to a right base after the crew requested that change. Shortly after intercepting the outbound track at about 2,000 ft, the crew of the 717 received a traffic alert and collision avoidance system (TCAS) traffic advisory (TA) and saw the 737 crossing from left to right on descent. The 717 crew's perception was that the expected track of the aircraft would place them on, or close to a collision course so they turned left and descended to avoid the 737 by passing behind it.

Analysis of air traffic control recorded radar data and aircraft flight data revealed that at 1619:15, after the 717 had turned left, the lateral and vertical distance between the aircraft was 1,112 m and 700 ft (737 above the 717). Both aircraft were fitted with a traffic alert and collision avoidance system (TCAS). The flight data recorder (FDR) in the 717 was only capable of recording TCAS resolution advisory (RA) parameters while the 737 FDR did not record any TCAS parameters. Data from the 717 revealed that there was no TCAS RA. The crew of the 717 changed the aircraft's heading by about 35 degrees and descended to 1,500 ft during the manoeuvre, before returning to their assigned track and climbing to 3,000 ft.

Hamilton Island Air Traffic Control

The ADC was responsible for providing air traffic control services in Class D airspace from ground level to 4,500 ft AMSL. In Class D airspace, air traffic control (ATC) is required to separate aircraft operating under the instrument flight rules (IFR) from other aircraft operating under the IFR or special visual flight rules (VFR). In addition, ATC is required to provide the crews of aircraft operating under the IFR with traffic information about aircraft operating under the VFR. The 737 and 717 were both operating under the IFR.

The circumstances were not related to any national airspace changes as both aircraft were IFR and in airspace being managed by ATC.

Clearances and separation

The pilot of the 737 had been issued with a clearance by the Brisbane Centre controller to track inbound to Hamilton Island via the 143 radial of the Hamilton Island VOR¹. The ADC issued a clearance to the pilot of the 717 to track via the 157 radial of the Hamilton Island VOR to Mackay and then the planned route to Sydney. The Aeronautical Information Publication En Route Supplement, Hamilton Island special procedures, nominated a right circuit for operations to runway 14. The use of runway 14 and the issued clearances would result in the aircraft tracks intersecting at some stage (see Figure 1).

Figure 1: Hamilton Visual Terminal Chart with aircraft tracks and times.

The ADC was aware of this and assigned altitudes to the crews that provided the 1,000 ft vertical separation standard required between two aircraft operating under the IFR. The application of that standard was necessary until an alternative separation standard was in place.

When a pilot of an arriving aircraft has been approved by ATC to make a visual approach, they are required to track in accordance with the assigned track clearance until within 5 NM of the aerodrome. From 5 NM, the pilot can diverge from the inbound track to join the circuit as directed by ATC for an approach to the nominated runway.

Air traffic control (ATC) visual separation standards and procedures

The Manual of Air Traffic Services (MATS) details the standards and procedures to be used by controllers to separate aircraft. Part 4, Section 1, Separation Standards stated that in the provision of separation, controllers shall place greater emphasis on traffic planning and conflict avoidance than on individual conflict resolution being achieved. This is to enable separation assurance to be achieved through planning traffic to ensure separation, executing the plan to achieve separation and monitoring the situation to ensure that the plan and the execution are effective. Section 5, Visual Separation stated that visual separation could be achieved by the use of visual procedures (by controllers) or by assigning visual separation responsibility to a pilot. Controllers are to consider aircraft performance characteristics when applying visual separation.

The application of visual separation by the ADC or either crew would have been an appropriate alternative to the vertical separation standard. When aircraft are operating at or below flight level (FL) 125² and will continue to do so during the application of visual separation by a pilot, the pilot of one aircraft is required to report sighting the other aircraft and has to be instructed by a controller to maintain visual separation with, or to follow, that aircraft. Also, if a pilot has been instructed to maintain separation from, but not to follow an IFR category aircraft, then the controller is required to provide traffic information to the pilot of the IFR category aircraft. That information should contain as much detail as possible including the aircraft type, altitude or flight level, position and intentions or direction of flight. If there is any doubt about a pilot's ability to either keep another aircraft in sight or to maintain separation, a controller shall issue alternative instructions to provide separation.

The ADC's reported expectation was that the aircraft tracks would cross such that the 737 would pass behind the 717. At 1615:15, when the 717 was lining up on the runway, the ADC advised the crew of the 737 that the 717 was due to depart direct to Mackay on climb to 3,000 ft and that they could expect to maintain 4,000 ft until the 717 was sighted [by the 737 crew]. The ADC did not advise either crew of the intention, after the 717 was seen by the crew of the 737, to assign separation responsibility to that crew and have them pass behind the 717 (see Appendix A.

While the 717 was taking off, the crew of the 737 reported approaching 4,000 ft and requested a clearance to track for left downwind. The ADC instructed the crew to track for left downwind. At 1618:26, the crew reported to the ADC that they could see the 717. The ADC issued a clearance for the crew to make a visual approach and to report turning a left base. The crew acknowledged the instruction and requested to track to a right base for the runway. The ADC instructed the crew to track for and to report on right base. At 1619:00, the ADC instructed the crew of the 717 to climb to FL310. A pilot approved to make a visual approach can descend as required to establish an aircraft on base or final to the assigned runway. The ADC's instruction to the crew of the 737 to make a visual approach and the instruction to the crew of the 717 to climb, cancelled the separation assurance provided by the application of the vertical separation standard.

Controllers can separate aircraft by visual observation of aircraft position and projected flight paths. The MATS advised that in providing visual separation, controllers should rely primarily on azimuth and if visual separation by judgement of relative distances or altitude is used, then it should be with such wide margins that there is no possibility of the aircraft being in close proximity. The MATS notes that 'experience has shown that a controller's visual determination of the relative distance of aircraft in close proximity can be in error, even to the extent of an apparent reversal of the positions of the two aircraft.' The convergence of the aircraft's respective tracks meant that the lateral spacing and the distance between them, from the ADC's perspective, was reducing.

At 1619:10, approximately 5 seconds before the crew of the 717 elected to turn to avoid the 737, the ADC queried the crew of the 737, 'just confirm that you will be passing behind the 717'. That query was the first time the ADC had mentioned to them that they were required to pass behind the 717. As they replied that they could pass behind, the pilot rolled the 737 right to a maximum bank angle of 26 degrees and 2 seconds later reversed the roll to the left to a maximum bank angle of 15 degrees in conjunction with a left turn when they saw that the 717 was turning away.

At the time of the occurrence, the ADC was managing the two jets and two other light aircraft operating remote from the Hamilton Island Airport area.

Rules of the air

The Civil Aviation Safety Authority Regulation 161 - Right of Way, states that 'an aircraft that is required by the rules in this Division to keep out of the way of another aircraft shall avoid passing over or under the other, or crossing ahead of it, unless passing well clear' and that 'the pilot in command of an aircraft that has the right of way must maintain its heading and speed, but nothing in the rules in this Division shall relieve the pilot in command of an aircraft from the responsibility of taking such action as will best avert collision'.

Regulation 162 - Rules for Prevention of Collision, states that 'when 2 aircraft are on converging headings at approximately the same height, the aircraft that has the other on its right shall give way' and 'when two aircraft are approaching head-on or approximately so and there is danger of collision, each shall alter its heading to the right'.

717 flight crew

The copilot was the pilot flying (PF) and the pilot in command (PIC) was the pilot not flying (PNF). They reported that they were both aware of the inbound 737 and understood the ADC's application of the vertical separation standard. The crew thought that the 737 was probably inbound from Brisbane. Prior to departure, the crew set both cockpit navigation displays³ to 10 NM range in accordance with company procedures.

After take-off, the crew saw on the 717's TCAS display that the 737 was presented as other traffic at about the 11 o'clock position⁴ at approximately 7 NM At that stage they could not see the 737. Shortly after retracting the aircraft's flaps from the take-off position of 18 degrees, they saw the 737 as proximate traffic, slightly above them, on the TCAS. They then saw the 737. The crew became concerned because:

• the 737 was stationary in their windscreen
• the vertical distance between the aircraft was reducing
• the track of the 737 did not appear to be changing
• the 737 was unlikely to be able to pass behind them.

They were also not sure of what action, if any, the crew of the 737 was taking. The copilot had previously reduced the rate of climb because he thought the rate of closure between the aircraft may cause a TCAS alert. The level of concern was such that the PIC instructed the copilot to turn left immediately, and he complied. During the turn the copilot descended the 717 and during that phase the crew received a slats 'overspeed exceedance warning'. The crew reported that the exceedance was 8 kts.

The time between the crew being issued with a clearance to take off and turning left from their outbound track was about 2 minutes.

737 flight crew

The PIC was the PF and the copilot was the PNF. They reported that they understood the effect of the ADC's altitude limits. They saw the 717 back track along the runway and line up as the visibility was 'quite clear.' The crew were initially unconcerned with the departure of the 717 as it was in plain view.

The copilot requested a clearance to track for left downwind in error and 25 seconds later amended it to a request to track for a right base. As the aircraft turned left to track for right base, the 717 was almost stationary in their windscreen where previously, while they were established on the 142 radial, it had appeared to be moving slowly from right to left. They later reported that they eventually realised the potential for conflict after the 717 turned left as they attempted to comply with the ADC's instruction to pass behind it. They received a TCAS TA after the 717 had turned away.

During the period that the crew was requesting approval to track for downwind, the aircraft's flight data indicated that its airspeed was 250 kts and that it had turned left 10 degrees. At that time, it was 9 NM from the aerodrome.

Radiotelephony procedures

Pilots are required to notify receipt of the current terminal information on first contact with ATC, either when taxiing for departure or when inbound for landing. If that advice is not provided, a controller is required to either confirm receipt of the information by the pilot or else provide the current terminal information. Part of the information normally provided is the local altimeter setting. That setting is required by a pilot to enable an aircraft's altimeter to provide the height above mean sea level. It is also needed to ensure the correct application of vertical separation standards between two aircraft. The crew of the 737 did not report receipt of the Hamilton Island terminal information. The ADC did not confirm with the crew that it had been received, nor did the ADC provide the information.

When a pilot is assigned and required to maintain separation with a sighted aircraft, a controller is required to instruct that pilot to, 'maintain separation with (or pass behind or follow) and include details of the aircraft type or identification and any restrictions'⁵. The ADC did not instruct the crew of the 737 to maintain separation or to pass behind the 717 after they reported that they could see that aircraft.

Pilots are required to read back some clearances and/or instructions issued by a controller. Readback items include any altitude or level assignments. If a required readback is not provided by a pilot, a controller is required to challenge the pilot to read back the necessary item. Following the initial inbound report by the crew of the 737, the ADC instructed the crew to descend to 5,000 ft. The crew did not read back that altitude and the ADC did not challenge the lack of a readback. The crew also did not read back the subsequent clearance to make a visual approach. The ADC did not challenge the lack of that readback.

Studies conducted by the US National Transportation Safety Board found that controllers have a tendency to relax their level of alertness in a low workload environment, which makes them vulnerable to operational errors and omissions. Similarly, pilots have been known to relax attentiveness and vigilance when under ATC control.⁶

Traffic alert and collision avoidance system

The Honeywell Incorporated, TCAS II Pilot's Handbook⁷ describes TCAS as:

...an independent on onboard collision avoidance system. It is designed as a backup to the ATC system and the "see-and-avoid" concept⁸. The [TCAS] system is designed to provide safe separation between aircraft predicted to be on collision trajectories while minimising ATC clearance deviation or excursions.

The TCAS does not replace the ATC system. TCAS II continually calculates and tracks the projected positions of air traffic control radar beacon system transponder equipped aircraft within 20 NM and within altitudes 10,000 ft of the aircraft's altitude. The system then generates Resolution Advisories (RA) and Traffic Advisories (TA) against intruder aircraft with ATC transponders.

The level of traffic information displayed is subject to the limits of TCAS, the aircraft's cockpit display and pilot display selections.

A TA is generated for aircraft which are predicted to be within 20 to 48 seconds of the aircraft's collision area⁹, while an RA is generated for an aircraft that is predicted to be within 15 to 35 seconds of the collision area. The timing for an alert is subject to sensitivity levels and altitude layers set in the system. These layers and levels prevent the system from initiating a descent RA when below 1,100 ft above ground level (AGL). For the altitudes of the aircraft at the time of the occurrence, a TA would be generated 25 seconds before the collision area and an RA would be generated 15 seconds before the collision area.

During an RA event TCAS should provide between 300 ft and 800 ft of vertical spacing between the aircraft involved.

¹ VOR - VHF omnidirectional radio range navigation aid.
² 12,500 ft.
³ The displays include details of navigation aids, TCAS and the route to be flown.
⁴ The numbers on a clock are commonly used by a pilot to refer to the relative position of another aircraft. For example, an aircraft observed abeam to the left would be said to be at 9 o'clock.
⁵ Manual of Air Traffic Services, Annex 6-12 Radiotelephony Phrases.
⁶ Shari Stamford Krouse, PhD Aircraft Safety, 1996, ISBN 0-07-036026-X.
⁷ Honeywell Inc, TCAS II Pilot's Handbook, 1.2 Introduction.
⁸ See the ATSB website for further information on see-and-avoid.
⁹ A volume of three dimensional airspace surrounding a TCAS equipped aircraft that varies in size depending upon the rate of closure of a conflicting aircraft.

ANALYSIS

The Hamilton Island Aerodrome Controller (ADC) had a plan to separate the aircraft, but did not clearly communicate the plan to the pilots and consequently it was not executed correctly. Having the 737 pass behind the 717 was going to present some difficulties due to the intended tracks of the aircraft, and required the ADC to advise the crew of the 737 as early as possible of that tracking requirement. While traffic information about the 717 was provided to the crew of the 737, the ADC did not communicate an important aspect; that is, that after reporting seeing the 717 they would have to pass behind it. Had that been the case, the crew of the 737 probably would have been able to turn right in sufficient time to safely pass behind the 717. Alternatively, they may have requested another means of separation as the position of the aircraft may have prevented them from passing behind it.

The use of visual separation, either by controllers or pilots, increases the likelihood of an apparent traffic alert and collision avoidance system (TCAS) alert between aircraft. Apparent alerts result from aircraft being within the TCAS alert parameters while complying with an air traffic control (ATC) clearance. In this occurrence, if the 717 crew had not turned away, it is possible that one or both of the aircraft's systems would have issued a traffic alert (TA) or a resolution advisory (RA) In the case of the latter, the pilots would have had to comply. That would have increased crew workload, particularly for the departing 717 crew, when the aircraft was in a critical stage of flight. As it was, the crew of the 717 had to descend to avoid the 737.

The limitations in using visual separation, highlighted in the Manual of Air Traffic Services, applied similarly to the ADC and both crews. The fact that the aircraft were tracking on almost reciprocal tracks, with little or no divergence when viewed by the ADC or the crews, made it difficult for those involved to obtain adequate cues about the situation. However, the crew of the 717 was able to appreciate the potential for conflict by using available visual and system information.

Despite the regulations stating that in a situation where aircraft are approaching head on a pilot shall alter the aircraft's heading to the right, the actions by the crew of the 717, in turning left, could be seen to be reasonable in the circumstances. A turn to the right may have increased the risk of collision. Similarly, if the crew of the 737 had turned their aircraft to the right in accordance with procedures, as the 717 turned left, the risk of a collision may have increased.

Once the ADC was under the impression that responsibility for separation had been transferred to the crew of the 737 and had issued instructions to the crews to climb/descend, the protection afforded by the vertical separation standard was lost. From that point on, the only defences available to the crews to prevent them being in close proximity were their awareness of the other aircraft and the use of TCAS. As they could clearly see the 717, the crew of the 737 did not perceive that there was a problem. The crew of the 717 were concerned at the developing situation and turned away from the 737. That action ensured that sufficient spacing was maintained between the two aircraft and probably prevented a subsequent TCAS RA.

Had the ADC maintained the 1,000 ft vertical separation standard between the aircraft until they had definitely passed, or else confirmed that one of the crews could maintain separation with the other aircraft, it is likely that the occurrence would have been prevented. The use of vertical or lateral separation standards instead of a visual standard would have also limited the possibility of a TCAS alert.

The occurrence highlighted the importance of using unambiguous radiotelephony phraseology to avoid misunderstandings and the need for pilots and controllers to remain vigilant at all times, especially when the dynamics of a situation require action to be implemented early to ensure that the safety of aircraft is not compromised.

SIGNIFICANT FACTORS

1. The ADC did not communicate to the crew of the 737 the requirement to pass behind the 717.
2. The crew of the 737 did not perceive the potential threat presented by the 717 until the crew of that aircraft acted to ensure that their flight path did not place them in a near collision situation.

SAFETY ACTION

Airservices Australia advised that the safety actions in place following the incident or planned for implementation include:

• The Group Tower Manager responsible for Hamilton Island has reinforced the need, through the Tower Manager, to ensure that the automatic terminal information system (ATIS) strip matches the actual ATIS broadcast
• A review of the requirements of the visual separation requirements in the Manual of Air Traffic Services (MATS) was conducted to assure that all pertinent limitations were referenced and determined that no changes to MATS were required
• A performance check will be completed every month for the first 3 months after an air traffic controller gets an initial rating, then at 6 months and then the checking regime will be in accordance with the requirements in the Civil Air Traffic Services Operations Administration Manual (target date for implementation is 30 June 2005).

Safety action update from Airservices Australia (dated 6 March 2008)

Following the ATSB investigation of this occurrence, Airservices advised of three safety actions in place or planned for implementation. All those actions were completed within the advised time frame. Airservices subsequently completed a review of the following action in respect of its application and benefit to the Air Traffic System.

A performance check will be completed every month for the first 3 months after an air traffic controller gets an initial rating, then at 6 months and then the checking regime will be in accordance with the requirements in the Civil Air Traffic Services Operations Administration Manual (target date for implementation is 30 June 2005).

This correspondence is to advise that Airservices has removed the prescriptive requirement to undertake these performance assessments and replaced it with a more generic requirement 'the assessor is encouraged to perform such additional performance assessment(s) as is/rare) thought appropriate to the individual circumstance in the early stages of the controller's consolidation in such a new Endorsement. This may for instance mean that a controller is formally re-assessed prior to the expiry of the normal currency period of 6 months'.

A follow up review indicated that additional guidance should be added such that 'particular consideration with regard to additional or more frequent assessments should be given to newly endorsed controllers who are not subject to regular supervision'. This guidance will be incorporated into the documents by end of May 2008.

APPENDIX A

Air Traffic Control Automatic Voice Recording - Hamilton Island ADC

Legend:

717 - Boeing 717 aircraft (`C/S 717' designates where the callsign of the aircraft is used in radio transmissions)
ADC - Aerodrome controller
737 - Boeing 737 aircraft (`C/S 737' designates where the callsign of the aircraft is used in radio transmissions)
SC - Brisbane sector controller
[…]- Unintelligible transmission

Only radio transmissions pertinent to the occurrence are included.