An infringement of separation standards occurred 70 NM east of Darwin, NT, between a descending Boeing 737-376 (737) and an Embraer EMB-120 (Brasilia) that was maintaining level flight. The event took place during the hours of darkness and in visual meteorological conditions. The crew of the 737 intentionally flew the aircraft through its assigned level in response to a traffic alert and collision avoidance system (TCAS) warning. The Brisbane sector controller also received a short-term conflict alert (STCA) between the two aircraft from the Australian Advanced Air Traffic System (TAAATS). The STCA alerted controllers when the radar trajectories of two aircraft indicated that separation standards might be infringed. The 737 and the Brasilia passed within 1.6 NM horizontally and 600 ft vertically. The required separation standard was either 3 NM or at least 1,000 ft.
TCAS is an airborne device that functions independently from the ground based air traffic control system and provides collision avoidance protection for a broad range of aircraft. The system fitted to the 737, TCAS II version 6.04, provided recommended escape manoeuvres in the vertical dimension, to either increase or maintain the existing vertical separation between the aircraft. The escape solution was communicated directly to the flight crew via a cockpit display and a synthesised voice attention getter. The TCAS alerts in the 737 used a liquid crystal display (LCD) instantaneous vertical speed indicator (IVSI) with red and green markings to indicate the vertical speeds to be avoided (red), and the desired vertical speed to be flown (green). The display was 70 mm square and had a fixed range of 6.5 NM. Aircraft were depicted using geometric symbols, depending on their threat status. A partial aircraft symbol was displayed at the extremity of the screen for aircraft detected beyond the display range. TCAS information could, in principle, have been incorporated in the electronic flight instrument system display (EFIS) of that aircraft but wiring and space available in the electronics racks of the aircraft would have required a major modification programme. TCAS information that is incorporated in the EFIS display allows the pilot to show traffic at longer ranges.
TCAS equipment interrogates the transponders of other aircraft to determine their range, bearing and altitude. Accordingly, the TCAS does not provide protection against aircraft that do not have an operating transponder. The Brasilia was fitted with an operating mode C transponder (altitude encoding) but was not TCAS equipped and the crew was not immediately aware of the infringement of separation standards. The Brasilia, which had 30 passenger seats and had a maximum take-off weight of 11,990 kg, was not required to be TCAS equipped by the Australian legislation current at the time. That legislation mandated the carriage of TCAS for all turbine-powered aircraft with more than 30 passenger seats or a maximum take-off weight greater than 15,000 kg. When both potentially conflicting aircraft are fitted with TCAS, both TCAS units co-ordinate their intentions to provide appropriate co-coordinated avoidance manoeuvres.
TCAS II can issue two types of alerts:
- Traffic Advisory (TA) to assist the pilot in the visual search for the intruder aircraft and to prepare the pilot for a potential RA; and
- Resolution Advisory (RA) to recommend manoeuvres that will either increase or maintain the existing vertical separation from an intruder aircraft.
Once the risk is over, the TCAS issues a synthesised voice `clear of conflict' message.
The 737 crew were on their fourth consecutive day of duty and completing the last sector of a four-sector day when the incident occurred. It took place at the transfer of control point between Brisbane Centre and Darwin Approach. The 737 was en-route from Cairns to Darwin and was on descent to FL220. The Brasilia was enroute to Groote Island, under the control of Brisbane Centre, with instructions to maintain flight level (FL) 210. Those routes placed the two aircraft on almost reciprocal tracks. As the 737 was passing FL235, the crew were instructed by the Brisbane sector controller to contact Darwin approach control for further descent, but were not advised of the opposite direction traffic.
The Manual of Air Traffic Services (MATS) gave guidance to controllers on how best to manage the situation and advised:
`6.6.5.5 Frequency change management in relation to the transit of an aircraft across airspace boundaries shall be arranged in a manner that normally enables pilot communication with the unit responsible for the airspace within which the aircraft is operating. A lateral tolerance of 10 NM either side of the boundary is permitted, except that when entering controlled airspace, the frequency change shall be within the 10 NM prior to the boundary.
6.6.5.10 Aircraft should normally remain on the frequency appropriate to the airspace in which it is operating. However, aircraft may be transferred to another ATS frequency provided that:
- significant operational advantage will be gained;
- workload, communications and equipment capabilities will permit the responsible controller to take such action as is necessary to preserve the separation without delay;
- the actual separation is in excess of the minimum;
- there is no possibility of separation being reduced to the minimum with the normal operation of the aircraft.'
MATS does not require controllers to provide traffic information to either crew in these circumstances.
In order to comply with MATS 6.6.5.10, the Brisbane sector controller could have either:
- Initially assigned the crew of the 737, FL230 on descent, instead of FL220, then assigned the responsibility for separation to the Darwin approach controller, coordinating the terms of the transfer of the 737 with him, before transferring the aircraft to the Darwin approach frequency; or
- Assumed the separation responsibility, coordinated the terms of the transfer of the 737 with the Darwin approach controller, and transferred the aircraft once the 737 had passed the Brasilia and a horizontal separation standard existed.
Airservices Australia believed that MATS section 6.6.5.10 did not apply to a change of frequency in these circumstances. According to Airservices Australia the action of the Brisbane sector controller, in assigning the crew of the 737 FL220, was consistent with MATS in that the controller applied a 1,000 ft separation standard between the two aircraft and the 737 crew was transferred to the Darwin approach controller within 10 NM of the lateral boundary between the Brisbane sector and Darwin airspace.
Shortly after acknowledging the instruction to change frequency, the 737 crew received an aural `traffic, traffic' warning and a display indication of an aircraft 5 NM ahead. The pilot in command stated that the TA quickly changed to a RA with a `descend, descend, descend' aural alert. As the aircraft was approaching its assigned level he disconnected the autopilot and pitched the aircraft nose down with the intention of following the RA commands. He stated that the required rate of descent shown on the IVSI was 1,200-1,500 ft/min. On passing FL220 the TCAS command abruptly reversed to a climb RA (aural `climb, climb now') which was followed positively. The climb annunciation continued until the aircraft was at FL225. No more commands were issued and there was no TCAS `clear of conflict', which is normally generated once a RA is removed.
Analysis of recorded data indicated that as the 737 descended through FL230, its rate of descent was approximately 2,900 ft/min. At FL227, the automatic flight system commenced a transition manoeuvre to achieve level flight at FL220. At approximately FL225 the autopilot was disengaged and the descent was continued manually at a rate of descent in excess of 3,200 ft/min to FL215. That was followed by a climb to FL225 at 2,900 ft/min as the pilot responded to the RA reversal (`climb now' advisory).
Maintenance files from the TCAS computer were examined and no indication of TCAS failure was found. Technical expertise was requested from the manufacturer of the TCAS equipment. Their evaluation of the event presented two possible scenarios.
`Explanation 1. The reported `descend' advisory was actually a `reduce descent' advisory that was misunderstood by the crew. A `reduce descent' would be consistent with the expected TCAS response per the reported geometry of the aircraft. Because the advisory was misinterpreted, the rate of descent was increased rather than decreased until the aircraft was below the TCAS required 700 feet vertical separation. Thus, the TCAS was required to issue a `climb now' advisory. The lack of a `clear of conflict' annunciation is explained in the following paragraph.
`Explanation 2. There is a possibility that the intruder aircraft's (equipped with mode C transponder) altitude report was not correctly received by the TCAS. There have been instances when a Mode C reply will not contain all the appropriate pulses in the message or it transmits pulses that are too narrow for the TCAS to detect. This could cause differing altitude reports and could result in multiple unstable tracks at different altitudes for the same intruder aircraft. This being the case, the TCAS could have issued a `descend' advisory for the intruder because it appeared (due to erroneous altitude report) that it was actually above its own aircraft. If subsequent replies had the correct altitude, the erroneous track would be dropped by the TCAS and the TCAS would issue a `climb now' advisory on the track with the correct altitude.
`The reason that `clear of conflict' was not annunciated can be attributed to low track firmness of the intruder aircraft. Mode C equipped aircraft are typically only equipped with a single antenna mounted on the lower hull. Since the intruder aircraft was below the 737 aircraft, it is likely that the TCAS was not able to receive regular replies at close proximity. The TCAS computer unit will `coast' the track of a previously established intruder file if it does not receive a valid or reasonable interrogation response. If the track of the intruder that generated the RA is coasted during the time of the associated RA, then the `clear of conflict' is not announced.'
Since the incident, the operator's TCAS software has been updated to Version 7. The objectives of the Version 7 update were to further increase the safety benefits of TCAS, make TCAS more compatible with the procedures used by ATC and to address operational concerns identified by pilots operating the older versions of TCAS. Improvements to the aural annunciations included a change from `reduce descent, reduce descent' to `adjust vertical speed, adjust'.