TCAS saves the day again - or not...
First there was the JAL incident, then Uberlingen. Now one on our own patch. From the ATSB:
"FACTUAL INFORMATION
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.
...
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.
...
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.’
...
ANALYSIS
As the 737 descended towards FL220, the crew was faced with the apparently conflicting demands of an ATC clearance and a TCAS resolution advisory. Given that the 737 was above the Brasilia, it would be normal for the initial TCAS advisory to have been a `reduce descent’ or a climb advisory. Although no evidence of a TCAS or transponder malfunction was found, the investigation could not exclude the possibility of an equipment failure contributing to this incident.
It is possible that the crew may have misidentified the TCAS aural warning. Prompt action was required to resolve the apparent ambiguity and the crew may have been guided more by the aural warning than by the IVSI display. That may have been, at least in part, due to the limitations of the IVSI display, where a pilot may initially rely more on the aural alert. Compared with a TCAS IVSI display, traffic information that is displayed on an EFIS screen increases the crew’s situational awareness. However, pilots are trained to use all the information at their disposal and an aural alert would be the trigger to look at the IVSI display immediately. Therefore if the green band of the IVSI was indicating a required rate of descent of 1200-1500 ft/min, then the correct procedure would be to disengage the autopilot and smoothly adjust the pitch to attain that rate of descent.
The probability that the crew of the 737 would receive a TCAS RA on the Brasilia could have been reduced had the Brisbane sector controller provided some indication to the crew of the 737 that there was another aircraft restricting further descent. That would have enabled the crew of the 737 to adjust their rate of descent in lieu of possibly maintaining a level, and would have provided additional information that the crew could have then used to improve their situational awareness and optimise their decision making."
So, despite plenty of genuine saves it is slowly beginning to sink in that TCAS is not infallible either. It can make a bad situation rotten or, as in this instance, make a fully controlled situation go pear-shaped.
Good thing we aren't relying on it for collision avoidance in the NAS airspace!