ATSB’s interim report of its ongoing investigation into the Sea World tragedy is already telling as to the limitations of anti-collision gizmos, particularly when they are not serviceable or are deliberately or unknowingly configured to filter out what are assumed to be ‘nuisances’. Key extracts, with my underlining:
VH-XH9 conspicuity devices
While operating in NZ, the helicopter was fitted with an L3 Lynx NGT 9000D+ transponder with Automatic Dependent Surveillance–Broadcast (ADS-B)-out capability on 27 September 2022. At that point the unit appeared to function with recorded data matching records of flights undertaken in NZ up to 19 October 2022.
Details of the new registration were to be updated in the unit, and some problems doing so were encountered. For flights from 2 December 2022 to 26 December 2022, externally recorded ADSB data was either showing no data (4 days) or the old NZ registration (11 days). From 27 December 2022 the new Australian registration was recorded but all tracking within Australia was intermittent.
Additionally, on 20 December 2022 air traffic control at Gold Coast Airport could not identify XH9 on secondary surveillance radar. Following this, pilots were aware that they would not be accepted into controlled airspace in XH9 without a functioning transponder. Troubleshooting of the transponder was not complete at the time of the accident. During inspection of XH9 post-accident, the ATSB lowered the belly panel and found that the transponder antenna was disconnected.
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The word “disconnected” is in an interesting one. It’s not one I would use if, for example, the antenna coax had been pulled out of the back of its fitted connector due to, for example, impact forces. I interpret it as meaning that the fitted connector remained on the antenna coax but it was not connected to the antenna. If that interpretation is correct, maybe the antenna had, in the course of fault finding or other maintenance, been deliberately disconnected and left disconnected, or not reconnected properly such that it vibrated loose and disconnected in subsequent operations.
I note that the installation manual for the L3 Lynx NGT 9000D+ system includes these CAUTIONS among others:
1. Operating the unit with the GPS antenna connected and the L-Band antenna NOT connected may damage the unit.
2. All antenna connections must be connected prior to operation.
My understanding is that t
he L-Band antenna is used to transmit/receive, among other frequencies, 1090 mHz – the frequency used for transponders and ADS-B in Australia. Happy to stand corrected. All of the avionics with transmitters that I’ve worked on could be damaged if they transmitted into the wrong impedance and, in any event, if no antenna’s connected the transmission doesn’t go far.
VH-XH9 detection equipment
The L3 Lynx NGT 9000D+ also provided traffic advisory (TA) alerts. The unit could visually display traffic although this was not configured. In addition, the system was also not linked to another display, therefore it gave audible alerts only. Pilots would hear warnings such as ‘traffic, traffic, 3 o’clock high, 2 miles’.
There were two sensitivity levels, A and B. Sensitivity level A reduced the number of nuisance TAs during take-off and landing. Sensitivity level B maximised the detection of TAs during the cruise phase of flight. If the aircraft was at or below 1,700 ft above ground level (AGL), the traffic advisory system (TAS) would not display or calculate alerts for other aircraft which were determined to be on ground (not airborne). Intruders determined to be below 380 ft AGL were considered to be on ground.
A configuration option called ‘Ground Filtering Altitude’ allowed the user to set the on ground (alternate) determination for 5 ft instead of 380 ft AGL.
At the time of publishing, the exact settings of this unit had not been determined and analysis is ongoing.
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My interpretation of the ATSB’s language here is that the TA alerting system was working in principle during the flight – “provided … alerts”; Pilots “would hear warnings” – and the only remaining question is whether the ‘ground filtering option’ was or was not configured such that XKQ would have a generated an alert. But I don’t see how the TA alerting system would have been working at all if XH9’s transponder antenna was disconnected. Again, happy to stand corrected if I’ve misconstrued ATSB’s language or misunderstand the technical consequences of XH9’s transponder antenna being disconnected.
(And I cannot help but point out the truly ghastly irony that these systems are deliberately designed and configurable to filter out the ‘nuisance’ of being warned about the very kinds of circumstances in which this collision occurred.)
VH-XKQ conspicuity equipment
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XKQ was equipped with a Garmin GTX327 Transponder which was detectable by secondary surveillance radar and was detected on the day of the accident.
VH-XKQ detection equipment
The helicopter was fitted with an Avidyne TAS600 TAS. The system had a ground mode and a flight mode. It would automatically switch between the 2 as the aircraft climbed or descended through 400 ft. In flight mode, when a detected intruder met the criteria for TA, the system used an annunciator light (marked ‘Traffic’ or ‘Traffic Alert’) and an audible voice annunciation. The announced phrase was preceded by a tone and began as ‘Traffic’, the clock position of the alert was given, and then the relative altitude of the intruder and range was announced. It was reported that no annunciator light was present in XKQ.
When the host aircraft descended below 400 ft AGL, the system transitioned to ‘ground mode’ and TA announcements were muted. While XKQ was on the ground, the TAS600 would be in ground mode. Aural annunciations would be muted and the aircraft did not have an interface from the TAS system to a display. After departure on the accident flight, XKQ did not reach an altitude that would have triggered the TAS600 to switch to flight mode.
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XKQ’s system was deliberately designed not to give an alert in the circumstances, even if XH9’s transponder had been working properly.
I look forward to seeing if the ATSB concludes that the much-hyped ADS-B IN functionality would have prevented this collision and terrible tragedy, and why.