The flight plan track, which was an on-screen indication showing the location of the Boeing 767 (B767) aircraft while outside radar coverage, disappeared from the air situation display while the crew was receiving an air traffic control service. This loss of display resulted in the controller loosing situational awareness and no separation or SAR alerting service being provided to the aircraft.
The controller observed the aircraft's flight plan track overhead Oodnadatta at 1727 Central Summer Time. At 1736 the crew reported their position overhead AGAGO, the controller had no on-screen indication of the B767 as the flight data record for the aircraft no longer existed. All indications of the aircraft had been removed from the controller's screen.
An investigation carried out by Airservices Australia revealed that the crew of an aircraft close to Sydney was cycling through SSR codes on its transponder and momentarily squawked the code that was assigned to the flight data record of the aircraft near Oodnadatta. The received transponder return was a valid code within the coupling corridor associated with that aircraft's track. That scenario caused a false radar coupling that resulted in The Australian Advanced Air Traffic System (TAAATS) automatically modifying the flight data record; which resulted in the deletion of the flight plan track and the controller's jurisdiction strip. That sequence of events displayed a coupled radar track for 10 seconds. However, the track was outside of the controllers displayed range.
A flight data record may couple on either the Previous SSR code (PSSR) or Assigned SSR code (ASSR), there was no indication to the controller on which of these codes the flight data record had coupled unless the controller selected "SSR ALL" to determine the squawked code. The PSSR is the code assigned by the previous control authority, in the receiving control authority TAAATS attempts to retain this code. The aircraft PSSR was the code assigned by the Singapore Control Authority and was retained by Brisbane and Melbourne TAAATS. This code was one normally used for code allocations to Visual Flight Rules flight data records.
In the TAAATS Eurocat Radar Data Processor, coupling between Radar tracks and Flight Plans was limited to a corridor around the flight plan route - ahead of the last overflown position. This significantly restricted the chances of false coupling to aircraft squawking incorrect SSR codes.
In geographically small systems with relatively saturated radar coverage, this was very effective and anomalies are usually easily determined and rectified. However, with the geographical extent of TAAATS en-route system, and the reliance in TAAATS on Flight Plan tracks for separation purposes, the possibility and ramifications of false couplings assumes greater significance.
A solution was to longitudinally restrict the coupling corridor to a time/distance ahead and behind the estimated position of the aircraft based on Flight Data Processor (FDP) time estimates.
Unless the non-radar controller observed the flight plan track and jurisdiction strip deletion, no cues other than the controllers memory or scratch pad notes existed of the presence of the flight and the controllers situational awareness was severely compromised.
The radar controllers in Sydney may not have noticed the newly coupled radar track and if they did, it may have been disregarded as a false coupling.
LOCAL SAFETY ACTION
Following this and other similar occurrences, Airservices Australia has changed the TAAATS system to reduce the possibility of similar false radar couplings. This change consisted of adding an additional longitudinal check that limits the size of the Coupling Corridor to within a defined Variable System Parameter (minutes) ahead and behind the estimated position of the aircraft based on FDP time estimates. Initially this parameter was set to 12 minutes. This additional longitudinal check was only to be applied for Flight plans that were "Active" or "Inhibited".
The software carrying this fix was installed in the ML FIR on the night of January 17th 2001 and in the BN FIR on the night of May 10th 2001.
|Date:||09 February 2000||Investigation status:||Completed|
|Time:||1736 hours CSuT|
|Release date:||05 September 2001||Occurrence class:||Infrastructure|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure time||1109 hours CSuT|