The holiday media excitement over a ‘near miss’ between a Cathay Pacific A330-300 and a Virgin Blue 737-800 south of Darwin last Tuesday was in fact a ‘far miss’.
This is how it is summarised by the Australian Transport Safety Bureau under its Level 5 investigation category.
An Airbus Industrie A330 was southbound at FL370 and a Boeing Company 737 was northbound on the reciprocal track also at non-standard FL370. When the crew of the A330 questioned the controller, the controller instructed the A330 crew to climb to FL380 and cleared the aircraft to divert right of track. The crew of the 737 then advised the controller they were diverting 10 NM right of track. There was a breakdown of separation standards. The investigation is continuing.
The incident is also classified as ‘serious’. Anything that departs from normal in air traffic control is indeed serious, but it was definitely not a near miss.
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The ‘serious’ element in the incident which the ATSB will undoubtedly address is the prevalence of non-standard flight levels being assigned to aircraft, in this case, the Virgin Blue jet, which was probably for purposes of maximising the fuel efficiency of the particular flight taking into account its load, flight plan and known head or tail winds.
Let’s recalibrate in lay terms the seriousness of the information published by the ATSB concerning this serious incident.
It is standard for flight paths that have an easterly element in their direction to be assigned to odd numbered flight levels like the 37,000 feet being flown by the Cathay Pacific jet which was on its way to Melbourne, or even numbers for those flown with a westerly component, such as the path being pursued by the Virgin Blue jet. Presumably Virgin Blue would have been assigned to say, 36,000 feet, or 38,000 feet, but asked for a non standard clearance of 37,000 feet because that was the ‘sweet spot’ the captain wanted for his or her particular flight . This happens all the time.
By way of example, a heavy 747 just departed on a 13 hour flight can be more efficient in still air at 29,000 feet under some circumstances than it would be at 28,000 feet, and so forth. The temporary use of a non standard flight level not only allows for a fuel saving, but a consequent benefit in terms of the distance it can fly with a particular load especially over long distances. Similarly, being able to get a clearance to a higher altitude, perhaps also a non-standard one, on a timely basis once the jet becomes lighter also saves on fuel burn.
However when non-standard flight levels are used there often comes a time when a conflict becomes apparent. Normally air traffic control will then call for the jet that has been given a non-standard clearance to revert to a standard level at a specified time and location to avoid that conflict. And quite often, another jet will raise the issue anyhow, as Cathay Pacific did. If there is no intervention by air traffic control, or if the pilots on the jets do not talk directly to each other in advance of a separation issue arising, say over oceanic air space well beyond the reach of ATC radars, the TCAS collision warning system unit on each jet will engage with each other and automatically generate an individual alert commanding a complimentary course change by them.
There is no reference to a TCAS alert in the ATSB report. If there had been one the inquiry would not be Level 5, but somewhere in the Level 1-Level 4 band which commands more investigative resources. TCAS course changes can be less than subtle. If the Cathay Pacific and Virgin Blue jets had resolved their conflict with TCAS alerts there would have been lots of “I thought I was about to die” interviews saturating the media within hours of the incident.
Level 1-4 investigations may generate one or more interim reports, and usually involve laboratory tests on equipment, or pieces of equipment, and the issuing of recommendations concerning safety issues to air transport regulators and investigators and airlines world wide.
The ATSB recently set up a special Level 5 unit to catch, for statistical purposes if nothing else, all the lesser incidents that may allow for the detection of trends in reportable air safety incidents that otherwise aren’t worth intensive investigations.
A more detailed explanation of the Level 5 unit can be found under Air Safety at the ATSB home page.