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Old 18th Oct 2003, 17:30
  #188 (permalink)  
Prof. Airport Engineer
Join Date: Oct 2000
Location: Australia (mostly)
Posts: 726
Airspace Occurrences in MBZ and CTA

As well as Class E, radar and USNAS, there is the CTAF/MBZ argument. This has been put off for a few months while the rest of Class 2b gets pushed forward. However it's got a fundamental flaw in it which may have got a few people on the wrong foot as far as NAS goes.

I read the ATSB 2003 draft discussion paper on Airspace-Related Occurrences Involving Regular Public Transport and Charter Aircraft within Mandatory Broadcast Zones. I came away thinking that these CTAFs weren't too bad, and wondering what the fuss was about MBZs/CTAFs. Just maybe all this opposition to NAS was a storm in the teacup and driven by self-interest . . blah . . blah . blah.

I had inferred from the draft Discussion Paper (Appendix C) that the MBZs are LESS SAFE than CTAFs for RPT aircraft. It was only later that the error in statistics was pointed out to me. The text of the ATSB Discussion Paper carried various appropriate warnings as to the limitations of the approach used, but the pictures allowed the Discussion Paper to become misleading.

After I saw the error, I got stuck into some analysis about the relative safety of MBZs and CTAFs (that must make Walley2 happy facts and figures at last and they are even Australian facts and figures).

I found that the opposite is true when the statistics are done properly, and CTAFs may be up to twice as risky as MBZs. It is a significant concern because it misled me and seems to have misled others involved with airspace reform.

How is this so, I hear you ask? I re-analysed the raw ATSB occurrence data over the last 8 years to take into account the actual number of movements in MBZs and CTAFs. Started with data on actual RPT movements in WA into MBZs and into CTAFs from current timetables, and used this as a first approximation to estimate relative rates of airspace-related occurrences and airmisses. Then I had Dr Clark [another PhD] extend the analysis to cover all of Australia using AVSTAT data. Both of us found CTAFs more risky than MBZs.

The executive summary of Dr Clark's report is below, and the full report is available upon request to serious scholars e-mail me BIK needn't bother give me a postal address because it's too big to e-mail. It has also been forwarded to ATSB as part of the requested commentary on their Discussion Paper. They have since replied "in order to remove possible confusion from around this issue, the Bureau has removed the Appendix from the Discussion Paper".

The objective of this analysis is to ascertain if there is any statistical difference in the rate of both airspace-related occurrences and airmisses between Common Traffic Advisory Frequency areas (CTAFs) and Mandatory Broadcast Zones (MBZs). The analysis is limited to Regular Public Traffic (RPT) aircraft.

The results of the analysis show that there is a significantly higher rate of both airspace-related occurrences and airmisses in CTAFs than in MBZs relative to the number of RPT movements.

Over the eight-year period 1994 to 2001, there was an average rate of 19.1 airspace-related occurrences per 100,000 RPT movements for CTAFs compared to 13.5 for MBZs. Similarly for airmisses the average rates for CTAFs and MBZs respectively were 8.4 and 4.8 incidents a year per 100,000 RPT movements.

These differences are significant at the 1 % level. In other words, there is only a one per cent chance that such a result is due to chance. Such a level of significance indicates there are significantly more incidents at CTAFs than at MBZs relative to the number of RPT aircraft movements. The level of airmisses at CTAFs, at an average rate of 8.4 incidents per year per 100,000 RPT aircraft movements, is 75% greater than for MBZs. Similarly, level of airspace-related occurrences in CTAFs, at 19.1 incidents per 100,000 RPT movements, is over 40% greater than for MBZs.

This significantly higher rate of incidents in CTAFs compared to MBZs is exacerbated by two facts. First, aerodromes operated as CTAFs have less RPT movements than aerodromes operated as MBZs. The median level of RPT movements at CTAF aerodromes is 2047 movements per year compared to 3741 movements per year at MBZ aerodromes. Thus on average aerodromes operating as CTAFs have only 55% of the RPT movements of aerodromes operating as MBZs (55% obtained from 2047 divided by 3741).

Secondly, across Australia, CTAFs carry much less traffic than MBZs. In detail, the average yearly volume of RPT movements in CTAFs (4.8% of total RPT movements) is much lower than the volume of RPT movements in MBZs (21.2% of total RPT movements). Thus despite the fact there are less RPT movements per aerodrome in CTAFs compared to MBZs, and despite the much lower yearly volume of RPT movements in CTAFs compared to MBZs, there is still a significantly higher incident rate in CTAFs.
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