Calculation of Circling MDA In Australia
 

I understand the basics of PAN-OPS approach design etc (and TERPS though not relevant to Aus)

However, I have never understood how some circling MDAs were calculated, I had assumed that in certain cases additional height was added using some additional heuristic. I believe PAN-OPS allows for this.

I can think of a few examples, though Wollongong is perhaps a good example relating to my question.

The original NDB approach has MDA of 1809 ft HAA. I would have guessed that this is based on the missed approach segment.

The newer RNAV approaches have a much lower straight in MDA of 888ft HAT (RWY 16) and 1309ft HAT (RWY 34), yet the circling MDA remains at 1809 ft HAA.

Given the obstacles in the circling area and my limited knowledge of PAN-OPS criteria, I would have thought the circling MDA would be lower than 1809ft, in fact similar or the same as the straight in MDA.

So my questions is, are some circling MDAs in Australia increased based on some additional safety criteria not specifically in PAN-OPS.

Notes on a postcard...

I have attached the charts, and here are the links to them:

NDB
RWY16
RWY32

The 1312' spot height is within 3NM of the southern threshold so it has to be at least 1612' for Cat B. The 2034 spot height, while not within the circling area, is close, and I suspect that might be responsible for the circling minima. What navigation tolerances are applied to circling area calculation? A 1NM tolerance would bring the hillside ramping up to that 2034' peak into play.

I thought the 1312' spot height was outside the circling area, or at least it could be using PAN-OPS criteria where a sector is used to prohibit circling. I understand Australia differs in this area, such that the prohibited area is reduced by a minimum of 2nm or more, in effect increasing the circling area. I don't think the high ground to the south west is within the 2nm.

Though my point was not to quibble, it's more related to an additional subjective criteria applied as a safety margin.

Circling MDA can never be lower than SI minima. So at YWOL the ndb missed approach is controlling forcing a minima higher than circling. In all cases circling minima is increased to match the higher approach minima.

CASA's view is that circling minima should be common for an aerodrome (PANSOPs view is its approach specific) so we publish same circling an all procedures. This helps support common published alternate minima (something only done in aus)

There are exceptions (like YBCS) where the minima are split, a look at the different values indicates why.

The end result of all this is that quite often, circling minima is determined based on an obstacle that is outside the circling area (as in the YWOL example)

There is no mysterious extra buffer.

Alpha

Thanks, that explains it. So this Australian criteria of keeping the circling MDA and I assume the prohibited section of the circling area constant for all approaches at the said airport has the effect of pushing up the circling MDA in some cases.

With the new RNAVs and withdrawal of older NPA associated with the recent nav aid decommissioning, will that mean some of the circling MDAs get reviewed/redesigned or has that already happened? e.g. Flinders Island

All approaches in oz are reviewed every 3 yrs. The circling minima will be checked at the next review, and if it can be dropped, it will be.

Alpha