TCAS protection volumes

[filmen]

Hi,
This thread is aimed particularly at pilots flying in Oceanic Control Areas. As an ATCO working in an OCA I'd like to clear some doubts regarding TCAS protection volumes. As you know, separation minima are way higher in oceanic areas than those applied where radar services are provided. Do the TCAS protection volumes also increase (lateral and longitudinally)? Are the TCAS settings changed when crossing from one area to another? Does this depend on the airlines' procedures?
I can't seem to find a clear answer to this on documents. Thanks for your help.
Cheers!

[gorter]

I can't see how or why. I honestly don't think TCAS knows where it is in the world. I think it only knows where it is relative to another transponder. I may be wrong and have no reference.

[wigglyamp]

TCAS provides an envelope of protection around the host aircraft -it doesn't know where it is geographically and doesn't need to.

[westhawk]

filmen:

From the TCAS II V7.1 intro booklet (http://www.faa.gov/documentLibrary/media/Advisory_Circular/TCAS%20II%20V7.1%20Intro%20booklet.pdf):

The horizontal dimensions of the protected airspace are not based on distance, but on tau. plus an estimate of the protected horizontal miss distance. Thus, the size of the protected volume depends on the speed and heading of the aircraft involved in the encounter. The horizontal miss distance filter seeks to constrain the volume in order to exclude RAs for aircraft with sufficient lateral separation, and uses both range and bearing information to accomplish this.

Tau is a measure of the time to intercept, thus distance will depend upon rate of closure. There is no provision mentioned in any documentation I am aware of for the protective TCAS "bubble" to expand in oceanic airspace. For a fuller understanding of TCAS concepts, I think you'll find the above linked FAA booklet quite informative.

[mad_jock]

TCAS doesn't do volumes only time to impact.

Slow things small volumes

High speed huge volumes.

its not a clue where it is operating apart from when you climb through 900ft on the rad alt or decend through 1100ft when it kicks into a different mode.

As for company procedures there is alot of fannying about changing between high/low 5/10/25 mile ranges on the display but that makes cock all difference in the way the unit operates as much as some pilots would like to think otherwise.

[filmen]

Thanks for the answers. @westhawk: I've already looked into that document, thanks for the tip. ;) It makes total sense for the protection bubbles to be 'managed' in terms of rates of closure. As for the TCAS settings, I thought it was possible for those to be changed manually (by the crew, knowing which kind of airspace they are flying in), not automatically by the system itself.
Cheers!

[westhawk]

As for the TCAS settings, I thought it was possible for those to be changed manually

As mad_jock points out above, there is a common misconception that manually adjusting the display range somehow affects the range at which the generation of TAs and RAs occurs. It does not. Company SOPs or manufacturer recommendations may specify which display range should be selected by the crew during different phases of flight so that the selected display range is appropriate to the true airspeed during that phase of flight. In terminal areas it is appropriate to select a smaller display range while during cruise flight at high altitude, a greater range should be displayed. In effect, the displayed range should be varied so as to provide an opportunity for the crew to observe intruder aircraft well prior to the generation of a TA or RA while allowing the display to be decluttered by selecting a smaller display range in more densely populated airspace. I hope that makes some kind of sense!

In high altitude oceanic airspace it is normal to select maximum display range.