TCAS II and TA/RA

[Guest webstyle]

Hi. I just got PMDG with TCAS running. Phantastic...But a little bug. The resolution advisory sounds verry late. I saw theaircraft right in front of my window, with is realy to late.Does somebody know the way to set the RA to a greater range.Thanks to all answers.

[Guest tmetzinger]

>Hi. I just got PMDG with TCAS running. Phantastic...>But a little bug. The resolution advisory sounds verry late. I>saw the>aircraft right in front of my window, with is realy to late.>Does somebody know the way to set the RA to a greater range.>>Thanks to all answers.The RA range is set correctly as far as I know - did you see the object get closer and generate a TA first, or did it magically appear and generate the RA?the RA should be a last-minute event - the time to act manually is to formulate a plan when you get the TA.

[Guest webstyle]

Hi!I got the TA first and thats correct. But as i am a Commercial Pilot in real i know that the RA must be earlier.I made a holding at 10000ft and a IVAO ATC Controller send me a Aircraft in the holding verry close to me in 9500ft. It producesno RA. Only when i disconnected my AP and steer manually to the Traffic. I have a picture enclosed to see what i mean.So if there is any possibility to set the range for RA, please let me know.Greatings ET

[744pilot 0]

I didn't read all this text in this topic because it's not necesary.RA will be activated if the impact is less then 20sec.So, first read the manuals next time.If you flying after an aircraft it's logical that RA will activate very late.

[ps1flyer 9]

Hi Ramon,> RA will be activated if the impact is less then 20sec.> If you flying after an aircraft it's logical that RA will> activate very late.unfortunately, that's not entirely correct. (Actually, the time to closest approach depends on airplane altitude and it's 25 seconds for the altitude in question which is 9400'. But that is not the point here.)The point is that you will ALSO get a RA when the time to closest approach is MORE than 25 seconds. This will happen if the distance to the intruder airplane gets LESS than a critical distance named dMod_RA. In our case, this distance is 0.55 nm so if you approach an airplane and the distance gets less that 0.55 nm you WILL get a RA even if you are flying parallel at the same speed. I assume that 'webstyle' who asked the questions is aware of that concecpt. However, it is difficult to judge the distance from the supplied image. It looks like the A320 intruder aircraft is some 0.3 to 0.4 nm away. That's not too far away from the correct alert distance of 0.55 nm, considering that there has been a delay in capturing the screenshot. So it might be just possible the the critical RA distance is correctly programmed into PMDG's TCAS system.Hope this helps, Markus

[Mats_J 2]

Markus - I also think the relative speed is a variable in the TCAS2 solution so can't calculate anything without it.Just a heads-up,

[Guest pagotan]

Hi,Just as a matter of curiosity since you mentioned IVAO did you by any chance flew online with the new IVap or whatever the name is for the SB replacement. If yes did you notice the same delay on the built in TCAS?If not it would be interesting to try and reproduce the situation and compare both TCAS advisories.Michael

[ps1flyer 9]

Hi Mats,> I also think the relative speed is a variable in the TCAS2> solution so can't calculate anything without it.yes, sure, relative speed is needed. Actually, the time to closest approach (usually named Tau) is just the radio of distance and relative speed.However, TCAS theory shows that the concept of time does not work anywore for very slow closure rates (e.g. one aircraft overtaking another aircraft). Hence, a new concept named 'Modified Tau' or 'Bramson criteria' was introduced to cover just these cases. In this new concept IN ANY CASE the (horizontal) distance between two airplanes gets LESS than a critical distance dMod, a Resolution Advisory is issued to the pilots. There is a nice mathematical formulae for that part of the TCAS algorithm which shows that the time concept is no longer important when the distance drops below that critical value. The RA is issued always then d < dMod. (Mathematically speaking, Tau gets negative even if distance is positive and closure rate negative, that is for airplanes closing in on each other. If Tau becomes negative, the airplanes are too close already, e.g. d < dMod!)Some nice background reading on TCAS and all the relevant parameters (including dMod) can be found in a nice booklet by the FAA:http://www.nak.no/flynytt/download/TCAS_II_V7.pdfMarkus

[Wolfko 0]

Ramon,since ET (webstyle) is a real world commercial pilot, you do not have to explain him how a TCAS works. As you see it is necessary to read the whole text sometimes.Wolfgang

[Guest webstyle]

Hi Michael!Thanks for the hint. I test this today and see what is the result in relation to the IVAP3.2 TCAS Computer.I just remeber one time, that TCAS of the IVAP was much quicker with the TA as PMDG. But as i understand, IVAP gives only TA not RA.As soon i have a test answer you can read it in the forum.Thanks ET.

[Guest TrafficTraffic]

I think my logic is likely running correctly in this case. This is a very low-closure-rate encounter, correct?If you look at the tables most of the way down http://www.sls.csail.mit.edu/ilh/fs/ILH_TCAS_doc/ in the section TCAS II version 7.0 Advisory Thresholds you will see that at your altitude, with a low-closure-rate encounter, you could get within 0.35nm (DMOD) of that aircraft you are following before an RA would be triggered. That is a real-world table from the TCAS II v7.0 specs and what is implemented in my TCAS2v7.dll that PMDG is using. I am using the values associated with altitudes 5000 - 10000 here.I went to great efforts to verify my logic second-by-second against TSIM, the TCAS simulator (referenced in that URL above), and many of the 250+ test cases were of this type of encounter.Lee Hetherington, PP-ASEL (KBED)

[jjeffreys 12]

Webstyle is correct, the IVAP only contains TA and not RA.

[Guest monsoon]

That why you the man Lee! Love your work.