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Old 14th Dec 2012, 03:26   #1 (permalink)
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TCAS TA vs TA/RA on ground

Soooo, I was having my briefs removed after my initial on a new jet and the point about selection of TA only when taxiing and TA/RA when cleared to line up apparently did not please my good fellow as much as one would expect. Habitual from a number of years and when trying to find a specific reference in a number of publications and a few google searches couldn't pin an answer down. Many references to 1030mhz and other tech things but can anyone give me the exact reference (and why more to the point) we have TA only for taxi and TA/RA for go? The more I think about the logic, the more it bakes my noodle.

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Old 14th Dec 2012, 07:44   #2 (permalink)
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Hi jtr!
Don't know which a/c you fly on, but I can tell you how it is on the F100. Basically you can select TA/RA all the time. RA only works above 900' AGL when taking off and below 1100' AGL when descending. Besides that, due to system logic, the system knows when the a/c is on ground (input of flight/ground switch) and therefor will only display traffic on ND and nothing else!
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Old 14th Dec 2012, 09:02   #3 (permalink)
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Most mode S have a XPDR mode.

XPDR which is the old mode Alpha with just the number being squirted but with some mode S trickery which means you don't show to aircraft in the air. Well at XPDR airports I never see anything on the ground on our honeywell.

RA is turned off below 1100ft rad alt decending and turns on again through 900ft climbing.

We have another installation where its just on and off. Apparently it does the XPDR thing with ground air logic maybe your jet is the same.

Must admit there seems to be some awful SOP's out there for TCAS and what to select when and what ranges etc and up down and normal. Most dont actually change what it does, just seem to be a beating stick for picking fault with people who really don't care what its set at but will do as it tells us when triggered.

The unit which is just on off is by far the least hassle it has one fixed range of 10 miles and no up/down selection. You just turn it on and leave it alone.
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Old 15th Dec 2012, 20:31   #4 (permalink)
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Transponder setting


I fear you were flying before an "old" aircraft which didn't have an squat-switch (on the landing gear) installed? So it was not able to switch automatically from GROUND to FLIGHT - Mode of the Mode-S TRSP?

What you have learned on your "old" aircraft must be aircraft specific - possibly as an automatic switching facility for the 2 possible Transponder Modes was not installed - was missing?

So, by switching on "TA" for taxying, you were actually activating the transponder for ground detection - "on ground" only. You were making sure that you are seen and detected by ground radar sensors (such as MLAT) - but not seen by en-route and APP-radars (no cluttering of en-route radars).

The ground radars, such as for SMGCS or A-SMGCS, Advanced Surface Movement Guidance & Control Systems, the switching-on of the TRSP for taxy is essential. Not only to be seen by the ground radars, but more so also to trigger alerts / alarms when you are by accident entering an active runways with another traffic operating on it (Safety Nets, such as RIMCAS or RWSL).

Interestingly, the TA position of your old transponder has nothing to do with TCAS itself - Nor with the correct functioning of TCAS (e.g. to issue or trigger TCAS RAs), not at all. But more so with the correct setting of the TRSP-mode for detection by either ground or all radars (in-flight and so by en-route radars).

I know this is quite confusing, but the best is that you read the FOM of you new aircraft, and then you will probably find out that you aircraft/TRSP is doing its job of selecting the correct mode (GROUND or AIR) automatically. And that you need as pilot to just sit there and observe what the system does - supervise if the transponder is in the correct Mode (e.g after take-off).

So, all you need to do is just to put on RA/TA or AUTO (or whatever it is in your new aircraft), and that's it (and then to forget all you have done on your previous old aircraft......)

Cheers Chris

Last edited by Olindaguy; 15th Dec 2012 at 20:32.
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Old 16th Dec 2012, 02:18   #5 (permalink)
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Thanks for the replies.

The "old A/C" is an A320.

The "new A/C" is a Bombardier Global (so similar era)

I realise the Global Vision is new tech but I can't imagine the "classic" global is any different from the bus. Have heard various stories over the years about TA on taxi and TA/RA lining up but was hoping for a definitive source.
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Old 16th Dec 2012, 14:15   #6 (permalink)
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Sorry, I can't help you with a definite source, but there is a number of links to various documents, both regulatory and from manufacturers, on Skybrary. It might be worth checking.
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Old 16th Dec 2012, 20:51   #7 (permalink)
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see EUROCONTROL's ACAS Bulletin No. 9
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