View Full Version : Xpndr only Vs. TaRa on the ground


Charles Darwin
30th Nov 2007, 03:51
My company is implementing the new procedure that before taxi we are to set the transponder on (altitude encoding, Ta only) but at line-up to put it in TaRa mode.
Is there any difference really when operating on ground wheather the xpndr is in Ta or TaRa modes?
Just wondering...



IRRenewal
30th Nov 2007, 06:44
From B738 FCOM:

RAs are inhibited below approximately 1,000 feet radio altitude. Below 1,000 feet when the TA/RA mode is selected on the transponder panel, TA only mode is enabled automatically and the TCAS message TA ONLY displays on the ND.

Right Way Up
30th Nov 2007, 06:48
Our SOPS have the transponder set to XPNDR only as it interferes with ground surveillance radar at bigger airfields.

Tail-take-off
30th Nov 2007, 08:56
We also have this SOP. I don't know much about how it works but our tech pilot tells me it is an ICAO recommendation because TCAS units, of airborne aircraft, have been known to miss an RA because they were interogating too many ground returns at the time.

Perhaps a more sensible compromise might have been to only switch on the TCAS when you are number 1 at the holding point.

Charles Darwin
30th Nov 2007, 11:24
I have heard similar explanations, but if there is an aircraft on final, and I'm at holding point, I don't know if there is any difference for the other one wheather I'm sqwaking TA only or TARA. He is only working his own settings and my altitude is showing anyway.
What puzzles me is really that I can't see any difference for me what I sqwak on ground (because RA is inhibited), nor the other airborne traffic (because they are working their own settings and can't see if I'm sqwaking TA or TARA).
:confused:

Piltdown Man
30th Nov 2007, 12:49
There is no difference for the one on finals, because he is TA only. The difference is that your transponder will only be dishing out Callsign, squawk and level. At worst, this will only generate a TA (right up to the point of impact). The important thing is that you are not part of any RA contract for those aircraft in your line of sight. When you are TA only, you will always be taken into account for another aircraft to calculate an RA to avoid you or any others.

PM

Dream Land
30th Nov 2007, 13:08
I agree with most that either way it's no problem for the aircraft, any ATC people out there with an opinion, I remember a few years ago how the ID tags go into a track drop or something and clog up the display.:confused:

fellman
30th Nov 2007, 18:02
See the latest EUROCONTROL ACAS Bulletin
http://www.eurocontrol.int/msa/gallery/content/public/documents/ACAS_Bulletin_9_Jul-07.pdf
which says the following:

Should TCAS be operated on the ground?
The Advanced Surface Movement Guidance and Control System (A-SMGCS) is to be introduced at a number of European airports. To enable A-SMGCS detection, aircraft must be operating their Mode S transponders, at least from the pushback and until reaching the stand on arrival.
TCAS also relies on Mode S technology and for this reason TCAS and Mode S control panels are usually combined in a single unit on the flight deck.
Experience shows that TCAS is sometimes turned ‘on’ and turned ‘off’ at the same time as the Mode S transponder. Except for turning TCAS on for a short period of time before crossing an active runway to double-check for the presence of any aircraft on approach, this should not be done for the following reasons:

TCAS II is not designed to provide any safety benefits for aircraft on the ground: the collision avoidance function is not active before being airborne and the TCAS traffic display does not depict intruders detected on the ground (Note: due to erroneous transponder data, some aircraft on the ground may be displayed).
TCAS II routine operation on the ground degrades safety. Performances of the ground surveillance equipment (e.g. SSR, A-SMGCS) and of the surveillance by airborne TCAS II units can be compromised if there are significant numbers of TCAS II units operated on the ground. As a result, ATC and the operation of airborne TCAS II could be adversely affected. (For more details, refer to www.eurocontrol.int/msa (http://www.eurocontrol.int/msa)).To address these issues, ICAO recommends in PANS-OPS (Doc 8168) and the ACAS manual (Doc 9863) not to operate TCAS II while taxiing.

At the gate: Transponder OFF/STBY, TCAS OFF
Pushback request: Transponder ON, TCAS OFF
Approaching holding position: Transponder ON, TCAS ON
Vacating runway: Transponder ON, TCAS OFF
Fully parked at the gate: Transponder OFF/STBY, TCAS OFF

kuwait340
30th Nov 2007, 18:36
in the A320 FCOM 3 (s.o.p) before takeoff : it says:

TCAS mode selector......................Select TA or TA/RA.

then it says :

the FAA recommends selecting TA in the following cases:

1-a known near by traffic that is in visual contact.

2-at particular airports and during particular procedures, identified by an operator as having a significant potential for unwanted or inappropriate RA (closely spaced runways,converging runways...)

Good Luck

Dream Land
30th Nov 2007, 19:04
Kuwait that's great except this thread is about ground ops. :ok:

Intruder
30th Nov 2007, 19:54
Several airports in various countries have implemented the requirement to squawk Transponder Only on the ground, either from receipt of a code from Clearance Delivery, or from pushback. These procedures are in accordance with ICAO and IFALPA recommendations that TA and RA be deselected until taking the runway for T/O (including Lineup and Wait) and after clearing the runway after landing.

kuwait340
30th Nov 2007, 20:59
Dream Land

yes you are correct....but what i wanted to say is ...this is the only place that the TCAS selection is mentioned..nothing mention in the taxi section .

thanks

Right Way Up
30th Nov 2007, 21:46
I have been told that aircraft with TA or TA/RA on the ground will affect the ground surveillance radar sweep.

Gary Lager
30th Nov 2007, 23:15
The general reasoning behind the recommendation is that every TCAS unit has to talk to every other TCAS unit in range, whether or not you see them on your traffic display. At busy airports or under busy airspace, the rate at which TCAS units can update their situation is reduced by having many other units to interrogate.

You don't need TCAS on the ground, so turn off TCAS (but leave on the relevant mode C/S squawk for Advanced Surface Movement Radar) until ready to get airborne, or to check the traffic on finals when at the hold. It's nothing to do with frequency interference or unwanted RAs, more to do with trying to keep the airbrne units working as efficiently as possible.

At quiet airports it's probably not such a problem, of course, but consistency of SOP is quite important so perhaps the recommendation is about encouraging 'good habits' for when you do visit somewhere busier.

Tail-take-off
1st Dec 2007, 13:16
Brilliantly put Gary!