TCAS/Transponder on the ground

[767-300ER]

In reading about transponder use on the ground for ground surv radar to track aircraft movements I see that when aircraft push back they want the crew to select the transponder on but not in a TCAS mode, what difference does it make to put the transponder into a TCAS mode vs a transponder mode???

What difference does it make to the aircraft on the ground, and what difference does it make for aircraft on takeoff/landing in the vicinity if an airplane on the ground has it's transponder in a TCAS vs Transponder mode??

Thanks

767

[Piper19]

I guess if every airplane on the ground sets his transponder in S mode, you get too much ground clutter. I can think of an airplane taking off on the runway, and another one holding on that runway to cross it midway. This situation can cause a traffic message. Corect me if I'm wrong here.
Anyway, when towing an aircraft I always set the xpndr/T2CAS to XPNDR and code 1000. They never told me to put it in TCAS mode, but I don't do it because of the reason above.
When I fly my Cessna, I put the transponder before lining up for takeoff.

[chemical alli]

if transponder is in tcas mode and someone fools with qnh setting then you can simulate an aircraft above airfield.but i think that no ra is given below 400 ft i may be wrong would have to chk the notes.but ta still active so just a safety issue with landings and toff i think
a

[Brain Potter]

TCAS, like the Mode C coding, is set to use the standard pressure setting. Changing the altimeter sub-scale does not affect the output. An aircraft squawking Mode C but not Mode S on the ground will still be seen as traffic by other aircraft on their TCAS. An aircraft squawking Mode A only will also be seen but without any altitude information.
RAs will be inhibited by the Radio Height input.

[EGPFlyer]

Some surface movement radars are affected by the frequencies given out by TCAS so Eurocontrol have asked that it be switched off until entering the runway.

[Gary Lager]

Additionally, if all traffic on the ground had TCAS on, since TCAS works by every unit talking to every other unit in the vicinity then it is likely that in high-density environments the effectiveness of TCAS for airborne units would be degraded.

[Paris Hilton]

The reply to a TCAS interrogation from another A/C includes a 'vertical status' bit, indicating if the A/C is on the ground or in the air.

[dv8]

In AMS you are required to leave the TXPDR on after landing until going on to stand
On departure turn it on from requesting start

[Gary Lager]

True Paris, there is no possibility of the TCAS units getting 'confused' if that is what you thought I meant; instead I was remarking that it is a limitation of the system that the more TCAS units a system has to interrogate (whether or not they are on ground or airborne) the less often it can update and so the less efficient the system is.

I know we're talking milliseconds but since it is of no use on the ground I can see no reason to potentially degrade airborne systems.

And before else anyone says "it's useful to see where landing traffic are/avoid runway incursions etc", I would suggest that they minimise going heads-down while taxiing and instead a) look out of the window, b) cross check with airport charts and signs and c) listen out and communicate effectively with ATC. TCAS is shouldn't be used for circumstances for which it wasn't designed - that can lead to serious complancency and misunderstanding of the risks and errors involved. It is an airborne collision avoidance system. You wnat one for taxiing? Push for implementation of RAAS!!

[Paris Hilton]

Check this out.
http://www.eurocontrol.int/epr/gallery/content/public/docs/skyway_autumn_2005/p32.pdf

[safetypee]

See the most recent (urgent) safety alert from Eurocontrol - ‘Loss of functionality of aircraft's collision avoidance System.’ (www.eurocontrol.int/safety/public/standard_page/safety_alert_board.html)
Perhaps selecting TCAS to 'On' when (or before) entering the runway and checking it before takeoff would be advantageous; but beware of the hazard of additional workload at a time when high vigilance is required.

[J.O.]

Switching the TCAS on should be part of the PNF's flow just prior to entering the runway. If it is trained that way, there's no reason for it to be missed, and it shouldn't be that much of a distraction. Been doing it that way for years and it works just fine.

[Ghostflyer]

It depends on the software modification state of the TCAS. When in standby, some transponders allow Mode S interogation only, which is what the latest spec of Ground Movement Radar (GMR) require. Other airports use other modes for the GMR. We have just had a software change which means that we no longer need to put the transponder on to comply with some airports GMR requirements.