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Transponder mode selector B744

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Transponder mode selector B744

Old 13th Mar 2018, 15:50
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Transponder mode selector B744

Can someone please tell me why we have to wait until entering the active runway to select RA/TA?
I seem to recall t had something to do with cluttering the ground radar but can find no reference.
Thanks
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Old 14th Mar 2018, 16:43
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Nothing to do with cluttering the ground radar. Only reason it has to be done is that they want to avoid a TA for landing aircraft.
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Old 14th Mar 2018, 17:11
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Transponder suppression.

The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.

If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.

Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.
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Old 14th Mar 2018, 17:32
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Originally Posted by BluSdUp
The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.

If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.

Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.
so the note on the FAA plans regarding ASDE-X is useless?
"ASDE-X in use. Operate transponders
with altitude reporting mode and ADS-B
(if equipped) enabled on all airport surfaces."
for example to find here:
http://aeronav.faa.gov/d-tpp/1803/00...nameddest=(JFK)

so it seems TCAS does the suppression because in mode-s on-ground is transmitted in the transponder response.

my guess would be switching ta/ra off means your TCAS isn't needlessly querying other transponders.

maybe this document can shed light on it ... https://www.eurocontrol.int/sites/de...t-20050403.pdf

Last edited by wiedehopf; 14th Mar 2018 at 17:53.
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Old 15th Mar 2018, 06:09
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Originally Posted by BluSdUp
The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.

If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.

Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.
I agree with you BluSdUp. The fact remains at least in my company is that the TA/RA mode is selected when cleared to line up. We are not 100% Boeing but I can not come to another conclussion than it would be a left over from the "old" days. Looking at the system there is no reason why you can not taxi around in TA/RA mode. As we do on the A330 but not on the B744 for some reason.

So I can not come to another conclussion that this a left over from the first days of TCAS
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Old 15th Mar 2018, 16:26
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Originally Posted by BluSdUp
The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.

If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.

Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.
Unfortunately, sometimes aircraft transmit a continuous signal due some problem in the air/ground logic, which then in turn can give false TA/RA when aircraft is taxiing on ground to airborne aircraft.

e.g. on several occasions I have requested ATC to ask an aircraft on ground to switch off their transponder as I could see them on taxiway when on short final, when flying in IMC.
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Old 15th Mar 2018, 17:50
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Boeing operator here (not on the B744)

Before starting the taxi-out we put the transponder in XPNDR, which basically sends signals to the ADS-B/Ground radar system. After vacating the runway, upon landing, we do the same. This mode shows a TCAS OFF message on my EHSI.
One airport in particular in our route network ask us not to do it, keep it STBDY, as their ground radar is an older generation and it would clutter the screens (or so I was told by an ATC chap).

When entering the runway, for departure, we select TA/RA, until vacating on arrival.

Now, en route, when asked to chance codes, most of the colleagues put it on STDBY, dial in the new code (or type it in, depending on transponder control panel type) and then back to TA/RA. I personally find this unnecessary and change the codes without selecting STDBY first.
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Old 15th Mar 2018, 20:39
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Never, ever, go Stby in the air when putting in a new squack.

I did that on departure long time ago, had a rather close call!
Trust me on this one!
Not smart!
Tcas dont work and on Radar you vanish!!
( I had the wrong squack, that was what started it all, long story, short answer: NO stby in the air.)

High on my Top 10 never again list!!
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Old 15th Mar 2018, 22:52
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So i found a definite reference by the FAA
https://www.faa.gov/other_visit/avia.../InFO11012.pdf

Purpose: This InFO informs air carriers and operators of the proper transponder selections during
aircraft taxi, in order to support air traffic control (ATC) ground surveillance systems as well as
minimize 1090 megahertz (MHZ) frequency congestion.
@coastrider26 so the FAA is not happy with your A330s

Discussion: TCAS II operation on the airport surface provides no safety benefit, except when
approaching or taxiing onto an active runway. Routine ground operations with the transponder selected
to TA-ONLY or TA/RA enables TCAS II to interrogate other aircraft at high power levels. This
unnecessarily contributes to 1090 MHZ frequency congestion. Routine operation of TCAS II on the
ground can degrade:
 Performance of ATC radars
 Surveillance performed by airborne TCAS II units in the vicinity of your aircraft
in the plates for EDDF you can find:
To ensure that performance of systems based on SSR frequencies (including airborne
TCAS units and SSR radars) is not compromised, TCAS shall not be activated
before approaching holding point. After landing, it shall be deselected after
vacating RWY.
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Old 16th Mar 2018, 10:15
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@coastrider26 so the FAA is not happy with your A330s

Luckily they were not operated to the US only to the far east and Oz. But Thank you for the reference, always thought it had to do with some older technology on the 744 vs A330 learned something new. Thanks for adding the reference as well.
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Old 16th Mar 2018, 19:57
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Skyjob
Just read your reply.
Have had that as well, when stuff dont work , indeed it is a distraction and more.
Also had heli ops holding short with transponder on, Rather un nerving .
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Old 16th Mar 2018, 22:00
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Originally Posted by BluSdUp
Never, ever, go Stby in the air when putting in a new squack.

High on my Top 10 never again list!!
BluSdUp, my thoughts exactly
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