"I have seen on my PFD a yellow tcas symbol showing alt 0 during take off many times(paris and Vienna)...I believe it was due to both of us (me taking off) having TA or TARA mode selected."
Capt STD
If you have seen that, Capt, your TCAS System has failed. Based on your own radio altimeter, any transponder return reporting altitude within 180' of ground is to be suppressed - not displayed. Now, if you have seen a yellow blip with no altitude tag (not 0), that is quite normal, as without an altitude in the reply, your TCAS cannot determine the other aircraft's altitude, only distance and rate of closure.
That is precisely why you are wrong in turning ALTOFF on the ground. You should never, ever, turn off ALT unless specifically instructed by ATC. There are only two conditions in which ATC will ask that:
1. Your altitude reporting is erroneous and you have no backup.
2. ATC SSR is overloaded. That's unlikely.
Putting your transponder in Standby will of course stop the altitude replies, so the ALT switch position is irrelevant.
When Mode S was in final development in 1987, I argued with the design engineers to remove the ALT switch from the control panels entirely, but they couldn't without an FAA mandated change.
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A bit off topic, but relevant:
The FAA stated that for Mode S (and TCAS) retrofits, the altitude source for the transponder be the most accurate in the airplane, i.e. the CADC digital or synchro output (with integrity monitoring), and not the Gilham code that is used by the older ATCRBS transponders. There is no monitoring of Gilham code, so errors in altitude reporting happen.
Sure enough, the KAL 747 fleet was retrofitted with TCAS (and Mode S), but they didn't use the best CADC altitude output. A few years later an error in the Gilham code had one KAL 747 reporting 10,000 when it was actually at 7,000, and it nearly collided with another 747; JAL, I believe.
GB