Xpndr only Vs. TaRa on the ground

[fellman]

See the latest EUROCONTROL ACAS Bulletin
http://www.eurocontrol.int/msa/galle...n_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).

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