"In the QRH for N-1 situations, it states "Transponder TA only." It is assumed this refers to the selector on the transponder panel, but the question has been asked that if the TFC switch is selected OFF, and the MAP shows TA only, why is this not sufficient? "
RAT 5, I'm not sure what "N-1 situations" are, but, generally speaking....
With the TFC switch off, you are not going to get any intruders at all painted on the EHSI/ND/IVSI (whatever type of display you have). That is, no diamonds, circles or squares. This is completely different to having TA ONLY selected, which will show intruders. Selecting "TA ONLY" will remove the resolution commands on the EADI/IVSI, inhibit RA aurals and, as I understand it, will show normally red targets/intruders on the EADI/PFD/IVSI in yellow. i.e. , with TA ONLY selected you will get some idea of what is around you, but the TCAS system won't tell you how to avoid conflicts. With TFC off, you lose your horizontal situational awareness (With TA/RA selected with TFC off, you have to rely purely on aural & display up/down guidance commands to get you out of trouble).
Because I don't know if N-1 situations occur on the ground or in the air, it makes it difficult to comment further. However, you shouldn't get TA ONLY annunciated if you are in the air and you have TA/RA selected).
Rgds.
Q.