Originally Posted by
Fortissimo
In many parts of the USA, pilots will often operate with TCAS in TA mode to avoid 'nuisance' RA because of parallel runway ops, mixed GA traffic etc. and the operators think this is quite acceptable as the airspace is designated as congested. The chances are that the UAL aircraft never got an RA because the automatics were not enabled. The danger comes when these guys operate over in Europe, which the US regards as congested in all areas, so it is quite possible that one half of a conflict is only receiving TA. So long as the other party follows the RA adequate separation should result, though the experts might be able to point out circumstances where this may not be good enough.
Why the rush to conclusion? Hopefully you saw the numerous references that debunked your "chances are" assumption.
Also, what parts of the USA do crews operate in TA mode? A mate telling you doesn't count.