Squeaker,
Avoding action instructions are determined by what action will best resolve the situation, safest and quickest (based upon non TCAS aircraft). This could be an immediate descent, an immediate climb, or a combination with turns. It doesn't do anyone any good to give you an immediate instruction that resolves one confliction to only put you into another situation.
When you factor in TCAS, it gets more complicated because we don't know what instruction that your TCAS is going to tell you to do. There have been many documented cases of TCAS giving instructions that resolves the initial confliction to only put the pilot in a worse situation with another aircraft.
I've had pilots receive TCAS resolutions (on traffic that did not exist or was not a factor) that had they followed the instructions of TCAS they would had flown into the ground or into other traffic.
I support TCAS and I think that it is a great safety aid, but it scares me to have a pilot take an action that can potentially make the situation a whole lot worse.
Mike
NATCA FWA