Sorry to be slow responding, kind of buried at work.
Galaxy, no probs mate. Regarding the whole TA only thing:
The reason its selected is a coordination thing that has most relevance to an encounter where manouevres in a particular sense are inhibited for one or both aircraft.
E.G. lets say 2 medium turboprop aircraft meet one another at high level. They will likely both have climb RA's performance inhibited.
Likewise a low level encounter both aircraft could be descent inhibited.
In these situations we can't have the aircraft manoeuvre in opposite senses, instead TCAS has to constrain one aircraft from doing anything with a preventative RA, whilst giving the other a corrective RA. This is an exercise in TCAS / TCAS coordination, and follows some established rules.
So, lets say aircraft A gets told to climb, and aircraft B gets told to stay level.
The strength of an RA, the time its issued, the response times, and the corrective RA thresholds are such that a single RA ought to be sufficient - if it weren't, the the system would be inadequate for protection against Non TCAS aircraft.
However, lets say that aircraft A has lost an engine, the crew are in the early stages of working the procedures and haven't yet gone to TA only, yet are unable to get anywhere near +1750 fpm.
This is going to be a bit of a shame, because there is an aircraft present - aircraft B - that has the performance required to avoid the collision, but its being specifically told not to climb.
Whereas, if aircraft A is at TA only, aircraft B will NOT coordinate its escape manoeuvre. It'll say to itself "need to manouvre - descent inhibited - I will climb". Problem solved.
In otherwords "TA only mode" frees up the other aircraft in the encounter.
Essentially, IF you are working in an 'all tcas equipped' environment, then using TA mode is a "I have priority, you get out of my way" mode.
To that end CJ driver, use of TA mode is safer if the aircraft has a situation where you know you can not respond fully or accurately to an RA. Various aircraft I have flown have had 'TA only mode' specified in several abnormal checklists relating to engine or flying control malfunctions.
Mad Flt:
Yes, lots of type dependancy in there. For sure, the TCAS may have inputs from various systems to help it know what the aircraft can and can't do. For example, on the ATR (which I was a skipper on whan TCAS was mandated) the system had inputs by making particular ice protection selections. Setting "level 2" protection (i.e. we are entering icing conditions) changed the TCAS perf inhibitions (as well as changing a bunch of other stall protection / minimum speeds etc).
Similarly in a jet, ice protection bleeds reduce performance in a quantifiable manner and presumeably quite accurate calculations can be made regarding perf inhibitions.
However, I was alluding to actual airframe ice, rather than airframe ice protection. I know a lot of folks here will be driving jets with good performance but spare a thought for the turboprop drivers who live in the teens of thousands rather than just punching through them in a couple of minutes
pb