Jumbojohn,
When you are at TA only mode the other aircrafts TCAS doesn't coordinate its escape manoeuvre with your TCAS. This gives it a greater range of responses.
Bear in mind that there are other system inhibitions. So in some scenarios there may not be that many options for avoidance. It is even possible to imagine a scenario (at low altitude - say whilst flying a circling approach) like this:
Aircraft 1 (engine out)
Can't descend (inhibited due Rad Alt)
Can't Climb (or not enough) due engine out, but TCAS doesn't know.
Aircraft 2
Can't descend (inhibited due Rad ALt)
Can Climb.
Both in RA mode:
One aircraft gets preventative RA (coloured band / arc says not to climb)
The other aircraft gets a climb.
Two possible solutions:
1:
Aircraft 1 gets preventative.
Aircraft 2 gets Climb.
All is well.
2:
Aircraft 2 gets preventative.
Aircraft 1 gets climb. Fails to do it.
Collision.
So, 50/50 chance of disaster.
However, if aircraft 1 had been at TA only mode, then aircraft 2 would have definitely been issued a Climb.
So what it boils down to is that when your aircraft is malfunctioning to the point where being able to comply with an RA is going to be difficult, its better to be non TCAS.
Regarding the freedom to disregard an RA on visual spotting:
There are some great axample around of why this is a bad idea.
My favourite is one from the FAA TCAS transition program, very abbreviated here:
Aircraft 1 has two RA's in quick succession, versus 2 different aircraft (aircraft 2 and 3)
Gets RA versus aircraft 2. Unable to see any aircraft. Follows RA. Gets another RA, this time versus aircraft 3. Coincidentally, crew happen to spot aircraft 2, passing clear down their side. They say 'This RA not needed'. Fortunately, they followed it anyway, and then spot aircraft 3 as it flashes 300 feet overhead!
The morale of this incident is 'So you think the RA is not necessary? Follow it anyway!"
Even if you can see the other aircraft, its virtually impossible to second guess TCAS's RA anyway.
See this thread
Tone-uncage-fire
Having said that, there are times when it may not be appropriate to follow an RA, and this is why most Ops manuals (and guidance from regulatory authorities and manufacturers) usually give final authority over RA compliance to the crew.
Remember, TCAS only know about transponding targets. It doesn't know about non-transponding aircraft, or terrain (other than straight down - a ridge will spoil your day) or antenna, so it is always possible to come up with a scenario where RA compliance is more dangerous than doing nothing. At the end of the day a certain terrain impact is worse than a possible air impact, so that is why TCAS is subserviant to GPWS and Windshear.
So I take issue with your statement that there are no exceptions, and that an RA must always be followed.
At the risk of repeating myself, the recent collision was not caused by
non-compliance with RA's. It was caused by manoeuvres
opposite to the RA's. Such manoeuvres are universally prohibited by manufacturers and regulators alike (and hopefully by all training departments!).
Ultimately, if the tupolev had stayed level, all would have been well (that would have been non compliance). It was the descent when told to climb, followed by increasing desccent when told to increase climb. Two manoeuvres opposite in sense to the issued RA.
CPB