Briefly,
1.Can anybody tell me if the TCAS 2 requires a mod S transponder?
(is there an aircraft equipped with A/C transponder only, having a TCAS2 on board)
A mode S transponder is essential for the aircraft fitted with TCAS.
To be detected, the other aircraft must have either a Mode S or Mode C transponder.
To be able to issue an RA the other aircraft must be altitude reporting.
(Note: It is commonly stated that TCAS can detect Mode A, this is false. It uses the empty pulse frames of a Mode C response when the intruder has a Mode A+C transponder with altitude reporting turned off or not fitted).
If yes- what happens if an ACAS equipped interrogates no mode s, but also acas equipped aircraft?
Not sure what you mean. If the intruder has no mode s, it can't have TCAS. If it has Mode S, but its turned off or U/S then the TCAS is non funtional.
And how many aircraft out there flying without mode s in European airspace?
In the open FIR? Lots.
In CAS? Not many, but some.
2. How often do the pilots switch off RA mode and use TA only,why and when?
How often? Whenever the circumstances require it to be in TA only.
When / Why?
Engine Out. To prevent unachievable RA's being given and to give the other aircraft the best number of options for its RA instead.
Parallel Runway Operations, at some locations, to avoid nuisance warnings.
Various Technical Malfunctions, varies by type. E.G. Problems with Flying controls or anything else that might prevent accurate compliance with an RA.
Automatic TA Only. At low altitude, or during windshear / GPWS activation, or due to input failures or deselection of altitude reporting.
I recommend using the search facility, in this forum and tech log.
pb