TCAS altitude information
 

I keep asking myself if the DHL 757 might have disappeared from the radar screen before the crash. Would have been possible if the transponder failed.
Now, is it possible that TCAS is working fine while the transponder isn´t? If yes, it would explain why both planes descended and why the 757 wasn´t given any instruction...the controller could simply have forgotten it because he couldn´t see it anymore. Maybe the descend instruction to the TU154 was just given because it was to be handed over to Geneva at FL 350.

That would be the only thing I could imagine to have happened. I just don´t believe a controller wouldn´t give it more than two tries to keep to conflicting airplanes apart.
Have had this kind of conflict myself, and I nearly got a heart attack and turned both of them 90° while giving traffic info...

caba, for an air traffic controller, I find your question and hypothesis somewhat strange, to say the least! However, to answer your question;TCAS will not operate when:

Mode-s transponder fails (display shows "TCAS FAIL")
Barometric altitude source fails (display shows "TCAS OFF")
Radio altitude fails (display shows "TCAS FAIL")

caba:

Interesting point, although perhaps not for the reason foremost on your mind.

Assuming you speak/read German, you might wish to read recently released (26 June 2002) Report No. 1744 of the Swiss Büro für Flugunfalluntersuchung which - among other items - points to the existence of screens at Skyguide from which planes have simply disappeared in the past (fortunately without any tragic consequences).

Caba,

Further to Hotdogs response, TCAS is dependant on having a functioning mode S transponder attached.

2 main reasons.

Firstly, the data linking process between the aircraft uses the transponders. I.E. your TCAS talks to his transponder, his TCAS talks to your transponder. Hence without the transponder their can be no TCAS to TCAS coordination.

Secondly, your TCAS actually gets its altitude data from your transponder.

Hence, if transponder turned off, TCAS stops working.

Incidentally, this is why it is important not to turn transponder to Standby when changing code, an arguemnt that pops up here every now and again.

CPB

Whilst ID squaks may momentarily disappear from ATC radar screens, the primary radar return remains in place. In case of transponder failure, which is not uncommon, a timely advice from ATC always facilitates the selection of the standby transponder. All the aircraft that I operated had two systems installed.

HotDog,

don´t know how it is in OZ, but it´s not uncommon here not to have primary radar data available for sectors working the upper airspace. In fact, most of the new antennas don´t have this capability anymore and are secondary only unless they are ASRs...

Regarding TCAS, I´m sorry for my "stupid" question, but our training doesn´t give us that many details...I think we are not supposed to know....

Hot Dog:

Second, if you had any idea of what you were talking about, you would know that the absence of primary radar data is one of the most critical issues at Skyguide.