Would an avionics question be pertinent here?
At our US company, the TCAS was out of service on the last leg, per MEL, on my last trip (a 100-seat turbofan). Only the normal, good old transponder mode C etc worked.
The MEL book had only one restriction, no PRM (parallel approaches, reduced lateral separation, radar monitored) approaches are allowed. No other limitations are required by the FAA. There was no mention of RVSM, altitude limitations (i.e. FL 290), autopilots etc.
It surprised both the First Officer and me that there were no other conditions, in order to operate 1,000' from aircraft at all altitudes. The RVSM is fairly new for us, and requires the digital altimeters and the large clean areas around static ports etc, but TCAS has been around many more years.
Does it seem like there is a contradiction here somewhere? Why would TCAS not be required to be operative, when we had no restrictions except for prohibiting PRM approaches?
This just now came back to me. One of our pilots who worked as a "Cruise FO" for two years with a 'north European' airline around '90-'91 was on our jumpseat about 14 years ago, going to ABQ. One dark night over eastern Russia the Captain went back to either sleep or puff on his pipe. This Cruise Pilot in the left seat was staring out ahead as they climbed to another altitude (before TCAS). "Kr****e" (his nickname over there) told me that he saw the lights of a large jet straight ahead on a collision course. He pushed the nose down a bit and missed the plane by not very much room! He told us something strange about ATC's response, when asked about the dangerous situation. Luckily they were in good VMC.