MD-83
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Aircraft
About half way into our flight; the VSI was showing 'TA only'.  I asked the Pilot Monitoring (PM) to inform ATC that only the TA function of our TCAS system was operational; and could they descend us into non-RVSM airspace. ATC said; that's okay; we have you on radar. At this point; the 'TA only' message was gone; so the PM did not say anything further to ATC about the TCAS being a company requirement for RVSM operations. About 10 minutes later the 'TA only' message reappeared; and I now requested to descend to FL280. Shortly after leveling at FL280; the 'TA only' message disappeared.
MD-83 Captain reported that only the TA function of the TCAS system was operational and requested clearance into non-RVSM airspace.