ACR DC9-30 XPONDER MALFUNCTION WHILE CONDUCTING PRECISION RADAR MONITOR (PRM) TEST PROC. FLC TO FOLLOW-UP WITH FAA AND COMPANY. FO CONCERNED WITH IMPACT OF THIS TEST PROC AND OTHER NEWLY IMPLEMENTED PERFORMANCE PROCS AT OTHER NATIONAL ARPTS.

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AFTER ARR; I WAS ADVISED BY MSP APCH (CALL TO MY ROOM) THAT THEY HAD LOST OUR ACFT FROM PRECISION RADAR MONITOR RADAR INSIDE OF SISSY FOR OVER 40 SECONDS. XPONDER HAD BEEN PLACED TO 'TA' MODE AT SISSY AS PER STANDARD PROCS; AND WAS NOT TOUCHED AGAIN UNTIL AFTER LNDG CHKLIST WAS DONE. CAPT WAS AT CTLS AND PRECISION RADAR MONITOR APCH WAS BRIEFED IN DETAIL. BOTH PRIMARY AND MONITOR FREQS WERE TESTED FOR VOLUME AND SELECTED AT APPROPRIATE TIME. NO CALL WAS MADE BY ATC TO ACFT TO INDICATE ANY PROB WITH THE XPONDER RECEPTION. DURING THE CALL FROM THE ATC SUPVR TO ME; IT WAS LEARNED THAT; ALTHOUGH THE ACFT DID DROP OFF THE PRECISION RADAR MONITOR SCOPE; THE ACFT XPONDER WAS RECEIVED ON MSP APCH 'NON-PRECISION RADAR MONITOR' RADAR. THE MSP SUPVR; SAID THAT THIS HAD HAPPENED BEFORE TO ANOTHER DC9 ACFT. HE INDICATED THAT HE WOULD WORK WITH ATC REPRESENTATIVE TO HELP SOLVE THE PROB. THIS EVENT HIGHLIGHTS THE EXISTENCE OF A TECHNICAL PROB AS WELL AS A PROB WITH PROCS FOR ACFT THAT DISAPPEAR FROM THE PRECISION RADAR MONITOR RADAR SCOPE. SUPPLEMENTAL INFO FROM ACN 431368: THE CAPT ADVISED ME THAT THERE WAS A QUESTION ABOUT OUR ILS PRECISION RADAR MONITOR RWY 30L APCH INTO MSP. CREW THOROUGHLY BRIEFED THE PRECISION RADAR MONITOR ILS RWY 30L PER OUR COMPANY. WE TURNED OUR TCASII FROM RA/TA MODE TO THE TA ONLY MODE AT THE SISSY INTXN AND IT REMAINED THERE UNTIL THE AFTER LNDG FLOW AND CHKLIST. MSP HAD MENTIONED THAT THEIR APCH RADAR NEVER LOST US BUT THEIR PRECISION RADAR MONITOR RADAR DID. THE MSP PRECISION RADAR MONITOR PROCS ARE IN A TESTING PHASE AND FROM WHAT THIS CREW EXPERIENCED IT IS NOT 100%. I THINK IT IS ERODING SAFETY WITH ALL THE LAHSO; NOISE ABATEMENT PROCS; CRAMMING OF RWYS; AND NOW ACFT FLYING IN FORMATION LESS THAN 1 MI APART SIDE-BY-SIDE IN IMC CONDITIONS.