CAPT OF A FK100 LEARNS THAT HE HAD OPERATED AN UNAIRWORTHY ACFT DUE TO A REDETERMINATION BY MAINT OF THE REAL ACFT DISCREPANCY. SUBSEQUENTLY FOUND COULD NOT BE MEL'ED!

Narrative

WHEN I ARRIVED AT THE ACFT THERE WAS A MAINT PLACARD IN THE ACFT LOG BOOK STATING; 'PITOT 3 FAULT LIGHT INOP PER MEL 30-9; AUTH 7-108C-C.' THE MEL SAYS; 'ALL [PITOT FAULT LIGHTS] MAY BE INOP PROVIDED MAINT VERIFIES THAT ASSOCIATED MFD FAULT MESSAGES AND RELATED ALERTS ARE OPERATIVE PER MPM 30-9.' I CALLED THE FK100 SPECIALIST AT OUR TECHNICAL CTR TO ASK WHAT THE MPM 30-9 PROC CONSISTED OF AND HOW THEY KNEW THE PROB WAS IN THE FAULT LIGHT AND NOT AN INOP PITOT HEATER. I WAS TOLD THAT MPM 30-9 WAS A VERY COMPLICATED; 2 PAGE DIAGNOSTIC PROC THAT CHKED THE ENTIRE SYS. THAT SOUNDED REASONABLE TO ME; SO I FLEW THE ACFT 3 LEGS. 4 DAYS LATER; I WAS NOTIFIED BY A CHIEF PLT THAT OUR MAINT HAD DECIDED THAT NOTWITHSTANDING THEIR MPM 30-9 PROC; THEY HAD COME TO THE WRONG CONCLUSION; AND THE #3 PITOT HEATER HAD BEEN INOP; NOT THE FAULT LIGHT. THE PLACARD WAS THEREFORE INVALID AND MAINT HAD RPTED THEIR ERROR TO THE APPROPRIATE FAA AIRWORTHINESS OFFICE. I AM WRITING THIS BECAUSE I AM NOW GUILTY; RETROACTIVELY; OF FLYING AN ACFT THAT WAS NOT AIRWORTHY. I DO NOT KNOW WHAT ELSE I COULD HAVE DONE TO CHK THE VALIDITY OF THE MAINT PLACARD. AT SOME POINT YOU JUST HAVE TO HAVE FAITH IN THE INTEGRITY OF THE SYS.