FLC OF ACR ACFT ARE ACCUSED OF OPERATING THE ACFT APU WHEN IT HAD BEEN PLACARDED OTS.

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DURING THE COURSE OF A 3-DAY TRIP WITH 12 LEGS OF FLT; MY COPLT AND I FLEW MLG SHIP ON 3 LEGS OF FLT. DURING THESE 3 LEGS ON THIS PARTICULAR ACFT; THE APU WAS PLACARDED INOP. AT NO TIME DURING THESE 3 FLTS DID MY COPLT OR I USE THE APU ON OCT/X OR Y/92. AFTER COMPLETION OF THE LAST LEG OF FLT FROM IAH TO DEN; OUR MAINT PERSONNEL; IN CORRECTING A WRITEUP I'D MADE ON ANOTHER ACFT SYS; NOTICED A MAJOR PROBLEM WITH THE APU. APPARENTLY; THE APU HAD HAD A FIRE AND SELF SHUTDOWN AS A RESULT OF A FIRE. OUR MAINT PEOPLE ASKED IF OUR CREW HAD USED THE APU AND HAD A FIRE. WE REPLIED IN THE NEGATIVE; OF COURSE. NOW; A FEW DAYS LATER; I'D LEARNED THAT THE FAA IS INVESTIGATING THE OCCURRENCE AND SUSPECTS ME AND MY COPLT OF HAVING CAUSED THE PROBLEM. INASMUCH AS THE APU HAD BEEN PLACARDED INOP FOR 2 1/2 DAYS WHILE THE ACFT WAS IN NORMAL SVC; THERE WERE SEVERAL FLCS AND NUMEROUS MAINT PERSONNEL AND OTHER AGENTS THAT EASILY COULD HAVE CAUSED THE PROBLEM. MY PHILOSOPHY OF FLYING FOR OVER 20 YRS IN MIL AND AIRLINE WORK HAS BEEN; AND CONTINUES TO BE; DO THINGS CORRECTLY AND BY THE BOOK; AND ONE HAS NOTHING TO FEAR FROM INSPECTORS; EVALUATORS OR ANYONE. IN THIS INSTANCE; HOWEVER; IF NO ONE ELSE COMES FORWARD TO ADMIT THEY CAUSED THE PROBLEM; IT BECOMES MY WORD AGAINST THE INSPECTOR'S GUESS OR ESTIMATE OF THE PROBLEM.