ACR FLC OPERATED MALFUNCTIONED APU ACFT EQUIP EVEN THOUGH THE APU HAD BEEN WRITTEN-UP IN THE ACFT RECORDS AS OTS DUE TO CAUSING SMOKE IN THE PAX CABIN.

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THE ACFT WE WERE USING TODAY HAD AN MEL ON THE APU FOR ITS HAVING PREVIOUSLY PUMPED SMOKE INTO THE CABIN. ALTHOUGH THE LOGBOOK WRITE-UP THAT PROMPTED ITS BEING TAKEN OTS DID NOT SAY SO SPECIFICALLY; THE CAPT ASSUMED THAT THE ACTUAL PROBLEM WAS ONE WHEREIN THE SMOKE HAD BEEN CREATED ONLY WHEN USING THE APU AIR SOURCE FOR AIR CONDITIONING ON THE GND; AND NOT MERELY FROM RUNNING THE APU AND PUMPING EXHAUST FUMES INSIDE. THE CAPT; THEREFORE; DIRECTED THAT THE APU BE STARTED AND USED FOR ENG START. SUBSEQUENT TO STARTING 1 ENG AND TAXIING OUT WITH THE OTHER ONE STILL SHUT DOWN; SO AS TO SAVE FUEL; I REVERTED TO OLD USUAL HABITS AND TURNED ON THE L AIR CONDITIONING PACK USING AIR FROM THE NOW-RUNNING L ENG AND THE R PACK USING THE AIR FROM THE APU. CLRLY; THIS SHOULD NOT HAVE BEEN DONE SINCE 3 OR 4 MINS AFTERWARD; THE AFT FLT ATTENDANT CAME FORWARD TO ADVISE THAT THERE WAS SMOKE COMING INTO THE CABIN AND BOTH LAVATORY SMOKE DETECTORS WERE SOUNDING. IMMEDIATELY; I TURNED OFF THE R AIR CONDITIONING PACK TO ELIMINATE THE SMOKE SOURCE. WE NEXT STARTED THE R ENG AND USED ITS AIR TO CLR OUT THE SMOKE AS RAPIDLY AS POSSIBLE. AS TO CAUSE; IT'S CLR THE PRIMARY ONE IS MY FAILURE TO REMEMBER NOT TO USE THE APU AS A SOURCE OF AIR AND TURNING ON THE R AIR CONDITIONING PACK AFTER STARTING ONLY THE L ENG. CONTRIBUTING ALSO I BELIEVE; WAS THE CAPT'S DEVIATING FROM MEL PROCS BY USING THE APU WHEN IT WAS NOT NECESSARY; AND SHOULD HAVE BEEN L OFF TO BEGIN WITH. UNFORTUNATELY; THIS WAS JUST ONE MORE OF THE INSTANCES DURING THIS 3 DAY TRIP WHEN THIS CAPT DEVIATED FROM RECOMMENDED OR REGULATED PROC. IN THIS CASE; I WAS CAUGHT COMPOUNDING THE DEV BY DOING JUST WHAT THE 'TOTALLY INOP' NATURE OF THE MEL PROCS TRIED TO AVOID. I FEEL THE FIRST CORRECTIVE ACTION WOULD BE TO FOLLOW PROCS IN ALL INSTANCES. HAD WE COMPLIED WITH THE MEL; AND L THE APU OFF TO BEGIN WITH; THERE NEVER WOULD HAVE BEEN A CHANCE FOR THIS PROBLEM OCCURRING.