CREW OF CL65-200 HAS SMOKE IN CABIN AND COCKPIT AFTER SHIFTING AIR CONDITIONING BLEED SOURCE TO APU DURING APCH PHASE OF FLT PER COMPANY PROC.

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DESCENDING THROUGH APPROX 10000 FT; THE AIR CONDITIONING PACKS WERE TRANSFERRED TO THE APU. SHORTLY AFTER THAT THE COCKPIT CREW SMELLED SMOKE. APPROX 30 SECONDS AFTER SMELLING SMOKE WE RECEIVED A 'SMOKE TOILET' CAUTION MESSAGE. THE FLT ATTENDANT WAS IMMEDIATELY NOTIFIED OF THE SIT. THE QRH PROC WAS FOLLOWED FOR THE CAUTION MESSAGE. THE FLT ATTENDANT THEN NOTIFIED US THAT THE LAV SMOKE ALARM WAS SOUNDING BUT THERE WAS NO SIGN OF BURNING EXCEPT FOR A 'FAINT WISPY SMOKE' IN THE CABIN. THE AIR CONDITIONING PACKS WERE TRANSFERRED TO THE ENG BLEED AIR AND THE APU WAS SHUT DOWN. AN EMER WAS DECLARED AND WE WERE IMMEDIATELY CLEARED DIRECTLY TO THE ARPT (CVG). A NORMAL LNDG WAS MADE WITH ALL NORMAL CHECKLISTS AS WELL AS QRH PROCS BEING ACCOMPLISHED. WE TAXIED TO THE GATE AND WERE MET BY CFR. THIS SEEMS TO BE A RECURRING PROB WITH THE CL65-200. IT SEEMS THAT OCCASIONALLY WHEN THE PACKS ARE BEING RUN BY THE ENGINES THEN TRANSFERRED TO THE APU; THE APU CONTAMINATES CABIN AIR WITH TURBINE OIL SMOKE. MY MAIN CONCERN IN DEALING WITH THIS EMER; SINCE WE DID NOT KNOW WHAT OR WHERE THE SMOKE WAS COMING FROM; WAS GETTING THE ACFT ON THE GND ASAP. CRM REALLY HELPED IN DEALING WITH THIS EMER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT IT IS NORMAL PROC TO START THE APU AND USE IT AS A BLEED SOURCE FOR AIR CONDITIONING DURING THE APCH PHASE OF FLT TO PROVIDE ADEQUATE COOLING WHEN REDUCED THRUST SETTINGS ON THE ENG WOULD NOT. IT IS COMMON TO SMELL OIL SMOKE FOR A VERY SHORT TIME AFTER THE SWITCH OF THE BLEED SOURCE; BUT THERE IS USUALLY NO VISIBLE SMOKE AND IT CLEARS QUICKLY. HE REPORTS THAT THERE HAVE BEEN THREE INSTANCES AT HIS COMPANY OF SIMILAR SMOKE IN THE CABIN PROBS ASSOCIATED WITH THE SWITCH TO APU BLEED SOURCE IN THE LAST YEAR. IN ONE INSTANCE; THE SMOKE WAS THICK ENOUGH TO REQUIRE AT LEAST ONE CREW MEMBER TO BE HOSPITALIZED FOR OBSERVATION. HIS CONVERSATION WITH MECHANICS AFTER THE INCIDENT REVEALED THAT THIS PROB IS MORE COMMON AFTER APU SERVICING. THEY REPORT THAT THERE ARE SEVERAL ACFT IN THE FLEET THAT ARE WORSE THAN OTHERS; BUT THEY HAVE NOT FOUND THE EXACT CAUSE YET. THE RPTR HAS FOLLOWED UP WITH THE SAFETY REPRESENTATIVE OF HIS UNION WHO IS ALSO LOOKING INTO THIS RECURRING PROB.