A CL65 IN CLBOUT AT 2000 FT WHEN SWITCHING FROM APU TO ENG PNEUMATICS R PACK TRIPPED TWICE AND L PACK WENT TO ZERO. RETURNED TO DEP ARPT.

Narrative

MAINT NOT BEING ABLE TO SIMULATE FLT PROBS ON THE GND. ON FIRST ATTEMPT TO SWITCH BLEEDS FROM APU TO ENGS; WE EXPERIENCED A R PACK HIGH PRESSURE ANNUNCIATION. RAN QRH AND PACK RESET. SECOND ATTEMPT TO SWITCH R PACK HIGH PRESSURE ANNUNCIATION AND L PACK WENT TO ZERO OUTPUT (UNPRESSURIZED FLT). NOTIFIED ATC LEVELED AT 8000 FT. WITHIN SECONDS OF THE PACK TRIPPING OFF WE RECEIVED AN APU OVER TEMP ANNUNCIATION. RAN QRH AND SHUT DOWN APU; AND WAS RESTR FROM RESTARTING. WE WERE ABLE TO RESET THE L PACK TO RE-ESTABLISH PRESSURIZATION AND AIR CONDITIONING. CONTACTED MAINT ON RADIO AND WAS DIRECTED TO RETURN TO FIELD. WHEN ADVISED OF OVERWT THEY ALSO INSTRUCTED US TO BURN OFF FUEL. AFTER BURNING DOWN TO MAX LNDG WT; 1 HR 30 MINS; LANDED; TAXIED; AND DEPLANED WITHOUT INCIDENT. MAINT HISTORY: PACKS WERE WRITTEN UP THE DAY BEFORE (SOMEWHAT RELATED PROB); MEL'ED EVENT MORNING AND CLRED BEFORE OUR FLT! MULTIPLE ANNUNCIATIONS. SAME AS ABOVE NARRATIVE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS DISPATCHED AFTER MAINT ACCOMPLISHED A GND RUN AND COULD NOT DUPLICATE THE PNEUMATIC PACK FAILURES RPTED THE PREVIOUS DAY. THE R PACK FAILED ON CLBOUT AT 2000 FT; BUT RESET ONLY TO TRIP OFF FOR OVERPRESSURE AGAIN AND THEN THE L PACK FAILED. THE AIRPLANE HAS BEEN FIXED AND IS FLYING; BUT THE MAINT ACTION IS UNKNOWN.