DURING FLT; AN EMB120 FLC EXPERIENCES A PROB IN ATTEMPTING TO MAINTAIN CABIN PRESSURE AFTER SHIFTING FROM APU AIR TO ENG BLEED AIR.

Narrative

WE CONDUCTED A BLEEDS CLOSED/APU ON DEP OUT OF BFL WITH 3 PAX ENRTE TO LAX. THE CABIN WAS PRESSURIZING NORMALLY ON CLBOUT. AT LEVELOFF WE BROUGHT THE ENG BLEEDS ON LINE AND DEACTIVATED THE APU. UPON XFER TO ENG BLEEDS FROM APU BLEED THE CABIN BEGAN TO CLB. WE IMMEDIATELY BEGAN A DSCNT; AUTH BY ATC; TO 11000 FT MSL INITIALLY. WE REFED THE QRH AND ACTED ACCORDINGLY; ATTEMPTING TO CTL THE SYS MANUALLY. THIS ACTION DID NOT CORRECT THE SIT AND THE CABIN CONTINUED TO CLB. WE RECEIVED FURTHER CLRNC TO 9000 FT MSL AND BEGAN A FURTHER DSCNT. WE RESTARTED THE APU AND ATTEMPTED TO PRESSURIZE USING APU BLEED SINCE IT HAD PROVEN USEFUL DURING THE CLBOUT. HOWEVER; XFERRING BACK TO APU BLEED FROM ENG BLEEDS DID NOT CORRECT THE SIT. SOMEWHERE IN THE DSCNT BTWN 11000 FT AND 10000 FT MSL WE RECEIVED A CABIN WARNING ON THE MULTIPLE ALARM PANEL AS WELL AS THE 'CABIN' AURAL WARNING (ACTIVATED AT 10000 FT CABIN PRESSURE ALT). BEING AWARE OF THE FACT THAT WE WERE LEGAL ABOVE 10000 FT MSL FOR 30 MINS AND WITHIN SECONDS OF PASSING THROUGH 10000 FT MSL ON OUR WAY TO 9000 FT MSL WE ELECTED TO KEEP THE OXYGEN MASKS STOWED. OUR DECISION WAS BASED ON THE FACT THAT REMOVING MASKS; DONNING MASKS; AND ESTABLISHING COMS AT THAT ALT AND IN THE DSCNT WOULD NOT BE AN IMMEDIATE PRIORITY AND WOULD ONLY SERVE TO INCREASE OUR WORKLOAD WHILE DECREASING OUR EFFECTIVENESS AS A CREW. REACHING 9000 FT MSL WE REQUESTED AND RECEIVED CLRNCS FOR A GRADUAL (FOR PAX COMFORT) DSCNT TO PROGRESSIVELY LOWER ALTS. INFLT; WE DEDUCED THAT A CABIN LEAKAGE WAS PRESENT SINCE: 1) THE ENG/APU BLEEDS AND AIR CONDITIONING PACKS BOTH WERE OPERABLE AND PROVIDING AIR TO THE CABIN AND 2) BOTH THE AUTO AND MANUAL PRESSURIZATION SYS FAILED TO HOLD THE CABIN ALT. UPON ARR AT LAX IT WAS DISCOVERED THAT THE CARGO DOOR SEAL HAD BEEN RIPPED AND A SMALL STRIP WAS HANGING DOWN FROM THE CARGO DOOR OUTSIDE OF THE ACFT. IN CONCLUSION; I BELIEVE THE SEAL WAS RIPPED PARTIALLY OR FULLY SOMETIME PRIOR TO DEP FROM BFL. UPON XFERRING FROM THE APU BLEEDS TO THE ENG BLEEDS; THE ADDITIONAL PRESSURE OF THE ENG BLEED AIR FORCED THE SEAL TO DISLODGE CREATING THE CABIN LEAKAGE THAT DISABLED THE PRESSURIZATION SYS. NO EMER WAS DECLARED AND THE FLT TERMINATED AT DEST WITHOUT FURTHER INCIDENT.