Narrative:

Medium large transport aircraft en route from MMMX, elevation 7340', to iah, performed a no bleed takeoff to meet performance requirements for runway 05R. The automatic controller for the pressurization system was placarded inoperative, consequently the system was placed in standby operation. After takeoff, I performed the after takeoff checklist. The aircraft was climbing at 3000 FPM, passing through 17000' MSL, the 10000' cabin altitude warning horn sounded. I analyzed the pressurization system to locate the problem, but could not arrest the cabin rate of climb. Meanwhile, the captain leveled the aircraft at 19000' MSL while I requested a descent to 14000' from mexico center. Descending through 15000' MSL the passenger O2 mask deployed. We returned to MMMX. After analyzing the pressurization system, I came to the conclusion that I may have depressurized the aircraft while transferring from APU to engine bleed air pressurization. Operations and maintenance decided to redesignate flight as ferry flight and return (with crew only) to iah. After takeoff, climbing through 10000' MSL, the cabin altitude warning horn sounded again. We immediately leveled at 11000' MSL. All control switches were verified in the proper positions. I switched to the manual pressurization mode and reduced the cabin altitude. Completed the flight in the manual mode. Suspect a failure of the standby pressurization system during the flight. Due to the inoperative automatic mode, then the failure of the standby mode, I was unable to analyze this failure in a timely manner to prevent the deployment of the passenger O2 mask system. Supplemental information from acn 93065: suspect that flight had a failure of the standby psi system (automatic was inoperative) and that first officer had not depressurized aircraft. Flight was climbing through 17000' when standby failed and first officer was just unsuccessful in manual mode.

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Original NASA ASRS Text

Title: ACR MLG PRESSURIZATION CONTROL FAILED CAUSING LOSS OF PRESSURIZATION AND PASSENGER OXYGEN MASK DEPLOYMENT.

Narrative: MLG ACFT ENRTE FROM MMMX, ELEVATION 7340', TO IAH, PERFORMED A NO BLEED TKOF TO MEET PERFORMANCE REQUIREMENTS FOR RWY 05R. THE AUTO CTLR FOR THE PRESSURIZATION SYS WAS PLACARDED INOP, CONSEQUENTLY THE SYS WAS PLACED IN STANDBY OPERATION. AFTER TKOF, I PERFORMED THE AFTER TKOF CHKLIST. THE ACFT WAS CLBING AT 3000 FPM, PASSING THROUGH 17000' MSL, THE 10000' CABIN ALT WARNING HORN SOUNDED. I ANALYZED THE PRESSURIZATION SYS TO LOCATE THE PROB, BUT COULD NOT ARREST THE CABIN RATE OF CLB. MEANWHILE, THE CAPT LEVELED THE ACFT AT 19000' MSL WHILE I REQUESTED A DSCNT TO 14000' FROM MEXICO CENTER. DSNDING THROUGH 15000' MSL THE PAX O2 MASK DEPLOYED. WE RETURNED TO MMMX. AFTER ANALYZING THE PRESSURIZATION SYS, I CAME TO THE CONCLUSION THAT I MAY HAVE DEPRESSURIZED THE ACFT WHILE TRANSFERRING FROM APU TO ENG BLEED AIR PRESSURIZATION. OPS AND MAINT DECIDED TO REDESIGNATE FLT AS FERRY FLT AND RETURN (WITH CREW ONLY) TO IAH. AFTER TKOF, CLBING THROUGH 10000' MSL, THE CABIN ALT WARNING HORN SOUNDED AGAIN. WE IMMEDIATELY LEVELED AT 11000' MSL. ALL CONTROL SWITCHES WERE VERIFIED IN THE PROPER POSITIONS. I SWITCHED TO THE MANUAL PRESSURIZATION MODE AND REDUCED THE CABIN ALT. COMPLETED THE FLT IN THE MANUAL MODE. SUSPECT A FAILURE OF THE STANDBY PRESSURIZATION SYS DURING THE FLT. DUE TO THE INOP AUTO MODE, THEN THE FAILURE OF THE STANDBY MODE, I WAS UNABLE TO ANALYZE THIS FAILURE IN A TIMELY MANNER TO PREVENT THE DEPLOYMENT OF THE PAX O2 MASK SYS. SUPPLEMENTAL INFO FROM ACN 93065: SUSPECT THAT FLT HAD A FAILURE OF THE STANDBY PSI SYS (AUTO WAS INOP) AND THAT F/O HAD NOT DEPRESSURIZED ACFT. FLT WAS CLBING THROUGH 17000' WHEN STANDBY FAILED AND F/O WAS JUST UNSUCCESSFUL IN MANUAL MODE.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.