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.