Narrative:

My first officer and I were relocating an airplane from oak to rdd. We had a dead heading pilot and a mechanic who was commuting home riding in the back during the part 91 flight. At 1/2 hour into the flight at FL180 the dead heading pilot came forward and informed me that the oxygen mask had deployed in the cabin. I then noticed the cabin altitude to be 12000 ft. The cabin annunciator never illuminated, even though it tested ok. Both flows were at 7. When I increased the left flow the cabin started to pressurize. It spiked the cabin rate to 1000-1500 FPM. I then turned the cabin rate down. The pilot and mechanic proceeded to replace the mask. All the masks were replaced by the time we landed. This has been a common event with this aircraft. I decided not to write it up as a maintenance discrepancy and informed the captain who was going to fly it next on a revenue flight. This was a common practice at our airline to keep the planes from being grounded at out-stations. The airplane did not have any problems that day. After further thought I decided that in the future I will write up all maintenance problems so recurring discrepancies will be fixed.

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Original NASA ASRS Text

Title: ACFT PRESSURIZATION PROB. CABIN ALT EXCEEDED 12000 FT WITHOUT THE ANNUNCIATOR ILLUMINATING. FIRST INDICATION OF EXCESSIVE CABIN ALT WAS DEPLOYED OXYGEN MASKS. FLC REGAINED PRESSURIZATION CTL.

Narrative: MY FO AND I WERE RELOCATING AN AIRPLANE FROM OAK TO RDD. WE HAD A DEAD HEADING PLT AND A MECH WHO WAS COMMUTING HOME RIDING IN THE BACK DURING THE PART 91 FLT. AT 1/2 HR INTO THE FLT AT FL180 THE DEAD HEADING PLT CAME FORWARD AND INFORMED ME THAT THE OXYGEN MASK HAD DEPLOYED IN THE CABIN. I THEN NOTICED THE CABIN ALT TO BE 12000 FT. THE CABIN ANNUNCIATOR NEVER ILLUMINATED, EVEN THOUGH IT TESTED OK. BOTH FLOWS WERE AT 7. WHEN I INCREASED THE L FLOW THE CABIN STARTED TO PRESSURIZE. IT SPIKED THE CABIN RATE TO 1000-1500 FPM. I THEN TURNED THE CABIN RATE DOWN. THE PLT AND MECH PROCEEDED TO REPLACE THE MASK. ALL THE MASKS WERE REPLACED BY THE TIME WE LANDED. THIS HAS BEEN A COMMON EVENT WITH THIS ACFT. I DECIDED NOT TO WRITE IT UP AS A MAINT DISCREPANCY AND INFORMED THE CAPT WHO WAS GOING TO FLY IT NEXT ON A REVENUE FLT. THIS WAS A COMMON PRACTICE AT OUR AIRLINE TO KEEP THE PLANES FROM BEING GNDED AT OUT-STATIONS. THE AIRPLANE DID NOT HAVE ANY PROBS THAT DAY. AFTER FURTHER THOUGHT I DECIDED THAT IN THE FUTURE I WILL WRITE UP ALL MAINT PROBS SO RECURRING DISCREPANCIES WILL BE FIXED.

Data retrieved from NASA's ASRS site as of July 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.