Narrative:

Departed las runway 19L bound for oak. During initial climb, both crew members noted higher than normal cockpit noise levels. I was the PNF, so upon completion of the climb checklist, during which pressurization was confirmed normal, I began investigating the source of the extra noise. All inflow and outflow controls were checked as were windows. All were found to be set appropriately for flight. At approximately 16000 ft MSL, during climb to FL280, as assigned by ZLA, I noticed a pressure change in my ears. I noted at that time that the cabin rate of climb indicator showed over 3000 FPM climb. I directed the first officer to stop the climb and rechked the pressurization components for both inflow and outflow. I determined that a descent was necessary and directed the first officer to descend to 12000 ft MSL. I also informed ZLA of the problem and told him we were descending to 12000 ft. After a long pause the controller asked if we could accept 14000 ft for a short time. Given that I had noise in the cockpit I couldn't explain and that we had also lost pressurization, I decided that I wanted to be at a safe altitude for normal breathing even though we had supplemental oxygen available. I told the controller we wanted 12000 ft. His response was to declare us to be an emergency aircraft citing the fact that we would descend below his MVA. I told him we were in visual conditions and would be able to stay clear of terrain. I then requested clearance back to las. After the cabin equalized with aircraft altitude, there was effectively no more operational problem and I informed ATC that the situation was under control and that no additional assistance was required. Landing in las was uneventful and normal. Investigation into the cause of the problem revealed no obvious defects and as of this writing, no cause has yet been found.

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Original NASA ASRS Text

Title: AN H25B CREW, DEPARTING LAS, EXPERIENCED A LOSS OF CABIN PRESSURE, SPAWNING AN EMER DSCNT TO 12000 FT.

Narrative: DEPARTED LAS RWY 19L BOUND FOR OAK. DURING INITIAL CLB, BOTH CREW MEMBERS NOTED HIGHER THAN NORMAL COCKPIT NOISE LEVELS. I WAS THE PNF, SO UPON COMPLETION OF THE CLB CHKLIST, DURING WHICH PRESSURIZATION WAS CONFIRMED NORMAL, I BEGAN INVESTIGATING THE SOURCE OF THE EXTRA NOISE. ALL INFLOW AND OUTFLOW CTLS WERE CHKED AS WERE WINDOWS. ALL WERE FOUND TO BE SET APPROPRIATELY FOR FLT. AT APPROX 16000 FT MSL, DURING CLB TO FL280, AS ASSIGNED BY ZLA, I NOTICED A PRESSURE CHANGE IN MY EARS. I NOTED AT THAT TIME THAT THE CABIN RATE OF CLB INDICATOR SHOWED OVER 3000 FPM CLB. I DIRECTED THE FO TO STOP THE CLB AND RECHKED THE PRESSURIZATION COMPONENTS FOR BOTH INFLOW AND OUTFLOW. I DETERMINED THAT A DSCNT WAS NECESSARY AND DIRECTED THE FO TO DSND TO 12000 FT MSL. I ALSO INFORMED ZLA OF THE PROB AND TOLD HIM WE WERE DSNDING TO 12000 FT. AFTER A LONG PAUSE THE CTLR ASKED IF WE COULD ACCEPT 14000 FT FOR A SHORT TIME. GIVEN THAT I HAD NOISE IN THE COCKPIT I COULDN'T EXPLAIN AND THAT WE HAD ALSO LOST PRESSURIZATION, I DECIDED THAT I WANTED TO BE AT A SAFE ALT FOR NORMAL BREATHING EVEN THOUGH WE HAD SUPPLEMENTAL OXYGEN AVAILABLE. I TOLD THE CTLR WE WANTED 12000 FT. HIS RESPONSE WAS TO DECLARE US TO BE AN EMER ACFT CITING THE FACT THAT WE WOULD DSND BELOW HIS MVA. I TOLD HIM WE WERE IN VISUAL CONDITIONS AND WOULD BE ABLE TO STAY CLR OF TERRAIN. I THEN REQUESTED CLRNC BACK TO LAS. AFTER THE CABIN EQUALIZED WITH ACFT ALT, THERE WAS EFFECTIVELY NO MORE OPERATIONAL PROB AND I INFORMED ATC THAT THE SIT WAS UNDER CTL AND THAT NO ADDITIONAL ASSISTANCE WAS REQUIRED. LNDG IN LAS WAS UNEVENTFUL AND NORMAL. INVESTIGATION INTO THE CAUSE OF THE PROB REVEALED NO OBVIOUS DEFECTS AND AS OF THIS WRITING, NO CAUSE HAS YET BEEN FOUND.

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.