Narrative:

In cruise at FL350 (approximately position 200 NM northeast of den VORTAC) we experienced a partial loss of cabin pressure requiring a diversion to denver (original destination phx). We had been cruising at FL350 for about 45 mins, 30 of which were in light to occasional mod turbulence, in the vicinity of a cold front which had numerous embedded thunderstorms. As we cleared the southwest end of the WX, the 'cabin pressure' warning illuminated and the cabin pressure indicator climbed to 10,500' and remained steady. The passenger masks had deployed and all passenger were receiving oxygen. Since the pressure had stabilized, we coordinated with den ARTCC and descended to FL310 and performed the 'cabin pressure loss' checklist. We hesitated to reduce engine thrust since the loss of high pressure engine bleed to the air cycle machines would only aggravate the situation. At FL310 the cabin pressure began to climb again at approximately +300 FPM at which point we elected to continue descending and divert to den which was the closest suitable airport considering the WX in the vicinity. The cabin altitude reached a maximum of about 12,500-13,000'. As we passed about 18,000' the cabin temperature and duct temperatures became noticeably warmer and passing about 15,000' the cabin pressure recovered rapidly. We leveled at 11,000', talked with the F/a to ensure that the passenger were suffering no ill effects and proceeded to den. According to maintenance in msp they determined that the probable cause was the freezing of moisture in the water separator bags in the air cycle machines and a possible problem with the automatic pressurization control box. There was never any difficulty handling the aircraft or dealing with the loss of cabin pressure. The biggest problem for a 2-MAN crew was coordinating with ATC, dispatch, and air carrier Y airlines operations (who handled us on the ground in den). For 15 mins prior to landing we were extremely busy with numerous radio calls, checklists, and talking to the F/a's. In many ways we were fortunate that it was night and that the den WX was VFR.

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Original NASA ASRS Text

Title: ACR MLG CABIN PRESSURIZATION PROBLEMS AT FL350. MASKS DROPPED AS DESIGNED AND ACFT DIVERTED TO ALTERNATE.

Narrative: IN CRUISE AT FL350 (APPROX POSITION 200 NM NE OF DEN VORTAC) WE EXPERIENCED A PARTIAL LOSS OF CABIN PRESSURE REQUIRING A DIVERSION TO DENVER (ORIGINAL DEST PHX). WE HAD BEEN CRUISING AT FL350 FOR ABOUT 45 MINS, 30 OF WHICH WERE IN LIGHT TO OCCASIONAL MOD TURBULENCE, IN THE VICINITY OF A COLD FRONT WHICH HAD NUMEROUS EMBEDDED TSTMS. AS WE CLRED THE SW END OF THE WX, THE 'CABIN PRESSURE' WARNING ILLUMINATED AND THE CABIN PRESSURE INDICATOR CLIMBED TO 10,500' AND REMAINED STEADY. THE PAX MASKS HAD DEPLOYED AND ALL PAX WERE RECEIVING OXYGEN. SINCE THE PRESSURE HAD STABILIZED, WE COORDINATED WITH DEN ARTCC AND DESCENDED TO FL310 AND PERFORMED THE 'CABIN PRESSURE LOSS' CHECKLIST. WE HESITATED TO REDUCE ENGINE THRUST SINCE THE LOSS OF HIGH PRESSURE ENGINE BLEED TO THE AIR CYCLE MACHINES WOULD ONLY AGGRAVATE THE SITUATION. AT FL310 THE CABIN PRESSURE BEGAN TO CLIMB AGAIN AT APPROX +300 FPM AT WHICH POINT WE ELECTED TO CONTINUE DESCENDING AND DIVERT TO DEN WHICH WAS THE CLOSEST SUITABLE ARPT CONSIDERING THE WX IN THE VICINITY. THE CABIN ALT REACHED A MAX OF ABOUT 12,500-13,000'. AS WE PASSED ABOUT 18,000' THE CABIN TEMPERATURE AND DUCT TEMPERATURES BECAME NOTICEABLY WARMER AND PASSING ABOUT 15,000' THE CABIN PRESSURE RECOVERED RAPIDLY. WE LEVELED AT 11,000', TALKED WITH THE F/A TO ENSURE THAT THE PAX WERE SUFFERING NO ILL EFFECTS AND PROCEEDED TO DEN. ACCORDING TO MAINT IN MSP THEY DETERMINED THAT THE PROBABLE CAUSE WAS THE FREEZING OF MOISTURE IN THE WATER SEPARATOR BAGS IN THE AIR CYCLE MACHINES AND A POSSIBLE PROBLEM WITH THE AUTO PRESSURIZATION CONTROL BOX. THERE WAS NEVER ANY DIFFICULTY HANDLING THE ACFT OR DEALING WITH THE LOSS OF CABIN PRESSURE. THE BIGGEST PROBLEM FOR A 2-MAN CREW WAS COORDINATING WITH ATC, DISPATCH, AND ACR Y AIRLINES OPERATIONS (WHO HANDLED US ON THE GND IN DEN). FOR 15 MINS PRIOR TO LNDG WE WERE EXTREMELY BUSY WITH NUMEROUS RADIO CALLS, CHECKLISTS, AND TALKING TO THE F/A'S. IN MANY WAYS WE WERE FORTUNATE THAT IT WAS NIGHT AND THAT THE DEN WX WAS VFR.

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.