Narrative:

While in cruise flight at FL310 in IMC conditions, icing condition with thunderstorms in the area, in constant light turbulence with occasional moderate turbulence. Received master warning light and cabin altitude annunciator light with cabin altitude indicating 9900 and climbing. Declared emergency by captain so as to ask lower altitude as ATC initially gave FL290. Captain wanted to get aircraft below 14000 ft to prevent oxygen poms from dispensing. Started rapid descent to 10000 ft, as approved by ATC. Meanwhile, the crew donned oxygen masks, and first officer performed checklist. Upon closing airfoil anti-ice valve, we recovered control over cabin and leveled off at FL190 and proceeded to destination.

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Original NASA ASRS Text

Title: DC9 LOSES PRESSURIZATION AT FL310 WHILE OPERATING ALL ACFT ANTI-ICE SYS. EMER DSCNT INITIATED, PRESSURIZATION RECOVERED AFTER ICING VALVES CLOSED PER CHKLIST.

Narrative: WHILE IN CRUISE FLT AT FL310 IN IMC CONDITIONS, ICING CONDITION WITH TSTMS IN THE AREA, IN CONSTANT LIGHT TURB WITH OCCASIONAL MODERATE TURB. RECEIVED MASTER WARNING LIGHT AND CABIN ALT ANNUNCIATOR LIGHT WITH CABIN ALT INDICATING 9900 AND CLBING. DECLARED EMER BY CAPT SO AS TO ASK LOWER ALT AS ATC INITIALLY GAVE FL290. CAPT WANTED TO GET ACFT BELOW 14000 FT TO PREVENT OXYGEN POMS FROM DISPENSING. STARTED RAPID DSCNT TO 10000 FT, AS APPROVED BY ATC. MEANWHILE, THE CREW DONNED OXYGEN MASKS, AND FO PERFORMED CHKLIST. UPON CLOSING AIRFOIL ANTI-ICE VALVE, WE RECOVERED CTL OVER CABIN AND LEVELED OFF AT FL190 AND PROCEEDED TO DEST.

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.