Narrative:

During descent from VMC at FL280 into cloud tops; OAT was such that cowl heat usage was required; and was selected. Cowl heat seemed to operate normally for a short period of time; I would estimate between 1 and 3 minutes. After which the cowl heat caution message posted at a time when the N2 was sufficient for normal operation; followed by an isolation valve caution message; and an abnormally high cabin rate of climb was noticed. Both pilots donned the oxygen mask as it was apparent that the cabin altitude was well beyond its normal range; and increasing rapidly. I then had the first officer begin the QRH procedure for the isolation valve caution message; and relieved him of all of his other non-flying pilot duties; as per company procedure. I immediately advised ATC that we were having an issue with a valve in the pressurization system and that we needed to continue our descent below our crossing altitude; but we were very close to the crossing intersection; and did have to briefly level as the controller coordinated descent. Upon stopping descent; cabin altitude began to increase above 8;500 and the cabin altitude caution message briefly posted; and before I was able to declare an emergency; the controller had cleared us to 10;000 ft. As we started a high normal rate of descent; the cabin altitude began descending; and through the combination of the descent and the QRH procedure completion; the cabin altitude returned to its normal range during descent. No emergency was declared by the crew; nor was priority requested; but we did advise ATC that we were operating without ice protection and that deviations may be required around clouds; but normal vectors and descent kept us free of icing conditions and we continued to a normal landing.

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Original NASA ASRS Text

Title: A CRJ-70 flight crew experienced a pneumatic isolation valve failure that led to pressurization issues. The First Officer mentioned the QRH for the problem was 'unclear.'

Narrative: During descent from VMC at FL280 into cloud tops; OAT was such that cowl heat usage was required; and was selected. Cowl heat seemed to operate normally for a short period of time; I would estimate between 1 and 3 minutes. After which the cowl heat caution message posted at a time when the N2 was sufficient for normal operation; followed by an isolation valve caution message; and an abnormally high cabin rate of climb was noticed. Both pilots donned the oxygen mask as it was apparent that the cabin altitude was well beyond its normal range; and increasing rapidly. I then had the First Officer begin the QRH procedure for the isolation valve caution message; and relieved him of all of his other non-flying pilot duties; as per company procedure. I immediately advised ATC that we were having an issue with a valve in the pressurization system and that we needed to continue our descent below our crossing altitude; but we were very close to the crossing intersection; and did have to briefly level as the Controller coordinated descent. Upon stopping descent; cabin altitude began to increase above 8;500 and the CABIN ALT caution message briefly posted; and before I was able to declare an emergency; the Controller had cleared us to 10;000 FT. As we started a high normal rate of descent; the cabin altitude began descending; and through the combination of the descent and the QRH procedure completion; the cabin altitude returned to its normal range during descent. No emergency was declared by the crew; nor was priority requested; but we did advise ATC that we were operating without ice protection and that deviations may be required around clouds; but normal vectors and descent kept us free of icing conditions and we continued to a normal landing.

Data retrieved from NASA's ASRS site as of July 2013 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.