After cabin pressure went to 11000 ft, the flight crew of a B737 descended to lower altitude when the engine anti-ice was turned off and cabin pressure was restored while on frequency with ZTL, GA. Distrs resulted in descent from assigned altitude.

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Attributes
ACN	622223
Time
Date	200303
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ztl.artcc
State Reference	GA
Altitude	msl bound lower : 15000 msl bound upper : 33000
Environment
Flight Conditions	IMC
Weather Elements	Ice
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl.artcc
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 235 flight time total : 23000 flight time type : 3500
ASRS Report	622223
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter : weather other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : cabin alt gauge other flight crewa other flight crewb
Resolutory Action	flight crew : exited adverse environment flight crew : overcame equipment problem other
Supplementary
Problem Areas	Flight Crew Human Performance Environmental Factor Aircraft Weather
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Cabin altitude warning horn sounded at FL330 (with engine anti-ice on, outside air temperature was -34 degrees static air temperature). Cabin altitude passed 11000 ft and an emergency descent was initiated. The cabin altitude warning checklist was accomplished. As we passed FL220, we entered VMC conditions and turned the engine anti-ice off. We leveled off at 15000 ft. The cabin altitude was now below 10000 ft and descending. The pressurization system returned to normal. A slow climb was initiated back to FL290 and a decision to continue to ewr was made. We climbed back up to FL330 and all indications (a cabin altitude of 6200 ft and pressure differential of 7.2 psi) were normal. Since engine and anti-ice were not required at cruise altitude, we could not duplicate the problem with the pressurization system. Engine anti-ice was used in the descent into ewr. All pressurization indications were normal.

Original NASA ASRS Text

Title: AFTER CABIN PRESSURE WENT TO 11000 FT, THE FLT CREW OF A B737 DSNDED TO LOWER ALT WHEN THE ENG ANTI-ICE WAS TURNED OFF AND CABIN PRESSURE WAS RESTORED WHILE ON FREQ WITH ZTL, GA. DISTRS RESULTED IN DSCNT FROM ASSIGNED ALT.

Narrative: CABIN ALT WARNING HORN SOUNDED AT FL330 (WITH ENG ANTI-ICE ON, OUTSIDE AIR TEMP WAS -34 DEGS STATIC AIR TEMP). CABIN ALT PASSED 11000 FT AND AN EMER DSCNT WAS INITIATED. THE CABIN ALT WARNING CHKLIST WAS ACCOMPLISHED. AS WE PASSED FL220, WE ENTERED VMC CONDITIONS AND TURNED THE ENG ANTI-ICE OFF. WE LEVELED OFF AT 15000 FT. THE CABIN ALT WAS NOW BELOW 10000 FT AND DSNDING. THE PRESSURIZATION SYS RETURNED TO NORMAL. A SLOW CLB WAS INITIATED BACK TO FL290 AND A DECISION TO CONTINUE TO EWR WAS MADE. WE CLBED BACK UP TO FL330 AND ALL INDICATIONS (A CABIN ALT OF 6200 FT AND PRESSURE DIFFERENTIAL OF 7.2 PSI) WERE NORMAL. SINCE ENG AND ANTI-ICE WERE NOT REQUIRED AT CRUISE ALT, WE COULD NOT DUPLICATE THE PROB WITH THE PRESSURIZATION SYS. ENG ANTI-ICE WAS USED IN THE DSCNT INTO EWR. ALL PRESSURIZATION INDICATIONS WERE NORMAL.

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.