Narrative:

While climbing through FL300 for final cruise altitude of FL330; the cabin pressurization warning horn sounded. The captain noted the cabin pressure between 10000 ft and 11000 ft and initiated an immediate descent while instructing the first officer to accomplish the QRH memory items for 'cabin altitude warning horn/abnormal pressurization' and emergency descent checklist items. Both the captain and first officer donned emergency oxygen masks and established crew communications while requesting an immediate descent to 14000 ft or below from ARTCC. The captain instructed the first officer to notify the flight attendants. The captain declared an emergency with ARTCC; who facilitated a vmmo descent to 9000 ft MSL. Once level at 9000 ft MSL; the captain and first officer removed their oxygen masks and completed all QRH checklist items. The first officer noticed the altitude set in the 'land altitude' and the 'cabin attendant altitude' indictor windows were both set incorrectly. The first officer intended to select 1118 ft. However; the first officer inadvertently set 11080 ft in the 'land altitude' and 'cabin attendant altitude' analog indicators. These entries likely caused the abnormal pressurization; which resulted in a maximum cabin altitude between 10000 ft and 11000 ft. The captain elected to switch the pressurization from manual to the automatic mode with the correct 'land altitude' and 'cabin attendant altitude' entries in the indicator windows. Upon this action; the aircraft pressurized normally. The captain instructed the first officer to complete all QRH and normal descent checklist items; and to evaluate the required fuel to complete the flight. After assessing all parameters; the captain elected to continue at 9000 ft and 250. En route company maintenance and operations were notified of an abnormal cabin pressurization event and were requested to meet the aircraft on arrival. Company maintenance completed a full test of the pressurization system of the aircraft and concluded that the incorrect settings in the 'land altitude' and 'cabin attendant altitude' indicators likely caused an off-scheduled cabin pressurization event. The aircraft was recertified to continue flight after passing all pressurization checks. The captain and first officer flew the aircraft uneventfully to our next city; as scheduled; with no further pressurization issues. Lessons learned: 1) first officer must ensure the correct destination airport altitude is entered into the aircraft system. 2) there is significant value in practicing donning emergency masks and establishing crew communications. The ambient noise during an emergency descent is significant and communications between crew members can be difficult. Additionally; boom mikes; headsets; and glasses can be impediments to rapidly donning the masks. Lastly; the first officer had recently completed a similar training event during his initial training as a new hire. This recency of training and exposure to a rapid descent was valuable training. 3) at certain seat heights; it is difficult to properly read the pressurization readouts and there are knobs that can obstruct the captain's and first officer xchk of the pressurization window. There is no substitute for aircraft system knowledge. This event appeared to be a result of improper 'land altitude' and 'cabin attendant altitude' indicator entries; but the possibility of such an event; or its consequences; is not addressed in the aircraft manual. 5) there is no substitute for xchk; check; and rechking even when one assumes all checklists have been confirmed.

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Original NASA ASRS Text

Title: B737 CREW EXPERIENCES CABIN ALT WARNING HORN AND INITIATES AN EMER DSCNT.

Narrative: WHILE CLBING THROUGH FL300 FOR FINAL CRUISE ALT OF FL330; THE CABIN PRESSURIZATION WARNING HORN SOUNDED. THE CAPT NOTED THE CABIN PRESSURE BTWN 10000 FT AND 11000 FT AND INITIATED AN IMMEDIATE DSCNT WHILE INSTRUCTING THE FO TO ACCOMPLISH THE QRH MEMORY ITEMS FOR 'CABIN ALT WARNING HORN/ABNORMAL PRESSURIZATION' AND EMER DSCNT CHKLIST ITEMS. BOTH THE CAPT AND FO DONNED EMER OXYGEN MASKS AND ESTABLISHED CREW COMS WHILE REQUESTING AN IMMEDIATE DSCNT TO 14000 FT OR BELOW FROM ARTCC. THE CAPT INSTRUCTED THE FO TO NOTIFY THE FLT ATTENDANTS. THE CAPT DECLARED AN EMER WITH ARTCC; WHO FACILITATED A VMMO DSCNT TO 9000 FT MSL. ONCE LEVEL AT 9000 FT MSL; THE CAPT AND FO REMOVED THEIR OXYGEN MASKS AND COMPLETED ALL QRH CHKLIST ITEMS. THE FO NOTICED THE ALT SET IN THE 'LAND ALT' AND THE 'CAB ALT' INDICTOR WINDOWS WERE BOTH SET INCORRECTLY. THE FO INTENDED TO SELECT 1118 FT. HOWEVER; THE FO INADVERTENTLY SET 11080 FT IN THE 'LAND ALT' AND 'CAB ALT' ANALOG INDICATORS. THESE ENTRIES LIKELY CAUSED THE ABNORMAL PRESSURIZATION; WHICH RESULTED IN A MAX CABIN ALT BTWN 10000 FT AND 11000 FT. THE CAPT ELECTED TO SWITCH THE PRESSURIZATION FROM MANUAL TO THE AUTO MODE WITH THE CORRECT 'LAND ALT' AND 'CAB ALT' ENTRIES IN THE INDICATOR WINDOWS. UPON THIS ACTION; THE ACFT PRESSURIZED NORMALLY. THE CAPT INSTRUCTED THE FO TO COMPLETE ALL QRH AND NORMAL DSCNT CHKLIST ITEMS; AND TO EVALUATE THE REQUIRED FUEL TO COMPLETE THE FLT. AFTER ASSESSING ALL PARAMETERS; THE CAPT ELECTED TO CONTINUE AT 9000 FT AND 250. ENRTE COMPANY MAINT AND OPS WERE NOTIFIED OF AN ABNORMAL CABIN PRESSURIZATION EVENT AND WERE REQUESTED TO MEET THE ACFT ON ARR. COMPANY MAINT COMPLETED A FULL TEST OF THE PRESSURIZATION SYS OF THE ACFT AND CONCLUDED THAT THE INCORRECT SETTINGS IN THE 'LAND ALT' AND 'CAB ALT' INDICATORS LIKELY CAUSED AN OFF-SCHEDULED CABIN PRESSURIZATION EVENT. THE ACFT WAS RECERTIFIED TO CONTINUE FLT AFTER PASSING ALL PRESSURIZATION CHKS. THE CAPT AND FO FLEW THE ACFT UNEVENTFULLY TO OUR NEXT CITY; AS SCHEDULED; WITH NO FURTHER PRESSURIZATION ISSUES. LESSONS LEARNED: 1) FO MUST ENSURE THE CORRECT DEST ARPT ALT IS ENTERED INTO THE ACFT SYS. 2) THERE IS SIGNIFICANT VALUE IN PRACTICING DONNING EMER MASKS AND ESTABLISHING CREW COMS. THE AMBIENT NOISE DURING AN EMER DSCNT IS SIGNIFICANT AND COMS BTWN CREW MEMBERS CAN BE DIFFICULT. ADDITIONALLY; BOOM MIKES; HEADSETS; AND GLASSES CAN BE IMPEDIMENTS TO RAPIDLY DONNING THE MASKS. LASTLY; THE FO HAD RECENTLY COMPLETED A SIMILAR TRAINING EVENT DURING HIS INITIAL TRAINING AS A NEW HIRE. THIS RECENCY OF TRAINING AND EXPOSURE TO A RAPID DSCNT WAS VALUABLE TRAINING. 3) AT CERTAIN SEAT HEIGHTS; IT IS DIFFICULT TO PROPERLY READ THE PRESSURIZATION READOUTS AND THERE ARE KNOBS THAT CAN OBSTRUCT THE CAPT'S AND FO XCHK OF THE PRESSURIZATION WINDOW. THERE IS NO SUBSTITUTE FOR ACFT SYS KNOWLEDGE. THIS EVENT APPEARED TO BE A RESULT OF IMPROPER 'LAND ALT' AND 'CAB ALT' INDICATOR ENTRIES; BUT THE POSSIBILITY OF SUCH AN EVENT; OR ITS CONSEQUENCES; IS NOT ADDRESSED IN THE ACFT MANUAL. 5) THERE IS NO SUBSTITUTE FOR XCHK; CHK; AND RECHKING EVEN WHEN ONE ASSUMES ALL CHKLISTS HAVE BEEN CONFIRMED.

Data retrieved from NASA's ASRS site as of January 2009 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.