A B737-400 with a MEL'd pack received a cabin altitude warning horn 55 min after takeoff. An emergen...

Attributes
ACN	700464
Time
Date	200606
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 24000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	B737-400
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
ASRS Report	700464
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	700465
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : declared emergency
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

While on the ramp; we had the right hand pack light illuminate and were issued MEL 21-1A. En route at FL240; we heard the cabin altitude horn. We put on our oxygen masks; declared an emergency; conducted our emergency checklists and descended to 10000 ft. At 10000 ft; we finished our checklists and communicated with dispatch and maintenance control what course of action they would like us to take. Since the checklist does not call for a divert to the nearest airport; all parties agreed to divert. We checked up on the welfare of our passenger and crew as workload permitted and all were reported as ok. We landed uneventfully and all passenger and crew moved to a different aircraft and proceeded to ZZZ1. Supplemental information from acn 700465: at FL240 approximately 55 mins after takeoff; the cabin warning horn came on and the cabin altitude indicated 10000 ft with a small (1.0) psid. Cabin altitude was steady. At 10000 ft; the cabin altitude was 8500 ft with no climb or descent on the cabin rate indicator and the outflow valve was full closed. Normal 2 pack indications would be a sea level cabin and approximately 4.0 psid as verified on our later flight.

Original NASA ASRS Text

Title: A B737-400 WITH A MEL'D PACK RECEIVED A CABIN ALT WARNING HORN 55 MIN AFTER TKOF. AN EMER WAS DECLARED FOLLOWED BY AN EMER DSCNT WITH FLT DIVERTING AT 10000 FT.

Narrative: WHILE ON THE RAMP; WE HAD THE R HAND PACK LIGHT ILLUMINATE AND WERE ISSUED MEL 21-1A. ENRTE AT FL240; WE HEARD THE CABIN ALT HORN. WE PUT ON OUR OXYGEN MASKS; DECLARED AN EMER; CONDUCTED OUR EMER CHKLISTS AND DSNDED TO 10000 FT. AT 10000 FT; WE FINISHED OUR CHKLISTS AND COMMUNICATED WITH DISPATCH AND MAINT CTL WHAT COURSE OF ACTION THEY WOULD LIKE US TO TAKE. SINCE THE CHKLIST DOES NOT CALL FOR A DIVERT TO THE NEAREST ARPT; ALL PARTIES AGREED TO DIVERT. WE CHKED UP ON THE WELFARE OF OUR PAX AND CREW AS WORKLOAD PERMITTED AND ALL WERE RPTED AS OK. WE LANDED UNEVENTFULLY AND ALL PAX AND CREW MOVED TO A DIFFERENT ACFT AND PROCEEDED TO ZZZ1. SUPPLEMENTAL INFO FROM ACN 700465: AT FL240 APPROX 55 MINS AFTER TKOF; THE CABIN WARNING HORN CAME ON AND THE CABIN ALT INDICATED 10000 FT WITH A SMALL (1.0) PSID. CABIN ALT WAS STEADY. AT 10000 FT; THE CABIN ALT WAS 8500 FT WITH NO CLB OR DSCNT ON THE CABIN RATE INDICATOR AND THE OUTFLOW VALVE WAS FULL CLOSED. NORMAL 2 PACK INDICATIONS WOULD BE A SEA LEVEL CABIN AND APPROX 4.0 PSID AS VERIFIED ON OUR LATER FLT.

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.

A B737-400 with a MEL'd pack received a cabin altitude warning horn 55 min after takeoff. An emergency was declared followed by an emergency descent with flight diverting at 10000 ft.