B737 crew had the cabin altitude warning horn sound on departure from mmlo.

Attributes
ACN	622390
Time
Date	200402
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : bjx.vortac
State Reference	FO
Altitude	msl single value : 11500
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mmlo.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 4000
ASRS Report	622390
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 270 flight time total : 9500 flight time type : 900
ASRS Report	622389
Events
Anomaly	aircraft equipment problem : critical non adherence : company policies non adherence : far non adherence : published procedure
Independent Detector	aircraft equipment other aircraft equipment : cabin altitude warning horn other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : landed as precaution flight crew : diverted to another airport other
Consequence	other other
Factors
Maintenance	contributing factor : schedule pressure performance deficiency : repair performance deficiency : testing performance deficiency : fault isolation performance deficiency : non compliance with legal requirements
Supplementary
Problem Areas	Aircraft Company Maintenance Human Performance
Primary Problem	Aircraft

Narrative:

While climbing on departure mmlo at 11500 ft, the altitude warning horn activated. Silenced horn and realized the cabin was at 10500 ft and climbing approximately 800-1000 FPM. Leveled off at 12500 ft and requested a 180 degree turn because of 14000 ft MEA on airway. ATC approved turn, and cleared us direct bjx VOR. We ran all checklists and prevented the cabin from climbing above 12000 ft. Descended to 10000 ft per ATC instructions. The left pack was inoperative per MEL, and right pack was configured per MEL with isolation valve closed. Duct pressure was approximately 45 psi, and the outflow valve was fully closed. Landed mmlo without incident. The left pack had multiple write-ups during previous week. I had requested an aircraft swap 45 mins prior to departure. This request was denied. I recommend that we not dispatch a pack inoperative aircraft to a high altitude airport, nor south of the border. Mmlo has only contract maintenance with mechanics who only speak spanish. Supplemental information from acn 622389: outflow valve full closed. Returned to land mmlo. The left pack had multiple write-ups and this aircraft, in my opinion, should not have been dispatched to a high altitude airport in mountainous terrain with its persistent air conditioning problems. Troubleshooting was a joke with spanish speaking contract maintenance that clearly didn't know the system.

Original NASA ASRS Text

Title: B737 CREW HAD THE CABIN ALT WARNING HORN SOUND ON DEP FROM MMLO.

Narrative: WHILE CLBING ON DEP MMLO AT 11500 FT, THE ALT WARNING HORN ACTIVATED. SILENCED HORN AND REALIZED THE CABIN WAS AT 10500 FT AND CLBING APPROX 800-1000 FPM. LEVELED OFF AT 12500 FT AND REQUESTED A 180 DEG TURN BECAUSE OF 14000 FT MEA ON AIRWAY. ATC APPROVED TURN, AND CLRED US DIRECT BJX VOR. WE RAN ALL CHKLISTS AND PREVENTED THE CABIN FROM CLBING ABOVE 12000 FT. DSNDED TO 10000 FT PER ATC INSTRUCTIONS. THE L PACK WAS INOP PER MEL, AND R PACK WAS CONFIGURED PER MEL WITH ISOLATION VALVE CLOSED. DUCT PRESSURE WAS APPROX 45 PSI, AND THE OUTFLOW VALVE WAS FULLY CLOSED. LANDED MMLO WITHOUT INCIDENT. THE L PACK HAD MULTIPLE WRITE-UPS DURING PREVIOUS WK. I HAD REQUESTED AN ACFT SWAP 45 MINS PRIOR TO DEP. THIS REQUEST WAS DENIED. I RECOMMEND THAT WE NOT DISPATCH A PACK INOP ACFT TO A HIGH ALT ARPT, NOR S OF THE BORDER. MMLO HAS ONLY CONTRACT MAINT WITH MECHS WHO ONLY SPEAK SPANISH. SUPPLEMENTAL INFO FROM ACN 622389: OUTFLOW VALVE FULL CLOSED. RETURNED TO LAND MMLO. THE L PACK HAD MULTIPLE WRITE-UPS AND THIS ACFT, IN MY OPINION, SHOULD NOT HAVE BEEN DISPATCHED TO A HIGH ALT ARPT IN MOUNTAINOUS TERRAIN WITH ITS PERSISTENT AIR CONDITIONING PROBS. TROUBLESHOOTING WAS A JOKE WITH SPANISH SPEAKING CONTRACT MAINT THAT CLRLY DIDN'T KNOW THE SYS.

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.