A B757-200 flight crew was dispatched with right pack inoperative; and experienced EICAS cabin altitude alert in cruise flight. They turned the right pack on to regain pressurization control; and returned to departure station.

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Attributes
ACN	711637
Time
Date	200609
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zob.artcc
State Reference	OH
Altitude	msl single value : 34000
Environment
Flight Conditions	Marginal
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	B757-200
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 : 180 flight time total : 16000 flight time type : 3000
ASRS Report	711637
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	aircraft equipment other aircraft equipment : eicas other flight crewa
Resolutory Action	flight crew : diverted to another airport flight crew : landed as precaution
Consequence	other
Supplementary
Problem Areas	Aircraft Maintenance Human Performance
Primary Problem	Aircraft

Narrative:

First officer discovered a small amount of water dripping forward of lower beacon on walkaround. Maintenance found small leak in right pack cold water injector(?) although we were dispatched with right pack inoperative; maintenance suggested right pack was operable. En route after leveloff at FL340; red cabin altitude EICAS and horn came on. First officer and I donned oxygen masks; noticed slight climb in cabin rate and cabin altitude at 10000 ft. I immediately turned on right pack which reversed the cabin into a 500 FPM rate of descent. About 3 mins later when cabin went below 8500 ft and light went out; we took off our masks and I contacted maintenance through dispatch. Maintenance advised us he thought operating pack en route shouldn't be a problem but it was my decision. My initial decision was to continue. After further discussion with my first officer we decided it was more prudent to either land in ord or jfk. We then advised dispatch and returned to jfk with the right pack on en route. This aircraft had multiple right pack write-ups over the prior month which may or may not have contributed to our experience.

Original NASA ASRS Text

Title: A B757-200 FLT CREW WAS DISPATCHED WITH R PACK INOP; AND EXPERIENCED EICAS CABIN ALT ALERT IN CRUISE FLT. THEY TURNED THE R PACK ON TO REGAIN PRESSURIZATION CTL; AND RETURNED TO DEP STATION.

Narrative: FO DISCOVERED A SMALL AMOUNT OF WATER DRIPPING FORWARD OF LOWER BEACON ON WALKAROUND. MAINT FOUND SMALL LEAK IN R PACK COLD WATER INJECTOR(?) ALTHOUGH WE WERE DISPATCHED WITH R PACK INOP; MAINT SUGGESTED R PACK WAS OPERABLE. ENRTE AFTER LEVELOFF AT FL340; RED CABIN ALT EICAS AND HORN CAME ON. FO AND I DONNED OXYGEN MASKS; NOTICED SLIGHT CLB IN CABIN RATE AND CABIN ALT AT 10000 FT. I IMMEDIATELY TURNED ON R PACK WHICH REVERSED THE CABIN INTO A 500 FPM RATE OF DSCNT. ABOUT 3 MINS LATER WHEN CABIN WENT BELOW 8500 FT AND LIGHT WENT OUT; WE TOOK OFF OUR MASKS AND I CONTACTED MAINT THROUGH DISPATCH. MAINT ADVISED US HE THOUGHT OPERATING PACK ENRTE SHOULDN'T BE A PROB BUT IT WAS MY DECISION. MY INITIAL DECISION WAS TO CONTINUE. AFTER FURTHER DISCUSSION WITH MY FO WE DECIDED IT WAS MORE PRUDENT TO EITHER LAND IN ORD OR JFK. WE THEN ADVISED DISPATCH AND RETURNED TO JFK WITH THE R PACK ON ENRTE. THIS ACFT HAD MULTIPLE R PACK WRITE-UPS OVER THE PRIOR MONTH WHICH MAY OR MAY NOT HAVE CONTRIBUTED TO OUR EXPERIENCE.

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.