Narrative:

I met the aircraft to begin day 2 of a pm trip. Contract maintenance was there working on the airplane. The departing captain had written up the prsov. Maintenance control had faxed diagrams and talked to the phone to ZZZ contract maintenance and instructed him to close the #1 engine bleed air shutoff valve and defer it under MEL 36-5. The aircraft also had MEL 74-1 #1 engine left ignition system. We loaded up and departed 7 mins late. MEL 36-5 has some altitude restrs so we were filed at FL230. I read the MEL text and configured for takeoff with the left pack using APU air; the right pack using the right engine bleed and the isolation valve closed. We climbed uneventfully to FL230. Once there; I realized we shouldn't be using the APU for bleed air above 17000 ft so we reconfigured (per MEL text) to left pack using right engine bleed; right pack off; and the isolation valve open. Shortly thereafter; the altitude warning horn sounded. We asked ATC for lower; but could not descend immediately because of crossing traffic 1000 ft below. The cabin was slowly climbing; so we immediately put the APU bleed back on and when allowed by ATC started a turn and descent. It took several mins for the altitude warning horn to silence. After leveling at 17000 ft; we configured with left pack on APU bleed; right pack using right engine bleed and isolation valve closed. Supplemental information from acn 762722: the cabin pressure did not exceed 10000 ft and the passenger oxygen masks did not drop. Continue with acn 762328: we continued on to ZZZ1 uneventfully. Company maintenance met the aircraft in ZZZ1 and in going over the original logbook write-up realized that the right engine bleed valve was the one that had low pressure and should have been closed. The logbook entry was 'significant pneumatic duct pressure split with isolation valve in automatic; at climb power; right side pressure 8 psi and left side 46 psi.' when I was talking to contract maintenance at our departure city; I asked if he was sure he was working on the correct engine. He laughingly said; 'wouldn't that be funny' and assured me that he was doing exactly as instructed by maintenance control. Continue with supplemental information from acn 762722: rptedly maintenance control had the information reversed from what was in the logbook. After further deliberation and consultation; it was determined by ZZZ1 maintenance personnel that we should run the engines to be certain which engine had the low pressure. After the engines were run; it was verified that the right engine had the low pressure. The ZZZ1 maintenance personnel ensured both engine bleed valves were correctly set. After the appropriate logbook entries were made and the MEL changed to 36-5 for the #2 engine; we completed our preflight duties and pushed back 57 mins late for the flight to our next city. The accepting crew was briefed. Aircraft number was scheduled to terminate in ZZZ1 after that 1 leg back. The wording in the MEL can be confusing. Suggest that the wording be reviewed for clrer phrasing.

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Original NASA ASRS Text

Title: A 737-300 FLIGHT CREW EXPERIENCES CABIN PRESSURIZATION LOSS. COMPANY MAINT CONTROL HAD CONTRACT MAINT DEFER AND CLOSE #1 ENGINE BLEED AIR S/O VALVE FOR LOW AIR PRESSURE WHEN; IN FACT; #2 ENG BLEED WAS FAILING.

Narrative: I MET THE ACFT TO BEGIN DAY 2 OF A PM TRIP. CONTRACT MAINT WAS THERE WORKING ON THE AIRPLANE. THE DEPARTING CAPT HAD WRITTEN UP THE PRSOV. MAINT CTL HAD FAXED DIAGRAMS AND TALKED TO THE PHONE TO ZZZ CONTRACT MAINT AND INSTRUCTED HIM TO CLOSE THE #1 ENG BLEED AIR SHUTOFF VALVE AND DEFER IT UNDER MEL 36-5. THE ACFT ALSO HAD MEL 74-1 #1 ENG L IGNITION SYS. WE LOADED UP AND DEPARTED 7 MINS LATE. MEL 36-5 HAS SOME ALT RESTRS SO WE WERE FILED AT FL230. I READ THE MEL TEXT AND CONFIGURED FOR TKOF WITH THE L PACK USING APU AIR; THE R PACK USING THE R ENG BLEED AND THE ISOLATION VALVE CLOSED. WE CLBED UNEVENTFULLY TO FL230. ONCE THERE; I REALIZED WE SHOULDN'T BE USING THE APU FOR BLEED AIR ABOVE 17000 FT SO WE RECONFIGURED (PER MEL TEXT) TO L PACK USING R ENG BLEED; R PACK OFF; AND THE ISOLATION VALVE OPEN. SHORTLY THEREAFTER; THE ALT WARNING HORN SOUNDED. WE ASKED ATC FOR LOWER; BUT COULD NOT DSND IMMEDIATELY BECAUSE OF XING TFC 1000 FT BELOW. THE CABIN WAS SLOWLY CLBING; SO WE IMMEDIATELY PUT THE APU BLEED BACK ON AND WHEN ALLOWED BY ATC STARTED A TURN AND DSCNT. IT TOOK SEVERAL MINS FOR THE ALT WARNING HORN TO SILENCE. AFTER LEVELING AT 17000 FT; WE CONFIGURED WITH L PACK ON APU BLEED; R PACK USING R ENG BLEED AND ISOLATION VALVE CLOSED. SUPPLEMENTAL INFO FROM ACN 762722: THE CABIN PRESSURE DID NOT EXCEED 10000 FT AND THE PAX OXYGEN MASKS DID NOT DROP. CONTINUE WITH ACN 762328: WE CONTINUED ON TO ZZZ1 UNEVENTFULLY. COMPANY MAINT MET THE ACFT IN ZZZ1 AND IN GOING OVER THE ORIGINAL LOGBOOK WRITE-UP REALIZED THAT THE R ENG BLEED VALVE WAS THE ONE THAT HAD LOW PRESSURE AND SHOULD HAVE BEEN CLOSED. THE LOGBOOK ENTRY WAS 'SIGNIFICANT PNEUMATIC DUCT PRESSURE SPLIT WITH ISOLATION VALVE IN AUTO; AT CLB PWR; R SIDE PRESSURE 8 PSI AND L SIDE 46 PSI.' WHEN I WAS TALKING TO CONTRACT MAINT AT OUR DEP CITY; I ASKED IF HE WAS SURE HE WAS WORKING ON THE CORRECT ENG. HE LAUGHINGLY SAID; 'WOULDN'T THAT BE FUNNY' AND ASSURED ME THAT HE WAS DOING EXACTLY AS INSTRUCTED BY MAINT CTL. CONTINUE WITH SUPPLEMENTAL INFO FROM ACN 762722: RPTEDLY MAINT CTL HAD THE INFO REVERSED FROM WHAT WAS IN THE LOGBOOK. AFTER FURTHER DELIBERATION AND CONSULTATION; IT WAS DETERMINED BY ZZZ1 MAINT PERSONNEL THAT WE SHOULD RUN THE ENGS TO BE CERTAIN WHICH ENG HAD THE LOW PRESSURE. AFTER THE ENGS WERE RUN; IT WAS VERIFIED THAT THE R ENG HAD THE LOW PRESSURE. THE ZZZ1 MAINT PERSONNEL ENSURED BOTH ENG BLEED VALVES WERE CORRECTLY SET. AFTER THE APPROPRIATE LOGBOOK ENTRIES WERE MADE AND THE MEL CHANGED TO 36-5 FOR THE #2 ENG; WE COMPLETED OUR PREFLT DUTIES AND PUSHED BACK 57 MINS LATE FOR THE FLT TO OUR NEXT CITY. THE ACCEPTING CREW WAS BRIEFED. ACFT NUMBER WAS SCHEDULED TO TERMINATE IN ZZZ1 AFTER THAT 1 LEG BACK. THE WORDING IN THE MEL CAN BE CONFUSING. SUGGEST THAT THE WORDING BE REVIEWED FOR CLRER PHRASING.

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.