Narrative:

Upon arrival at the gate in ZZZ, the previous captain explained that the aircraft's 'a' electric pump had intermittently come on and pressurized the 'a' system even though the 'a' electric pump switch was off. He wrote the problem up in the aircraft logbook and called dispatch to obtain MEL information. They conferred and agreed MEL 29-10 applied and crew deferred the problem. The information was hand copied on the dispatch release. I took over the aircraft, confirmed with dispatch about the MEL and confirmed the release had the information copied on it. I requested a facsimile copy of the MEL procedure. This facsimile did not arrive before our departure time. We departed for ZZZ1, our next stop. Upon getting the dispatch release for the next leg ZZZ1 to ZZZ, I noticed the MEL information, the information applied only to the engine driven pump of the 'a' system. It was the electric pump that was written up. I called dispatch and we conference-called with maintenance to clarify the problem. It turns out the electric pump can't be MEL'ed for this condition. The aircraft had been improperly MEL'ed in ZZZ, and we had flown it to ZZZ1 in this condition. The aircraft was taken OTS in ZZZ1 for repairs. If I had read the MEL in the operations manual closer, I may have caught this mistake in ZZZ.

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Original NASA ASRS Text

Title: A B737-500 WAS DISPATCHED IN NON COMPLIANCE WITH AN 'A' SYS ELECTRIC HYD PUMP DEFERRED AS INOP IN CONFLICT WITH THE MEL.

Narrative: UPON ARR AT THE GATE IN ZZZ, THE PREVIOUS CAPT EXPLAINED THAT THE ACFT'S 'A' ELECTRIC PUMP HAD INTERMITTENTLY COME ON AND PRESSURIZED THE 'A' SYS EVEN THOUGH THE 'A' ELECTRIC PUMP SWITCH WAS OFF. HE WROTE THE PROB UP IN THE ACFT LOGBOOK AND CALLED DISPATCH TO OBTAIN MEL INFO. THEY CONFERRED AND AGREED MEL 29-10 APPLIED AND CREW DEFERRED THE PROB. THE INFO WAS HAND COPIED ON THE DISPATCH RELEASE. I TOOK OVER THE ACFT, CONFIRMED WITH DISPATCH ABOUT THE MEL AND CONFIRMED THE RELEASE HAD THE INFO COPIED ON IT. I REQUESTED A FAX COPY OF THE MEL PROC. THIS FAX DID NOT ARRIVE BEFORE OUR DEP TIME. WE DEPARTED FOR ZZZ1, OUR NEXT STOP. UPON GETTING THE DISPATCH RELEASE FOR THE NEXT LEG ZZZ1 TO ZZZ, I NOTICED THE MEL INFO, THE INFO APPLIED ONLY TO THE ENG DRIVEN PUMP OF THE 'A' SYS. IT WAS THE ELECTRIC PUMP THAT WAS WRITTEN UP. I CALLED DISPATCH AND WE CONFERENCE-CALLED WITH MAINT TO CLARIFY THE PROB. IT TURNS OUT THE ELECTRIC PUMP CAN'T BE MEL'ED FOR THIS CONDITION. THE ACFT HAD BEEN IMPROPERLY MEL'ED IN ZZZ, AND WE HAD FLOWN IT TO ZZZ1 IN THIS CONDITION. THE ACFT WAS TAKEN OTS IN ZZZ1 FOR REPAIRS. IF I HAD READ THE MEL IN THE OPS MANUAL CLOSER, I MAY HAVE CAUGHT THIS MISTAKE IN ZZZ.

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.