Narrative:

Aircraft was en route. The crew experienced a failure of the automatic pressurization system. Manual pressurization was fully functional so the flight continued to ZZZ. Maintenance control was notified and I contacted contract maintenance vendor. A&P mechanic; an approved mechanic on the vendor list; came to the airport and was instructed to follow the procedures to apply MEL 21-16 to disable the automatic pressurization system per the MEL procedures. The MEL contains instructions and a diagram for the procedures and amm references were sent to provide documentation and procedure for entering the equipment space. Upon the mechanic's report of successful completion of the procedure; I issued MEL 21-16. Aircraft departed ZZZ and was forced to divert to ZZZ1 because the crew found manual pressurization inoperative during climb and they were unable to control cabin altitude. Contract maintenance at ZZZ1 investigated and found that the MEL procedure had not been correctly applied. The MEL calls for one line on the electromechanical outflow valve to be disconnected. Instead; the contract mechanic had apparently disconnected 2 lines; thus disabling pressurization. Note: the MEL calls for 1 line to be disconnected and 2 caps to be installed. I suspect the mechanic misunderstood the written procedure. To continue; ZZZ1 maintenance corrected the MEL application; performed an operations check of the manual pressurization and the aircraft was returned to service. I later called and spoke with mechanic and made him aware of the diversion and reviewed the MEL procedure and explained the error as well as the correct procedure. He conceded the error and now understands the correct procedure.

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Original NASA ASRS Text

Title: AN EMB WITH AUTO PRESSURE SYS INOP AND DEFERRED PER THE MEL DIVERTED DUE TO UNABLE TO PRESSURIZE WITH MANUAL SYS. FOUND MEL SPECIAL PROC INOP'ING THE OUTFLOW NOT ACCOMPLISHED CORRECTLY.

Narrative: ACFT WAS ENRTE. THE CREW EXPERIENCED A FAILURE OF THE AUTOMATIC PRESSURIZATION SYS. MANUAL PRESSURIZATION WAS FULLY FUNCTIONAL SO THE FLT CONTINUED TO ZZZ. MAINT CTL WAS NOTIFIED AND I CONTACTED CONTRACT MAINT VENDOR. A&P MECH; AN APPROVED MECH ON THE VENDOR LIST; CAME TO THE ARPT AND WAS INSTRUCTED TO FOLLOW THE PROCS TO APPLY MEL 21-16 TO DISABLE THE AUTOMATIC PRESSURIZATION SYS PER THE MEL PROCS. THE MEL CONTAINS INSTRUCTIONS AND A DIAGRAM FOR THE PROCS AND AMM REFS WERE SENT TO PROVIDE DOCUMENTATION AND PROC FOR ENTERING THE EQUIP SPACE. UPON THE MECH'S RPT OF SUCCESSFUL COMPLETION OF THE PROC; I ISSUED MEL 21-16. ACFT DEPARTED ZZZ AND WAS FORCED TO DIVERT TO ZZZ1 BECAUSE THE CREW FOUND MANUAL PRESSURIZATION INOP DURING CLB AND THEY WERE UNABLE TO CTL CABIN ALT. CONTRACT MAINT AT ZZZ1 INVESTIGATED AND FOUND THAT THE MEL PROC HAD NOT BEEN CORRECTLY APPLIED. THE MEL CALLS FOR ONE LINE ON THE ELECTROMECHANICAL OUTFLOW VALVE TO BE DISCONNECTED. INSTEAD; THE CONTRACT MECH HAD APPARENTLY DISCONNECTED 2 LINES; THUS DISABLING PRESSURIZATION. NOTE: THE MEL CALLS FOR 1 LINE TO BE DISCONNECTED AND 2 CAPS TO BE INSTALLED. I SUSPECT THE MECH MISUNDERSTOOD THE WRITTEN PROC. TO CONTINUE; ZZZ1 MAINT CORRECTED THE MEL APPLICATION; PERFORMED AN OPS CHK OF THE MANUAL PRESSURIZATION AND THE ACFT WAS RETURNED TO SVC. I LATER CALLED AND SPOKE WITH MECH AND MADE HIM AWARE OF THE DIVERSION AND REVIEWED THE MEL PROC AND EXPLAINED THE ERROR AS WELL AS THE CORRECT PROC. HE CONCEDED THE ERROR AND NOW UNDERSTANDS THE CORRECT PROC.

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.