A B737-800 HAD A FAILED DIGIT ON THE STANDBY SIDE OF THE 'NAV' CTL PANEL. MAINT DEFERRED THE FO'S ILS SYS TO CONTINUE IN SVC.

Narrative

ACFT ARRIVED AT ZZZ FROM ZZZ1 WITH AN MEL DEFERRAL OF ITEM 78 IN THE ACFT LOG WHICH HAD BEEN WRITTEN UP AT ZZZ1 PRIOR TO DEP ON FLT ABC TO ZZZ. THE PROB WAS WITH IN INOP DIGIT ON THE STANDBY SIDE OF THE 'NAV' CTL PANEL ON THE FO'S SIDE. THE ITEM WAS DEFERRED BY USING AN MEL ITEM THAT STATED THAT THE FO'S ILS WAS INOP. THE CODE THAT WAS USED TO WRITE UP THIS PROB WAS FOR THE 'ILS CTL PANEL OR DISPLAY.' THERE IS NO ILS CTL PANEL IN THE B737. THERE IS ALSO A CODE FOR THE 'VOR CTL PANEL OR DISPLAY.' THERE IS NO VOR CTL PANEL IN THE B737. THERE IS A 'NAV' CTL PANEL WITH AN 'ACTIVE' AND A 'STANDBY' DISPLAY. THE 'NAV' CTL PANEL CTLS THE FREQ SELECTION FOR EITHER/BOTH THE ILS AND THE VOR. THE PROB WAS WITH THE DISPLAY ON THE STANDBY SIDE OF THE FO'S NAV CTL PANEL. THE ILS; THE VOR; AND THE HUD ALL WERE OPERATING PERFECTLY. BUT THE SIGNOFF WAS TO DEFER THE ILS AS INOP AND ALSO DOWNGRADE THE HUD TO CAT I ONLY; IN SPITE OF THE FACT THAT WITH THE ILS INOP; THE ACFT COLD NOT BE DISPATCHED TO AN ARPT WHERE THE ILS WAS REQUIRED. ENRTE; I EXAMINED THE MEL AND THE LOGBOOK AT LENGTH AND DECIDED ALL WAS NOT WELL. IN SPITE OF THE FACT THAT THE ILS AND THE VOR WERE IN FACT OPERATING PERFECTLY; THE 'NAV' CTL PANEL WAS NOT. BUT THERE IS NO VALID MEL PROVISION FOR OPERATING THE ACFT WITH A MALFUNCTIONING NAV CTL PANEL. IT WAS SIGNED OFF USING AN INOP ILS MEL PROVISION; APPARENTLY; IN ORDER TO MOVE THE AIRPLANE IN SPITE OF THE FACT THAT THERE WAS A NAV CTL PANEL IN STOCK IN ZZZ1. WHEN WE ARRIVED AT ZZZ1; I WROTE UP THE PROB AGAIN. THE NAV CTL PANEL WAS REPLACED; AND WE WERE ON OUR WAY TO ZZZ2 AFTER TAKING A MAINT DELAY OF APPROX 20 MINS. BUT THE ACFT HAD OPERATED 'ILLEGALLY' FOR 2 FLTS BECAUSE AN INAPPROPRIATE MEL PROVISION HAD BEEN USED TO DEFER MAINT ON AN ITEM NOT COVERED BY THE MEL.