Narrative:

We were scheduled to operate ZZZ-ZZZ1 on aircraft X. During the preflight; the first officer (first officer) noticed that his CDU would not accept normal button inputs and we informed our mechanic. The airplane has had a history of CDU/FMC anomalies and the right CDU had just been replaced on the prior flight. The mechanic and we both believed the proper MEL deferral would be flight management computer systems (fmcs) - FMC (including CDU/hmcdu/mcdu) (-200/-300): [fans - rnp - non-er operations]; 2 installed and 1 required; as the left CDU was operating normally. When the mechanic contacted [maintenance control]; they decided to defer per 34-xx-X-xx flight management computer systems (fmcs) - FMC (including CDU/hmcdu/mcdu) (-200/-300): [approach - fans - rnp - non-er operations]; 2 installed and 0 required. I do not know why [maintenance control] decided to defer the CDU per the -xy MEL vs. The -xx MEL since we did have an operative left CDU but that was what they had decided. After the deferral was completed; I decided to further review the MEL and noticed that we would be restricted to non-RNAV ATC routing and we were filed on the [RNAV routes for both departure and arrival] and would thus be non-compliant and would need to be refiled. I contacted the dispatcher and informed him of this and he agreed. During this time; since we were on a ground stop to ZZZ1; the mechanic attempted to acquire a spare CDU to replace the defective unit; but there were no spare cdus located in ZZZ. He stated that they had contacted ZZZ1 maintenance which had a serviceable CDU and it would be replaced in ZZZ1 upon our arrival. After realizing that there was no spare CDU in ZZZ and that the -xx MEL was the applicable MEL for our situation per our continued discussion; the mechanic informed [maintenance control] that we should close out the -xy deferral and initiate the -xx deferral. [Maintenance control] initially agreed with this decision; as there had been a shift change at [maintenance control] and the current [aircraft X] maintenance lead was unfamiliar with the decision to apply MEL 34-xx-X-xy initially. The mechanic began this process but shortly thereafter; [maintenance control] informed him that they had decided to keep the existing -xy deferral active so that we would not incur a further delay as the ground stop to ZZZ1 had been lifted. Unfortunately I did not notice that the -xy MEL also required either chief pilot or director of operations approval to dispatch the flight. We completed the preflight and engine start and taxied to the runway uneventfully. ZZZ tower cleared us for the RNAV departure but I reminded him that we were dispatched non RNAV 1 capable and we were given a heading to fly instead. The remainder of the flight was uneventful. Upon landing in ZZZ1; I asked the mechanic there if he had the CDU ready for replacement and he told me that he had not received any information about replacing the CDU and instead he would continue the deferral under the existing MEL (-xy). Once the mechanic and I agreed that the wrong MEL was being applied in ZZZ; I should have insisted on having the correct MEL applied even if it would have resulted in a further delay. I felt that since the -xy MEL was more restrictive than the -xx MEL that we were not compromising safety by applying it even though it wasn't the exact situation we had. I do not feel that flight safety was compromised as we did have a functioning CDU and were not degraded as stated per MEL 34-xx-X-xy. It is unfortunate that [maintenance control] overrode both myself and the first officer; as well as the mechanic in applying MEL 34-xx-X-xy. I have trust in our maintenance professionals as this is their job to apply proper maintenance practices including MEL deferrals but realize that there are times when we as pilots will need to insist on correcting their errors. Additionally; perhaps these mels could be further clarified to prevent future occurrences.

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Original NASA ASRS Text

Title: B767-300 Captain reported after a failure with the navigation system; there was confusion choosing the right MEL.

Narrative: We were scheduled to operate ZZZ-ZZZ1 on Aircraft X. During the preflight; the FO (First Officer) noticed that his CDU would not accept normal button inputs and we informed our Mechanic. The airplane has had a history of CDU/FMC anomalies and the right CDU had just been replaced on the prior flight. The Mechanic and we both believed the proper MEL deferral would be Flight Management Computer Systems (FMCS) - FMC (Including CDU/HMCDU/MCDU) (-200/-300): [FANS - RNP - NON-ER OPS]; 2 installed and 1 required; as the Left CDU was operating normally. When the Mechanic contacted [Maintenance Control]; they decided to defer per 34-XX-X-XX Flight Management Computer Systems (FMCS) - FMC (INCLUDING CDU/HMCDU/MCDU) (-200/-300): [APPROACH - FANS - RNP - NON-ER OPS]; 2 installed and 0 required. I do not know why [Maintenance Control] decided to defer the CDU per the -XY MEL vs. the -XX MEL since we did have an operative left CDU but that was what they had decided. After the deferral was completed; I decided to further review the MEL and noticed that we would be restricted to non-RNAV ATC routing and we were filed on the [RNAV Routes for both departure and arrival] and would thus be non-compliant and would need to be refiled. I contacted the Dispatcher and informed him of this and he agreed. During this time; since we were on a ground stop to ZZZ1; the Mechanic attempted to acquire a spare CDU to replace the defective unit; but there were no spare CDUs located in ZZZ. He stated that they had contacted ZZZ1 Maintenance which had a serviceable CDU and it would be replaced in ZZZ1 upon our arrival. After realizing that there was no spare CDU in ZZZ and that the -XX MEL was the applicable MEL for our situation per our continued discussion; the Mechanic informed [Maintenance Control] that we should close out the -XY deferral and initiate the -XX deferral. [Maintenance Control] initially agreed with this decision; as there had been a shift change at [Maintenance Control] and the current [Aircraft X] Maintenance lead was unfamiliar with the decision to apply MEL 34-XX-X-XY initially. The Mechanic began this process but shortly thereafter; [Maintenance Control] informed him that they had decided to keep the existing -XY deferral active so that we would not incur a further delay as the ground stop to ZZZ1 had been lifted. Unfortunately I did not notice that the -XY MEL also required either Chief Pilot or Director of Operations approval to Dispatch the flight. We completed the preflight and engine start and taxied to the runway uneventfully. ZZZ Tower cleared us for the RNAV departure but I reminded him that we were dispatched non RNAV 1 capable and we were given a heading to fly instead. The remainder of the flight was uneventful. Upon landing in ZZZ1; I asked the Mechanic there if he had the CDU ready for replacement and he told me that he had not received any information about replacing the CDU and instead he would continue the deferral under the existing MEL (-XY). Once the Mechanic and I agreed that the wrong MEL was being applied in ZZZ; I should have insisted on having the correct MEL applied even if it would have resulted in a further delay. I felt that since the -XY MEL was more restrictive than the -XX MEL that we were not compromising safety by applying it even though it wasn't the exact situation we had. I do not feel that flight safety was compromised as we did have a functioning CDU and were not degraded as stated per MEL 34-XX-X-XY. It is unfortunate that [Maintenance Control] overrode both myself and the First Officer; as well as the Mechanic in applying MEL 34-XX-X-XY. I have trust in our Maintenance professionals as this is their job to apply proper maintenance practices including MEL deferrals but realize that there are times when we as pilots will need to insist on correcting their errors. Additionally; perhaps these MELs could be further clarified to prevent future occurrences.

Data retrieved from NASA's ASRS site 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.