Had an interesting situation a few months back regarding dispatch with both FMC's inoperative (767-300ER). To my knowledge, our MEL is a virtual copy of the Boeing master MEL, but the procedures outlined to cover this situation did not work.
Our paperwork indicated dispatch (short domestic flight) with both FMC's inoperative. On arrival at aircraft, we had two placarded inop (Pegasus) FMC's, but both appeared to operate normally (maintenance procedures do not call for disabling FMC's in this situation. Write up for the problem was 'keyboard problems" with both FMC's. Crew must have been new to the Pegasus and not familiar with the slow(er) keypad entry to the acratchpad on the Pegasus units. This causes some crews to double enter keystrokes.
Maintenance took the easy way out and deferred the FMC's for the short domestic leg. MEL procedures (don't have the book in front of me)call for programing present position in IRS overhead panel and switching to CDU L/CDU R to activate STANDBY NAV. Followed procedures and - no standby (IRS only) nav. After discsussion, we elected to operate the flight using /A procedures with the operating (but deferred) FMC's programmed for the flight.
Further study of the manuals gave no clue as to why we could not get into Standby Nav. The only thing I could think of was that, while deferred as inop, the FMC's did not know they were inop (no fail light). To test this theory, after block in we pulled the FMC breakers and - WALLA - we could access the Stanby Nav function. ONE CANNOT ACCESS THE STANDBY NAV FUNCTION UNLESS BOTH FMC'S KNOW THEY ARE INOPERATIVE (FAIL LIGHT ON).
I wrote this up to our MEL folks (no response yet) and conferred with a couple of our overwater check airman, who were also unaware of this scenario. My concern would be that if FMC's are degraded or Inop on an overwater flight, and do not realize it (no fail lights), access to STANDBY NAV is not possible (without pulling the FMC circuit breakers, not an inflight procedure I am aware of).
Any comments out there?