So what do you do when A pilot claims his V-speeds disappeared (typically pilot error)? Or they screw up an NDB update and a way-point or even airport is missing from it? FMS maintenance IS poorly supported. Shotgunning FMC's that come back as NFF from overhaul get's expensive. On the MD-11 you need an active flight plan inserted for the FMA's to turn to white boxes (active), if you can catch the flight crew in time (and the FCC's have not faulted themselves) knowing how to completely enter a flight plan makes for easy T/S, no approved maintenance data to do this. I am an avionics tech working line MX of 20 years and have hundreds of inbd and outbd flights daily in my sandbox, I see some wierd stuff.
I see enough of it to know the FIM/AMM of any aircraft will never cover every scenario.