FMS use below MSA?

[PEI_3721]

Pontius's Copilot, take care with the generalised statement “'Map Shift' error only happens to single-source RNAV equipment, …” as in many situations this can be incorrect. From you own example of an FMS with GPS/DME input; when flying in an area without DME or poorly located DMEs, a degraded GPS can result in the FMS position ‘slipping’ and the accuracy being less than required for an NPA.

Incident 3 in "Celebrating TAWS ‘Saves’: But lessons still to be learnt" illustrates a hazard in programming FMS approaches negating safety monitoring, as well as presenting a confused navigation display.