AOPA wrote a letter to the FAA last week saying that if a new policy outlined in Advisory Circular (AC) 90-100A is allowed to remain as it is currently written, more than 25,000 GPS users will not be able to use the unit as a substitute for DME or Rnav procedures. According to the association, only the Garmin 400-, 500- and 1000-series units comply with the new policy and allow operators to use the GPS in lieu of DME and ADF, fly Rnav arrival and departure procedures or Rnav routes. The FAA has also issued a detailed list of FMS units also affected by the new AC. Above all, the FAA says, operators can confirm their aircraft’s compliance by contacting the aircraft or avionics manufacturer. Additionally, the AC offers guidance to flight crews on proper utilization and training, including flight manual and minimum equipment list considerations.