The June 20, 2019 update to the Aeronautical Chart User’s Guide explains the new MON designator added to basic airport information displayed on IFR enroute charts
The new label is added to airports that are part of the VOR Minimum Operational Network plan that FAA is implementing as it gradually decommissions about 30 percent of the existing VOR network. (To review the latest update on the MON plan, see Next Round of VOR Shutdowns here at BruceAir.)
Effective June 20, 2019, IFR US Enroute Charts will symbolize VOR Minimum Operational Network (MON) airports with thedesignator placed above the airport name in reverse negative text. The intent of the MON designation is to alert pilots, in the event of a GPS outage, of those airports that have retained ILS and VOR instrument approach procedures for safe recovery during such an outage.
The VOR MON will retain sufficient VORs and increase VOR service volume to ensure that pilots will have nearly continuous signal reception of a VOR when flying at 5,000 feet AGL. A key concept of the MON is to ensure that an aircraft will always be within 100 NM of an airport with an instrument approach that is not dependent on GPS…If the pilot encounters a GPS outage, the pilot will be able to proceed via VOR-to-VOR navigation at 5,000 feet AGL through the GPS outage area or to a safe landing at a MON airport or another suitable airport, as appropriate.