Function XPLMSetFMSFlightPlanEntryInfo 

pub unsafe extern "C" fn XPLMSetFMSFlightPlanEntryInfo(
    inFlightPlan: XPLMNavFlightPlan,
    inIndex: c_int,
    inRef: XPLMNavRef,
    inAltitude: c_int,
)

XPLMSetFMSFlightPlanEntryInfo

This routine changes an entry in the FMS to have the destination navaid passed in and the altitude specified. Use this only for airports, fixes, and radio-beacon navaids. Currently of radio beacons, the FMS can only support VORs, NDBs and TACANs. Use the routines below to clear or fly to a lat/lon.