Function XPLMSetDestinationFMSFlightPlanEntry 

pub unsafe extern "C" fn XPLMSetDestinationFMSFlightPlanEntry(
    inFlightPlan: XPLMNavFlightPlan,
    inIndex: c_int,
)

XPLMSetDestinationFMSFlightPlanEntry

This routine changes which entry the FMS is flying the aircraft toward. The track is from the n-1’th point to the n’th point.