change the interface

AircraftReport.proto

@@ -10,5 +10,5 @@ message AircraftReport {
     Coordinate position           = 3; // current WGS84 position
     Dynamics   dynamics           = 4; // current aircraft dynamics
     string     destination        = 5; // the destination airport
-    string     distanceToIAF      = 6; // distance to IAF in NM
+    int32      distanceToIAF      = 6; // distance to IAF in NM
 }