When reseting the aircraft position, the system looks at a number of the

requested parameters to determine if this should be an on-ground vs. in-air
start.  The problem was that we never defaulted the value to anything so
if we didn't match an in-air condition, we simply inherited whatever value
was there from before.

src/Main/fg_init.cxx

@@ -979,6 +979,10 @@ fgInitNav ()
 bool fgInitPosition() {
     bool set_pos = false;
 
+    // Default to starting on the ground unless one of the other
+    // conditions implies an in-air start.
+    fgSetBool("/sim/presets/onground", true);
+
     // If glideslope is specified, then calculate offset-distance or
     // altitude relative to glide slope if either of those was not
     // specified.