diff --git a/ArduCopter/mode.h b/ArduCopter/mode.h
index 4b142409bc..7ced9b0575 100644
--- a/ArduCopter/mode.h
+++ b/ArduCopter/mode.h
@@ -101,6 +101,8 @@ public:
         AUTO_RTL =     27,  // Auto RTL, this is not a true mode, AUTO will report as this mode if entered to perform a DO_LAND_START Landing sequence
         TURTLE =       28,  // Flip over after crash
 
+        // Mode number 30 reserved for "offboard" for external/lua control.
+
         // Mode number 127 reserved for the "drone show mode" in the Skybrush
         // fork at https://github.com/skybrush-io/ardupilot
     };