Plane: added a value for RTL_AUTOLAND to disable arming check

set RTL_AUTOLAND=3 to get go-around but not RTL DO_LAND_START usage

ArduPlane/AP_Arming.cpp

@@ -368,7 +368,7 @@ bool AP_Arming_Plane::mission_checks(bool report)
 {
     // base checks
     bool ret = AP_Arming::mission_checks(report);
-    if (plane.mission.get_landing_sequence_start() > 0 && plane.g.rtl_autoland.get() == 0) {
+    if (plane.mission.get_landing_sequence_start() > 0 && plane.g.rtl_autoland == RtlAutoland::RTL_DISABLE) {
         ret = false;
         check_failed(ARMING_CHECK_MISSION, report, "DO_LAND_START set and RTL_AUTOLAND disabled");
     }

ArduPlane/Parameters.cpp

@@ -716,10 +716,10 @@ const AP_Param::Info Plane::var_info[] = {
 
     // @Param: RTL_AUTOLAND
     // @DisplayName: RTL auto land
-    // @Description: Automatically begin landing sequence after arriving at RTL location. This requires the addition of a DO_LAND_START mission item, which acts as a marker for the start of a landing sequence. The closest landing sequence will be chosen to the current location. 
-    // @Values: 0:Disable,1:Enable - go HOME then land,2:Enable - go directly to landing sequence
+    // @Description: Automatically begin landing sequence after arriving at RTL location. This requires the addition of a DO_LAND_START mission item, which acts as a marker for the start of a landing sequence. The closest landing sequence will be chosen to the current location. If this is set to 0 and there is a DO_LAND_START mission item then you will get an arming check failure. You can set to a value of 3 to avoid the arming check failure and use the DO_LAND_START for go-around without it changing RTL behaviour. For a value of 1 a rally point will be used instead of HOME if in range (see rally point documentation).
+    // @Values: 0:Disable,1:Fly HOME then land,2:Go directly to landing sequence, 3:OnlyForGoAround
     // @User: Standard
-    GSCALAR(rtl_autoland,         "RTL_AUTOLAND",   0),
+    GSCALAR(rtl_autoland,         "RTL_AUTOLAND",   float(RtlAutoland::RTL_DISABLE)),
 
     // @Param: CRASH_ACC_THRESH
     // @DisplayName: Crash Deceleration Threshold

ArduPlane/Parameters.h

@@ -362,7 +362,7 @@ public:
     AP_Int16 sysid_my_gcs;
     AP_Int8 telem_delay;
 
-    AP_Int8  rtl_autoland;
+    AP_Enum<RtlAutoland> rtl_autoland;
 
     AP_Int8  crash_accel_threshold;
 

ArduPlane/defines.h

@@ -50,6 +50,15 @@ enum class StickMixing {
     VTOL_YAW = 3,
 };
 
+// values for RTL_AUTOLAND
+enum class RtlAutoland {
+    RTL_DISABLE = 0,
+    RTL_THEN_DO_LAND_START = 1,
+    RTL_IMMEDIATE_DO_LAND_START = 2,
+    NO_RTL_GO_AROUND = 3,
+};
+    
+
 enum ChannelMixing {
     MIXING_DISABLED = 0,
     MIXING_UPUP     = 1,

ArduPlane/events.cpp

@@ -261,7 +261,7 @@ void Plane::handle_battery_failsafe(const char *type_str, const int8_t action)
 #endif
             if (!already_landing) {
                 // never stop a landing if we were already committed
-                if (g.rtl_autoland == 2 && plane.mission.is_best_land_sequence()) {
+                if (g.rtl_autoland == RtlAutoland::RTL_IMMEDIATE_DO_LAND_START && plane.mission.is_best_land_sequence()) {
                     // continue mission as it will reach a landing in less distance
                     plane.mission.set_in_landing_sequence_flag(true);
                     break;

ArduPlane/mode_rtl.cpp

@@ -75,7 +75,7 @@ void ModeRTL::navigate()
     }
 #endif
 
-    if (plane.g.rtl_autoland == 1 &&
+    if (plane.g.rtl_autoland == RtlAutoland::RTL_THEN_DO_LAND_START &&
         !plane.auto_state.checked_for_autoland &&
         plane.reached_loiter_target() && 
         labs(plane.altitude_error_cm) < 1000) {
@@ -90,7 +90,7 @@ void ModeRTL::navigate()
         // on every loop
         plane.auto_state.checked_for_autoland = true;
     }
-    else if (plane.g.rtl_autoland == 2 &&
+    else if (plane.g.rtl_autoland == RtlAutoland::RTL_IMMEDIATE_DO_LAND_START &&
         !plane.auto_state.checked_for_autoland) {
         // Go directly to the landing sequence
         if (plane.mission.jump_to_landing_sequence()) {