Plane: fixed height_above_ground() for case when rangefinder is below min

this prevents a rangefinder that goes below min distance from causing
the calculations that depend on height above ground to fail

ArduPlane/altitude.cpp

@@ -128,6 +128,13 @@ float Plane::relative_ground_altitude(bool use_rangefinder_if_available)
         return rangefinder_state.height_estimate;
    }
 
+   if (use_rangefinder_if_available && quadplane.in_vtol_land_final() &&
+       rangefinder.status_orient(ROTATION_PITCH_270) == RangeFinder::RangeFinder_OutOfRangeLow) {
+       // a special case for quadplane landing when rangefinder goes
+       // below minimum. Consider our height above ground to be zero
+       return 0;
+   }
+
 #if AP_TERRAIN_AVAILABLE
     float altitude;
     if (target_altitude.terrain_following &&

ArduPlane/quadplane.cpp

@@ -2795,7 +2795,7 @@ int8_t QuadPlane::forward_throttle_pct(void)
         // lidar could cause the aircraft not to be able to
         // approach the landing point when landing below the takeoff point
         vel_forward.last_pct = vel_forward.integrator;
-    } else if (in_vtol_land_descent() && poscontrol.state == QPOS_LAND_FINAL && motors->limit.throttle_lower) {
+    } else if (in_vtol_land_final() && motors->limit.throttle_lower) {
         // we're in the settling phase of landing, disable fwd motor
         vel_forward.last_pct = 0;
         vel_forward.integrator = 0;
@@ -3109,3 +3109,11 @@ bool QuadPlane::in_vtol_land_descent(void) const
     }
     return false;
 }
+
+/*
+  see if we are in the final phase of a VTOL landing
+ */
+bool QuadPlane::in_vtol_land_final(void) const
+{
+    return in_vtol_land_descent() && poscontrol.state == QPOS_LAND_FINAL;
+}

ArduPlane/quadplane.h

@@ -541,6 +541,11 @@ private:
      */
     bool in_vtol_land_descent(void) const;
 
+    /*
+      are we in the final landing phase of a VTOL landing?
+     */
+    bool in_vtol_land_final(void) const;
+    
 public:
     void motor_test_output();
     MAV_RESULT mavlink_motor_test_start(mavlink_channel_t chan, uint8_t motor_seq, uint8_t throttle_type,