Plane: added Q_LAND_ALTCHG parameter

this is the threshold height change over 4 seconds for a landing to be
detected. It can be raised if landing detection is very slow

ArduPlane/quadplane.cpp

@@ -434,6 +434,15 @@ const AP_Param::GroupInfo QuadPlane::var_info2[] = {
     // @Path: ../libraries/AC_AttitudeControl/AC_WeatherVane.cpp
     AP_SUBGROUPPTR(weathervane, "WVANE_", 30, QuadPlane, AC_WeatherVane),
 
+    // @Param: LAND_ALTCHG
+    // @DisplayName: Land detection altitude change threshold
+    // @Description: The maximum altitude change allowed during land detection. You can raise this value if you find that landing detection takes a long time to complete. It is the maximum change in altitude over a period of 4 seconds for landing to be detected
+    // @Units: m
+    // @Range: 0.1 0.6
+    // @Increment: 0.05
+    // @User: Standard
+    AP_GROUPINFO("LAND_ALTCHG", 31, QuadPlane, landing_detect.detect_alt_change, 0.2),
+    
     AP_GROUPEND
 };
 
@@ -2995,7 +3004,7 @@ bool QuadPlane::land_detector(uint32_t timeout_ms)
     // we only consider the vehicle landed when the motors have been
     // at minimum for timeout_ms+1000 and the vertical position estimate has not
     // changed by more than 20cm for timeout_ms
-    if (fabsf(height - landing_detect.vpos_start_m) > 0.2) {
+    if (fabsf(height - landing_detect.vpos_start_m) > landing_detect.detect_alt_change) {
         // height has changed, call off landing detection
         landing_detect.land_start_ms = 0;
         return false;

ArduPlane/quadplane.h

@@ -402,6 +402,9 @@ private:
         uint32_t lower_limit_start_ms;
         uint32_t land_start_ms;
         float vpos_start_m;
+
+        // landing detection threshold in meters
+        AP_Float detect_alt_change;
     } landing_detect;
 
     // throttle mix acceleration filter