AP_TECS: add TECS_LAND_TDAMP for land damp

+     // @Description: This is the damping gain for the throttle demand loop during and auto-landing. Same as TECS_THR_DAMP but only in effect during an auto-land. Increase to add damping to correct for oscillations in speed and height. When set to 0 landing throttle damp is controlled by TECS_THR_DAMP.

libraries/AP_TECS/AP_TECS.cpp

@@ -202,6 +202,14 @@ const AP_Param::GroupInfo AP_TECS::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("LAND_SRC", 22, AP_TECS, _land_sink_rate_change, 0),
 
+    // @Param: LAND_TDAMP
+    // @DisplayName: Controller throttle damping when landing
+    // @Description: This is the damping gain for the throttle demand loop during and auto-landing. Same as TECS_THR_DAMP but only in effect during an auto-land. Increase to add damping to correct for oscillations in speed and height. When set to 0 landing throttle damp is controlled by TECS_THR_DAMP.
+    // @Range: 0.1 1.0
+    // @Increment: 0.1
+    // @User: Advanced
+    AP_GROUPINFO("LAND_TDAMP", 23, AP_TECS, _land_throttle_damp, 0),
+
     AP_GROUPEND
 };
 
@@ -586,7 +594,11 @@ void AP_TECS::_update_throttle(void)
         ff_throttle = nomThr + STEdot_dem / (_STEdot_max - _STEdot_min) * (_THRmaxf - _THRminf);
 
         // Calculate PD + FF throttle
-        _throttle_dem = (_STE_error + STEdot_error * _thrDamp) * K_STE2Thr + ff_throttle;
+        float throttle_damp = _thrDamp;
+        if (_is_doing_auto_land && !is_zero(_land_throttle_damp)) {
+            throttle_damp = _land_throttle_damp;
+        }
+        _throttle_dem = (_STE_error + STEdot_error * throttle_damp) * K_STE2Thr + ff_throttle;
 
         // Constrain throttle demand
         _throttle_dem = constrain_float(_throttle_dem, _THRminf, _THRmaxf);

libraries/AP_TECS/AP_TECS.h

@@ -150,6 +150,7 @@ private:
     AP_Float _ptchDamp;
     AP_Float _landDamp;
     AP_Float _thrDamp;
+    AP_Float _land_throttle_damp;
     AP_Float _integGain;
     AP_Float _vertAccLim;
     AP_Float _rollComp;