Plane: added THR_PASS_STAB parameter

this allows direct passthru of throttle in STABILIZE and FBWA, which
is useful for nitro planes wher you have a throttle cut switch that
drops the throttle below normal minimum.

ArduPlane/Attitude.pde

@@ -454,6 +454,7 @@ static void set_servos(void)
                                                  g.throttle_max.get());
 
         if (suppress_throttle()) {
+            // throttle is suppressed in auto mode
             g.channel_throttle.servo_out = 0;
             if (g.throttle_suppress_manual) {
                 // manual pass through of throttle while throttle is suppressed
@@ -461,7 +462,13 @@ static void set_servos(void)
             } else {
                 g.channel_throttle.calc_pwm();                
             }
+        } else if (g.throttle_passthru_stabilize && 
+                   (control_mode == STABILIZE || control_mode == FLY_BY_WIRE_A)) {
+            // manual pass through of throttle while in FBWA or
+            // STABILIZE mode with THR_PASS_STAB set
+            g.channel_throttle.radio_out = g.channel_throttle.radio_in;
         } else {
+            // normal throttle calculation based on servo_out
             g.channel_throttle.calc_pwm();
         }
 #endif

ArduPlane/Parameters.h

@@ -170,6 +170,7 @@ public:
         k_param_gcs_heartbeat_fs_enabled,
         k_param_throttle_slewrate,
         k_param_throttle_suppress_manual,
+        k_param_throttle_passthru_stabilize,
 
         //
         // 200: Feed-forward gains
@@ -286,6 +287,7 @@ public:
     AP_Int8 throttle_max;
     AP_Int8 throttle_slewrate;
     AP_Int8 throttle_suppress_manual;
+    AP_Int8 throttle_passthru_stabilize;
     AP_Int8 throttle_fs_enabled;
     AP_Int16 throttle_fs_value;
     AP_Int8 throttle_cruise;

ArduPlane/Parameters.pde

@@ -288,6 +288,13 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Advanced
     GSCALAR(throttle_suppress_manual,"THR_SUPP_MAN",   0),
 
+    // @Param: THR_PASS_STAB
+    // @DisplayName: Throttle passthru in stabilize
+    // @Description: If this is set then when in STABILIZE or FBWA mode the throttle is a direct passthru from the transmitter. This means the THR_MIN and THR_MAX settings are not used in these modes. This is useful for petrol engines where you setup a throttle cut switch that suppresses the throttle below the normal minimum.
+	// @Values: 0:Disabled,1:Enabled
+    // @User: Advanced
+    GSCALAR(throttle_passthru_stabilize,"THR_PASS_STAB",   0),
+
     // @Param: THR_FAILSAFE
     // @DisplayName: Throttle Failsafe Enable
     // @Description: The throttle failsafe allows you to configure a software failsafe activated by a setting on the throttle input channel