AP_Airspeed: Correct airspeed calibration issue for gliders with sens…

…ors positioned behind the propeller

libraries/AP_Airspeed/Airspeed_Calibration.cpp

@@ -13,6 +13,7 @@
 #include <AP_Math/AP_Math.h>
 #include <GCS_MAVLink/GCS.h>
 #include <AP_AHRS/AP_AHRS.h>
+#include <SRV_Channel/SRV_Channel.h>
 
 #include "AP_Airspeed.h"
 
@@ -122,6 +123,13 @@ void AP_Airspeed::update_calibration(uint8_t i, const Vector3f &vground, int16_t
         return;
     }
 
+    if (param[i].use == 2 && !is_zero(SRV_Channels::get_output_scaled(SRV_Channel::k_throttle))) {
+        // special case for gliders with airspeed sensors behind the
+        // propeller. Allow airspeed to be disabled when throttle is
+        // running
+        return;
+    }
+
     // set state.z based on current ratio, this allows the operator to
     // override the current ratio in flight with autocal, which is
     // very useful both for testing and to force a reasonable value.