diff --git a/libraries/AP_Airspeed/AP_Airspeed.h b/libraries/AP_Airspeed/AP_Airspeed.h
index 1af6c88282..23ded9da80 100644
--- a/libraries/AP_Airspeed/AP_Airspeed.h
+++ b/libraries/AP_Airspeed/AP_Airspeed.h
@@ -7,13 +7,13 @@
 #include <AP_HAL.h>
 #include <AP_Param.h>
 #include <GCS_MAVLink.h>
-#include <AP_SpdHgtControl.h>
+#include <AP_Vehicle.h>
 
 class Airspeed_Calibration {
 public:
     friend class AP_Airspeed;
     // constructor
-    Airspeed_Calibration(const AP_SpdHgtControl::AircraftParameters &parms);
+    Airspeed_Calibration(const AP_Vehicle::FixedWing &parms);
 
     // initialise the calibration
     void init(float initial_ratio);
@@ -29,14 +29,14 @@ private:
     const float Q1; // process noise matrix bottom right element
     Vector3f state; // state vector
     const float DT; // time delta
-    const AP_SpdHgtControl::AircraftParameters &aparm;
+    const AP_Vehicle::FixedWing &aparm;
 };
 
 class AP_Airspeed
 {
 public:
     // constructor
-    AP_Airspeed(const AP_SpdHgtControl::AircraftParameters &parms) : 
+    AP_Airspeed(const AP_Vehicle::FixedWing &parms) : 
         _ets_fd(-1),
         _EAS2TAS(1.0f),
         _calibration(parms)
diff --git a/libraries/AP_Airspeed/Airspeed_Calibration.cpp b/libraries/AP_Airspeed/Airspeed_Calibration.cpp
index 13c97e39e3..c456c0c92a 100644
--- a/libraries/AP_Airspeed/Airspeed_Calibration.cpp
+++ b/libraries/AP_Airspeed/Airspeed_Calibration.cpp
@@ -14,7 +14,7 @@
 extern const AP_HAL::HAL& hal;
 
 // constructor - fill in all the initial values
-Airspeed_Calibration::Airspeed_Calibration(const AP_SpdHgtControl::AircraftParameters &parms) :
+Airspeed_Calibration::Airspeed_Calibration(const AP_Vehicle::FixedWing &parms) :
     P(100,   0,         0,
       0,   100,         0,
       0,     0,  0.000001f),
@@ -119,18 +119,27 @@ void AP_Airspeed::update_calibration(const Vector3f &vground)
         // auto-calibration not enabled
         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. 
+    float ratio = constrain_float(_ratio, 1.0f, 4.0f);
+
+    _calibration.state.z = 1.0 / sqrtf(ratio);
+
     // calculate true airspeed, assuming a airspeed ratio of 1.0
     float dpress = get_differential_pressure();
     float true_airspeed = sqrtf(dpress) * _EAS2TAS;
-    float ratio = _calibration.update(true_airspeed, vground);
 
-    if (isnan(ratio) || isinf(ratio)) {
+    float zratio = _calibration.update(true_airspeed, vground);
+
+    if (isnan(zratio) || isinf(zratio)) {
         return;
     }
 
     // this constrains the resulting ratio to between 1.0 and 4.0
-    ratio = constrain_float(ratio, 0.5f, 1.0f);
-    _ratio.set(1/sq(ratio));
+    zratio = constrain_float(zratio, 0.5f, 1.0f);
+    _ratio.set(1/sq(zratio));
     if (_counter > 60) {
         if (_last_saved_ratio > 1.05f*_ratio || 
             _last_saved_ratio < 0.95f*_ratio) {
diff --git a/libraries/AP_Airspeed/examples/Airspeed/Airspeed.pde b/libraries/AP_Airspeed/examples/Airspeed/Airspeed.pde
index c69a01fda7..79bd5d4e35 100644
--- a/libraries/AP_Airspeed/examples/Airspeed/Airspeed.pde
+++ b/libraries/AP_Airspeed/examples/Airspeed/Airspeed.pde
@@ -30,7 +30,7 @@
 #include <Filter.h>
 #include <AP_Buffer.h>
 #include <AP_Airspeed.h>
-#include <AP_SpdHgtControl.h>
+#include <AP_Vehicle.h>
 #include <AP_Notify.h>
 #include <DataFlash.h>
 #include <GCS_MAVLink.h>
@@ -43,7 +43,7 @@ AP_ADC_ADS7844 apm1_adc;
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;
 
-static AP_SpdHgtControl::AircraftParameters aparm;
+static AP_Vehicle::FixedWing aparm;
 
 AP_Airspeed airspeed(aparm);