diff --git a/libraries/AP_Navigation/AP_Navigation.h b/libraries/AP_Navigation/AP_Navigation.h
index 9e0703a116..857bcd4388 100644
--- a/libraries/AP_Navigation/AP_Navigation.h
+++ b/libraries/AP_Navigation/AP_Navigation.h
@@ -51,9 +51,14 @@ public:
 	
 	// return the distance in meters at which a turn should commence
 	// to allow the vehicle to neatly move to the next track in the
-	// mission when approaching a waypoint
+	// mission when approaching a waypoint. Assumes 90 degree turn
 	virtual float turn_distance(float wp_radius) const = 0;
 
+	// return the distance in meters at which a turn should commence
+	// to allow the vehicle to neatly move to the next track in the
+	// mission when approaching a waypoint
+	virtual float turn_distance(float wp_radius, float turn_angle) const = 0;
+
 	// update the internal state of the navigation controller, given
 	// the previous and next waypoints. This is the step function for
 	// navigation control for path following between two points.  This