Plane: tilt motors fast in final stages of quadplane transition

when transition of tilt quadplane has reached the stage where the tilted
motors are used solely for fwd thrust and rear motors are off we should
move the tilt to full forward rapidly

ArduPlane/tiltrotor.cpp

@@ -17,9 +17,12 @@ float QuadPlane::tilt_max_change(bool up)
     } else {
         rate = tilt.max_rate_down_dps;
     }
-    if (plane.control_mode == MANUAL && tilt.tilt_type != TILT_TYPE_BINARY) {
-        // allow a minimum of 90 DPS in manual, to give fast control
-        rate = MAX(rate, 90);
+    if (tilt.tilt_type != TILT_TYPE_BINARY && !up) {
+        if (plane.control_mode == MANUAL || (!in_vtol_mode() && !assisted_flight)) {
+            // allow a minimum of 90 DPS in manual or if we are not
+            // stabilising, to give fast control
+            rate = MAX(rate, 90);
+        }
     }
     return rate * plane.G_Dt / 90.0f;
 }