diff --git a/ArduPlane/AP_Arming.h b/ArduPlane/AP_Arming.h
index 26b0fb63f8..1a4d4f3628 100644
--- a/ArduPlane/AP_Arming.h
+++ b/ArduPlane/AP_Arming.h
@@ -28,7 +28,7 @@ public:
     bool arm(AP_Arming::Method method, bool do_arming_checks=true) override;
 
     void update_soft_armed();
-    bool get_delay_arming() { return delay_arming; };
+    bool get_delay_arming() const { return delay_arming; };
 
 protected:
     bool ins_checks(bool report) override;
diff --git a/ArduPlane/Plane.h b/ArduPlane/Plane.h
index aa88d6feef..24e0701a3f 100644
--- a/ArduPlane/Plane.h
+++ b/ArduPlane/Plane.h
@@ -786,7 +786,7 @@ private:
     void update_load_factor(void);
     void adjust_altitude_target();
     void setup_glide_slope(void);
-    int32_t get_RTL_altitude();
+    int32_t get_RTL_altitude() const;
     float relative_ground_altitude(bool use_rangefinder_if_available);
     void set_target_altitude_current(void);
     void set_target_altitude_current_adjusted(void);
@@ -800,7 +800,7 @@ private:
     void reset_offset_altitude(void);
     void set_offset_altitude_location(const Location &loc);
     bool above_location_current(const Location &loc);
-    void setup_terrain_target_alt(Location &loc);
+    void setup_terrain_target_alt(Location &loc) const;
     int32_t adjusted_altitude_cm(void);
     int32_t adjusted_relative_altitude_cm(void);
     float mission_alt_offset(void);
@@ -903,7 +903,7 @@ private:
 
     // control_modes.cpp
     void read_control_switch();
-    uint8_t readSwitch(void);
+    uint8_t readSwitch(void) const;
     void reset_control_switch();
     void autotune_start(void);
     void autotune_restore(void);
@@ -924,7 +924,7 @@ private:
     Vector2l get_fence_point_with_index(uint8_t i) const;
     void set_fence_point_with_index(const Vector2l &point, unsigned i);
     void geofence_load(void);
-    bool geofence_present(void);
+    bool geofence_present(void) const;
     void geofence_update_pwm_enabled_state();
     bool geofence_set_enabled(bool enable);
     bool geofence_enabled(void);
@@ -1039,13 +1039,13 @@ private:
     void servos_auto_trim(void);
     void servos_twin_engine_mix();
     void force_flare();
-    void throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle);
+    void throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle) const;
     void throttle_watt_limiter(int8_t &min_throttle, int8_t &max_throttle);
     void throttle_slew_limit(SRV_Channel::Aux_servo_function_t func);
     bool suppress_throttle(void);
     void update_throttle_hover();
     void channel_function_mixer(SRV_Channel::Aux_servo_function_t func1_in, SRV_Channel::Aux_servo_function_t func2_in,
-                                SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out);
+                                SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out) const;
     void flaperon_update(int8_t flap_percent);
 
     // is_flying.cpp
diff --git a/ArduPlane/altitude.cpp b/ArduPlane/altitude.cpp
index 9a0af4c5b5..abae31545e 100644
--- a/ArduPlane/altitude.cpp
+++ b/ArduPlane/altitude.cpp
@@ -137,7 +137,7 @@ void Plane::setup_glide_slope(void)
 /*
   return RTL altitude as AMSL altitude
  */
-int32_t Plane::get_RTL_altitude()
+int32_t Plane::get_RTL_altitude() const
 {
     if (g.RTL_altitude_cm < 0) {
         return current_loc.alt;
@@ -459,7 +459,7 @@ bool Plane::above_location_current(const Location &loc)
   modify a destination to be setup for terrain following if
   TERRAIN_FOLLOW is enabled
  */
-void Plane::setup_terrain_target_alt(Location &loc)
+void Plane::setup_terrain_target_alt(Location &loc) const
 {
 #if AP_TERRAIN_AVAILABLE
     if (g.terrain_follow) {
diff --git a/ArduPlane/control_modes.cpp b/ArduPlane/control_modes.cpp
index 9a21bd0935..2bcb8ca4e0 100644
--- a/ArduPlane/control_modes.cpp
+++ b/ArduPlane/control_modes.cpp
@@ -146,7 +146,7 @@ void Plane::read_control_switch()
 #endif
 }
 
-uint8_t Plane::readSwitch(void)
+uint8_t Plane::readSwitch(void) const
 {
     uint16_t pulsewidth = RC_Channels::get_radio_in(g.flight_mode_channel - 1);
     if (pulsewidth <= 900 || pulsewidth >= 2200) return 255;            // This is an error condition
diff --git a/ArduPlane/geofence.cpp b/ArduPlane/geofence.cpp
index 4ad76208f4..f5bcdc7794 100644
--- a/ArduPlane/geofence.cpp
+++ b/ArduPlane/geofence.cpp
@@ -145,7 +145,7 @@ void Plane::geofence_disable_and_send_error_msg(const char *errorMsg)
  * return true if a geo-fence has been uploaded and
  * FENCE_ACTION is 1 (not necessarily enabled)
  */
-bool Plane::geofence_present(void)
+bool Plane::geofence_present(void) const
 {
     //require at least a return point and a triangle
     //to define a geofence area:
diff --git a/ArduPlane/mode.h b/ArduPlane/mode.h
index 886a22aff2..62f25577ac 100644
--- a/ArduPlane/mode.h
+++ b/ArduPlane/mode.h
@@ -382,7 +382,7 @@ public:
 
     void navigate() override;
 
-    bool get_target_heading_cd(int32_t &target_heading);
+    bool get_target_heading_cd(int32_t &target_heading) const;
 
     bool does_auto_throttle() const override { return true; }
 
diff --git a/ArduPlane/mode_cruise.cpp b/ArduPlane/mode_cruise.cpp
index a1223c5358..4b3b76b250 100644
--- a/ArduPlane/mode_cruise.cpp
+++ b/ArduPlane/mode_cruise.cpp
@@ -69,7 +69,7 @@ void ModeCruise::navigate()
     }
 }
 
-bool ModeCruise::get_target_heading_cd(int32_t &target_heading)
+bool ModeCruise::get_target_heading_cd(int32_t &target_heading) const
 {
     target_heading = locked_heading_cd;
     return locked_heading;
diff --git a/ArduPlane/quadplane.cpp b/ArduPlane/quadplane.cpp
index b5deae7836..dd55f89e07 100644
--- a/ArduPlane/quadplane.cpp
+++ b/ArduPlane/quadplane.cpp
@@ -2328,7 +2328,7 @@ bool QuadPlane::init_mode(void)
 /*
   handle a MAVLink DO_VTOL_TRANSITION
  */
-bool QuadPlane::handle_do_vtol_transition(enum MAV_VTOL_STATE state)
+bool QuadPlane::handle_do_vtol_transition(enum MAV_VTOL_STATE state) const
 {
     if (!available()) {
         gcs().send_text(MAV_SEVERITY_NOTICE, "VTOL not available");
diff --git a/ArduPlane/quadplane.h b/ArduPlane/quadplane.h
index 717991d272..0a343672fe 100644
--- a/ArduPlane/quadplane.h
+++ b/ArduPlane/quadplane.h
@@ -89,7 +89,7 @@ public:
     */
     bool in_tailsitter_vtol_transition(uint32_t now = 0) const;
 
-    bool handle_do_vtol_transition(enum MAV_VTOL_STATE state);
+    bool handle_do_vtol_transition(enum MAV_VTOL_STATE state) const;
 
     bool do_vtol_takeoff(const AP_Mission::Mission_Command& cmd);
     bool do_vtol_land(const AP_Mission::Mission_Command& cmd);
@@ -568,8 +568,8 @@ private:
     bool is_motor_tilting(uint8_t motor) const {
         return (((uint8_t)tilt.tilt_mask.get()) & (1U<<motor));
     }
-    bool tiltrotor_fully_fwd(void);
-    float tilt_max_change(bool up);
+    bool tiltrotor_fully_fwd(void) const;
+    float tilt_max_change(bool up) const;
 
     void afs_terminate(void);
     bool guided_mode_enabled(void);
diff --git a/ArduPlane/servos.cpp b/ArduPlane/servos.cpp
index 8e85f4f817..6a065bc2dd 100644
--- a/ArduPlane/servos.cpp
+++ b/ArduPlane/servos.cpp
@@ -142,7 +142,7 @@ bool Plane::suppress_throttle(void)
   SERVOn_* parameters
  */
 void Plane::channel_function_mixer(SRV_Channel::Aux_servo_function_t func1_in, SRV_Channel::Aux_servo_function_t func2_in,
-                                   SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out)
+                                   SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out) const
 {
     // the order is setup so that non-reversed servos go "up", and
     // func1 is the "left" channel. Users can adjust with channel
@@ -378,7 +378,7 @@ void Plane::set_servos_manual_passthrough(void)
 /*
   Scale the throttle to conpensate for battery voltage drop
  */
-void Plane::throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle)
+void Plane::throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle) const
 {
     // return if not enabled, or setup incorrectly
     if (g2.fwd_thr_batt_voltage_min >= g2.fwd_thr_batt_voltage_max || !is_positive(g2.fwd_thr_batt_voltage_max)) {
diff --git a/ArduPlane/tiltrotor.cpp b/ArduPlane/tiltrotor.cpp
index 534f926676..05df5cdf88 100644
--- a/ArduPlane/tiltrotor.cpp
+++ b/ArduPlane/tiltrotor.cpp
@@ -9,7 +9,7 @@
 /*
   calculate maximum tilt change as a proportion from 0 to 1 of tilt
  */
-float QuadPlane::tilt_max_change(bool up)
+float QuadPlane::tilt_max_change(bool up) const
 {
     float rate;
     if (up || tilt.max_rate_down_dps <= 0) {
@@ -310,7 +310,7 @@ void QuadPlane::tilt_compensate(float *thrust, uint8_t num_motors)
 /*
   return true if the rotors are fully tilted forward
  */
-bool QuadPlane::tiltrotor_fully_fwd(void)
+bool QuadPlane::tiltrotor_fully_fwd(void) const
 {
     if (tilt.tilt_mask <= 0) {
         return false;