diff --git a/libraries/SRV_Channel/SRV_Channel.h b/libraries/SRV_Channel/SRV_Channel.h
index 20ee360b23..4ebd141a21 100644
--- a/libraries/SRV_Channel/SRV_Channel.h
+++ b/libraries/SRV_Channel/SRV_Channel.h
@@ -270,7 +270,7 @@ private:
     AP_Int16 servo_trim;
     // reversal, following convention that 1 means reversed, 0 means normal
     AP_Int8 reversed;
-    AP_Int16 function;
+    AP_Enum16<Aux_servo_function_t> function;
 
     // a pending output value as PWM
     uint16_t output_pwm;
diff --git a/libraries/SRV_Channel/SRV_Channel_aux.cpp b/libraries/SRV_Channel/SRV_Channel_aux.cpp
index 6f77ca8c9d..a25311192a 100644
--- a/libraries/SRV_Channel/SRV_Channel_aux.cpp
+++ b/libraries/SRV_Channel/SRV_Channel_aux.cpp
@@ -34,13 +34,13 @@ void SRV_Channel::output_ch(void)
     int8_t passthrough_from = -1;
 
     // take care of special function cases
-    switch(function)
+    switch(function.get())
     {
     case k_manual:              // manual
         passthrough_from = ch_num;
         break;
     case k_rcin1 ... k_rcin16: // rc pass-thru
-        passthrough_from = int8_t(function - k_rcin1);
+        passthrough_from = int8_t((int16_t)function - k_rcin1);
         break;
     }
     if (passthrough_from != -1) {
@@ -87,7 +87,7 @@ void SRV_Channels::output_ch_all(void)
 SRV_Channel::Aux_servo_function_t SRV_Channels::channel_function(uint8_t channel)
 {
     if (channel < NUM_SERVO_CHANNELS) {
-        return (SRV_Channel::Aux_servo_function_t)channels[channel].function.get();
+        return channels[channel].function;
     }
     return SRV_Channel::k_none;
 }
@@ -100,7 +100,7 @@ void SRV_Channel::aux_servo_function_setup(void)
     if (type_setup) {
         return;
     }
-    switch (function) {
+    switch (function.get()) {
     case k_flap:
     case k_flap_auto:
     case k_egg_drop:
@@ -181,11 +181,14 @@ void SRV_Channels::update_aux_servo_function(void)
 
     // set auxiliary ranges
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
-        if ((uint16_t)channels[i].function.get() < SRV_Channel::k_nr_aux_servo_functions) {
-            channels[i].aux_servo_function_setup();
-            function_mask.set((uint16_t)channels[i].function.get());
-            functions[channels[i].function.get()].channel_mask |= 1U<<i;
+        const uint16_t function_num = channels[i].function.get();
+        if (function_num >= SRV_Channel::k_nr_aux_servo_functions) {
+            // invalid function number
+            continue;
         }
+        channels[i].aux_servo_function_setup();
+        function_mask.set(function_num);
+        functions[function_num].channel_mask |= 1U<<i;
     }
     initialised = true;
 }
@@ -204,19 +207,19 @@ void SRV_Channels::enable_aux_servos()
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
         SRV_Channel &c = channels[i];
         // see if it is a valid function
-        if ((uint16_t)c.function.get() < SRV_Channel::k_nr_aux_servo_functions) {
+        if (c.function < SRV_Channel::k_nr_aux_servo_functions) {
             hal.rcout->enable_ch(c.ch_num);
         }
 
         // output some servo functions before we fiddle with the
         // parameter values:
-        if (c.function.get() == SRV_Channel::k_min) {
+        if (c.function == SRV_Channel::k_min) {
             c.set_output_pwm(c.servo_min);
             c.output_ch();
-        } else if (c.function.get() == SRV_Channel::k_trim) {
+        } else if (c.function == SRV_Channel::k_trim) {
             c.set_output_pwm(c.servo_trim);
             c.output_ch();
-        } else if (c.function.get() == SRV_Channel::k_max) {
+        } else if (c.function == SRV_Channel::k_max) {
             c.set_output_pwm(c.servo_max);
             c.output_ch();
         }
@@ -277,7 +280,7 @@ void SRV_Channels::set_output_pwm(SRV_Channel::Aux_servo_function_t function, ui
         return;
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
-        if (channels[i].function.get() == function) {
+        if (channels[i].function == function) {
             channels[i].set_output_pwm(value);
             channels[i].output_ch();
         }
@@ -296,7 +299,7 @@ SRV_Channels::set_output_pwm_trimmed(SRV_Channel::Aux_servo_function_t function,
         return;
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
-        if (channels[i].function.get() == function) {
+        if (channels[i].function == function) {
             int16_t value2;
             if (channels[i].get_reversed()) {
                 value2 = 1500 - value + channels[i].get_trim();
@@ -320,7 +323,7 @@ SRV_Channels::set_trim_to_servo_out_for(SRV_Channel::Aux_servo_function_t functi
         return;
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
-        if (channels[i].function.get() == function) {
+        if (channels[i].function == function) {
             channels[i].servo_trim.set_and_save_ifchanged(channels[i].get_output_pwm());
         }
     }
@@ -336,7 +339,7 @@ SRV_Channels::copy_radio_in_out(SRV_Channel::Aux_servo_function_t function, bool
         return;
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
-        if (channels[i].function.get() == function) {
+        if (channels[i].function == function) {
             RC_Channel *c = rc().channel(channels[i].ch_num);
             if (c == nullptr) {
                 continue;
@@ -378,7 +381,7 @@ SRV_Channels::set_failsafe_pwm(SRV_Channel::Aux_servo_function_t function, uint1
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
         const SRV_Channel &c = channels[i];
-        if (c.function.get() == function) {
+        if (c.function == function) {
             hal.rcout->set_failsafe_pwm(1U<<c.ch_num, pwm);
         }
     }
@@ -395,7 +398,7 @@ SRV_Channels::set_failsafe_limit(SRV_Channel::Aux_servo_function_t function, SRV
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
         const SRV_Channel &c = channels[i];
-        if (c.function.get() == function) {
+        if (c.function == function) {
             uint16_t pwm = c.get_limit_pwm(limit);
             hal.rcout->set_failsafe_pwm(1U<<c.ch_num, pwm);
         }
@@ -413,10 +416,10 @@ SRV_Channels::set_output_limit(SRV_Channel::Aux_servo_function_t function, SRV_C
     }
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
         SRV_Channel &c = channels[i];
-        if (c.function.get() == function) {
+        if (c.function == function) {
             uint16_t pwm = c.get_limit_pwm(limit);
             c.set_output_pwm(pwm);
-            if (c.function.get() == SRV_Channel::k_manual) {
+            if (c.function == SRV_Channel::k_manual) {
                 RC_Channel *cin = rc().channel(c.ch_num);
                 if (cin != nullptr) {
                     // in order for output_ch() to work for k_manual we
@@ -458,7 +461,7 @@ SRV_Channels::move_servo(SRV_Channel::Aux_servo_function_t function,
     v = constrain_float(v, 0.0f, 1.0f);
     for (uint8_t i = 0; i < NUM_SERVO_CHANNELS; i++) {
         SRV_Channel &c = channels[i];
-        if (c.function.get() == function) {
+        if (c.function == function) {
             float v2 = c.get_reversed()? (1-v) : v;
             uint16_t pwm = c.servo_min + v2 * (c.servo_max - c.servo_min);
             c.set_output_pwm(pwm);
@@ -481,7 +484,7 @@ bool SRV_Channels::set_aux_channel_default(SRV_Channel::Aux_servo_function_t fun
         }
         hal.console->printf("Channel %u already assigned function %u\n",
                             (unsigned)(channel + 1),
-                            (unsigned)channels[channel].function);
+                            (unsigned)channels[channel].function.get());
         return false;
     }
     channels[channel].type_setup = false;
@@ -579,8 +582,8 @@ void SRV_Channels::set_trim_to_min_for(SRV_Channel::Aux_servo_function_t functio
 void SRV_Channels::set_default_function(uint8_t chan, SRV_Channel::Aux_servo_function_t function)
 {
     if (chan < NUM_SERVO_CHANNELS) {
-        int8_t old = channels[chan].function;
-        channels[chan].function.set_default((uint16_t)function);
+        const SRV_Channel::Aux_servo_function_t old = channels[chan].function;
+        channels[chan].function.set_default(function);
         if (old != channels[chan].function && channels[chan].function == function) {
             function_mask.set((uint16_t)function);
         }
@@ -606,7 +609,7 @@ void SRV_Channels::adjust_trim(SRV_Channel::Aux_servo_function_t function, float
     }
     for (uint8_t i=0; i<NUM_SERVO_CHANNELS; i++) {
         SRV_Channel &c = channels[i];
-        if (function != (SRV_Channel::Aux_servo_function_t)(c.function.get())) {
+        if (function != c.function) {
             continue;
         }
         float change = c.reversed?-v:v;
diff --git a/libraries/SRV_Channel/SRV_Channels.cpp b/libraries/SRV_Channel/SRV_Channels.cpp
index aebbfa9bcd..7184d037c6 100644
--- a/libraries/SRV_Channel/SRV_Channels.cpp
+++ b/libraries/SRV_Channel/SRV_Channels.cpp
@@ -317,7 +317,8 @@ void SRV_Channels::calc_pwm(void)
             channels[i].set_override(true);
             override_counter[i]--;
         }
-        channels[i].calc_pwm(functions[channels[i].function].output_scaled);
+        const uint16_t function_num = channels[i].function.get();
+        channels[i].calc_pwm(functions[function_num].output_scaled);
     }
 }