diff --git a/libraries/RC_Channel/RC_Channel_aux.cpp b/libraries/RC_Channel/RC_Channel_aux.cpp
index 9101eeb740..9e70a246f7 100644
--- a/libraries/RC_Channel/RC_Channel_aux.cpp
+++ b/libraries/RC_Channel/RC_Channel_aux.cpp
@@ -20,9 +20,23 @@ const AP_Param::GroupInfo RC_Channel_aux::var_info[] = {
 };
 
 RC_Channel_aux *RC_Channel_aux::_aux_channels[RC_AUX_MAX_CHANNELS];
-uint64_t RC_Channel_aux::_function_mask;
+uint64_t RC_Channel_aux::_function_mask[2];
 bool RC_Channel_aux::_initialised;
 
+void
+RC_Channel_aux::set_function_mask(uint8_t fn)
+{
+    uint8_t idx = fn / 64;
+    uint8_t bit = fn % 64;
+    _function_mask[idx] |= (1ULL<<(uint8_t)bit);
+}
+
+void
+RC_Channel_aux::clear_function_mask(void)
+{
+    memset(_function_mask, 0, sizeof(_function_mask));
+}
+
 /// map a function to a servo channel and output it
 void
 RC_Channel_aux::output_ch(void)
@@ -123,7 +137,7 @@ void RC_Channel_aux::aux_servo_function_setup(void)
     }
 
     if (function < k_nr_aux_servo_functions) {
-        _function_mask |= (1ULL<<(uint8_t)function);
+        set_function_mask((uint8_t)function.get());
     }
 }
 
@@ -136,7 +150,7 @@ void RC_Channel_aux::aux_servo_function_setup(void)
 /// (do not call this twice with different parameters, the second call will reset the effect of the first call)
 void RC_Channel_aux::update_aux_servo_function(void)
 {
-    _function_mask = 0;
+    clear_function_mask();
 
     // set auxiliary ranges
     for (uint8_t i = 0; i < RC_AUX_MAX_CHANNELS; i++) {
@@ -381,7 +395,10 @@ bool
 RC_Channel_aux::function_assigned(RC_Channel_aux::Aux_servo_function_t function)
 {
     if (function < k_nr_aux_servo_functions) {
-        return (_function_mask & (1ULL<<function)) != 0;
+        uint8_t fn = (uint8_t)function;
+        uint8_t idx = fn / 64;
+        uint8_t bit = fn % 64;
+        return (_function_mask[idx] & (1ULL<<bit)) != 0;
     }
 	 return false;
 }
diff --git a/libraries/RC_Channel/RC_Channel_aux.h b/libraries/RC_Channel/RC_Channel_aux.h
index 011146aa3d..af5e4089ba 100644
--- a/libraries/RC_Channel/RC_Channel_aux.h
+++ b/libraries/RC_Channel/RC_Channel_aux.h
@@ -163,9 +163,11 @@ public:
     static bool find_channel(Aux_servo_function_t function, uint8_t &chan);
     
 private:
-    static uint64_t _function_mask;
+    static uint64_t _function_mask[2];
     static bool _initialised;
     static RC_Channel_aux *_aux_channels[RC_AUX_MAX_CHANNELS];
 
     void aux_servo_function_setup(void);
+    static void set_function_mask(uint8_t function);
+    static void clear_function_mask(void);
 };