Arducopter Flight Mode

In HEARTBEAT( Msg id 0) consist of Flightmode stored in custom_mode field
but in MAVLink document didn't define this field because it depend on

which flight controller software was using in this case Arducopter

it was define in file "defines.h"

// Auto Pilot modes
// ----------------
#define STABILIZE 0                     // hold level position
#define ACRO 1                          // rate control
#define ALT_HOLD 2                      // Altitude Hold
#define AUTO 3                          // Waypoint
#define GUIDED 4                        // Hold a single location from command
#define LOITER 5                        // Hold a single location
#define RTL 6                           // return to launch
#define CIRCLE 7                        // orbit around a single location
#define LAND 9                          // Landing
#define OF_LOITER 10                    // Hold a single location using optical flow sensor
#define DRIFT 11                        // DRIFT mode (Note: 12 is no longer used)
#define SPORT 13                        // earth frame rate control
#define FLIP        14                  // flip the vehicle on the roll axis
#define AUTOTUNE    15                  // autotune the vehicle's roll and pitch gains
#define POSHOLD     16                  // position hold with manual override
#define NUM_MODES   17

Ref:

https://github.com/diydrones/ardupilot/blob/master/ArduCopter/defines.h