MAVlink - Change altitude

Ground Control Software MAVLink
1

I have tried to set target altitude with these functions to no avail:

MAV_CMD_CONDITION_CHANGE_ALT (113)
MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT (30)
MAV_CMD_DO_CHANGE_ALTITUDE (186)
MAV_CMD_NAV_LOITER_TO_ALT (31)

Specifically:

mavlink_msg_command_long_pack(2, 0, &msg, 1, 1, 31, 0, 0, 0, 0, 0, 0, 0, 420);
uint16_t len = mavlink_msg_to_send_buffer(buf, &msg);
Serial.write(buf,len);

I try this with SITL and the altitude won’t change.
According to Mission Commands — Plane documentation, at least commands 30 and 31 seem to be supported.

Is this even a correct way to control altitude with a companion computer (Arduino)? If so, how should this be implemented?
Thank you very much.

2

Yes it is possible, but you need to be in guided mode.

3

Doesn’t seem to be working. The plane takes off with TAKEOFF and holds an altitude of 50m. After changing mode to Guided, it still holds this altitude even though I periodically send

mavlink_msg_command_long_pack(2, 0, &msg, 1, 1, 31, 0, 0, 0, 0, 0, 0, 0, 420);