In the heli’s wiki page, at accelerometer calibration it is said that ‘Before you attach your APM to the heli I would recommend you do an accelerometer calibration which is well described here. It’s easier to do if the APM is not mounted in the heli and it’s essential when running firmware 2.9 or higher’.
The purpose of the calibration is to keep the autopilot and with it the helicopter level in the flight. If due to some reason the autopilot is not at the same horizontal level as the helicopter, the autopilot may keep level but the helicopter does not. In helicopters, the autopilot often comes with vibration damping mount, and with this mount it is difficult to install the autopilot at the same horizontal level as the helicopter. I had the experience of not having both at the same horizontal level, the helicopter then leans on one side in the flight. So it is important to keep both on the same horizontal level. IMHO, one solution is to do accel calibration with the autopilot mounted on the helicopter.
It may be more difficult than calibrate the autopilot alone, but you can design various stands helping the calibration. It should be better than calibrate only the autopilot then don’t know how level it is with the helicopter.
After thinking carefully, I’ve come to the conclusion that the accelerometer calibration still can be done with the autopilot alone, but then we need to check that the autopilot is mounted levelly with the helicopter. This can be done the next way:
Put the helicopter standing on the level surface. Connect to the Mission Planner. Go to Flight data->Action->Raw Sensor View. Mark to display Accel X, Accel Y. You need to adjust the autopilot mounting that these data are shown around 0.
I reinvented the wheel. That’s the way Mission Planner displays the horizon. No problem. Now I know exactly the horizon is horizontal.