The accelerometer calibration is saved; reboot before flying

What ArduPilot says:

• PreArm: Accels calibrated requires reboot

The accelerometer calibration has just finished and its values are saved. ArduPilot will not arm until it has been restarted, so that every part of it, the EKF (its position and attitude estimator) above all, starts from the new values rather than from an estimate built on the old ones.

What to do, most likely first

1. An accelerometer calibration was completed since the last boot.

   FixReboot the flight controller: from the ground station, or by unplugging the battery and USB and plugging in again.

Where to do it:Setup & Tuning Guide