I am unable to complete the Accelerometer calibration using Mission Planner 1.3.49.
When I complete the orientation steps, I see a message in the console ‘Info: Calibration Succesful’. When I try to arm it, I see a message ‘Critical: PreArm: Accelerometers calibrated requires reboot.’ However after calibrating my Pixhawk 2.4.8, when I reconnect Mission Planner, I still receive a message: ‘PreArm: Accelerometers calibrated requires reboot’. I’m using Mission Planner on Windows 8.1. Grateful for any guidance for my problem.
You need to reboot your aicraft electronics not Mission planner 
Shift F4 in Mission Planner and Reboot Pixhawk should do the trick.
Thank friends. My problem solved