IMU info:
Many of us have heard about the dreaded fly a ways. One way to help prevent them is through calibrations. A lot of controversy over what causes them. But I can guarantee you one thing a proper IMU calibration will help prevent some of them. What is an IMU you say. Inertial Measurment Unit is the brains of the quad when it comes to a collection of the sensors and is typically comprised of at least a three-axis accelerometer and a 3-axis gyroscope and employes magnetometers (compass), barometers and other sensor to determine the angular position or attitude of the quadcopter. Basically it's location, direction and altitude at any determined time. Remove any one of the sensors information and you can have a fly a way.
IMU information is data and data can be corrupted. Sensors can be disoriented. How you say, the main way is by rough handling (shipping), hard landings, crashes and changes to the sensors an compass. These quads are shipped all over the world and are roughly handled so new out of the box they need to be re-calibrated and have fresh data at their new home. After being sent out for repairs and shipped back to you the same is needed. DJI even sends you a email telling you to re-calibrate. Calibration is also need after new firmware updates as new data is introduced.
Calibration of the IMU must be done with the Quadcopter perfectly level from back to back and side to side. Easy way is to put a piece of glass, cutting board or simular flat panel on top of the quad covering the 4 motors spindles and then using a level front to back, side to side, and diagonally. Placing spacers or paper where needed under the legs to get her perfectly level. Then power up the assistant and the quad and fairly quickly do the advanced IMU calibration. This needs to be done before the main controller gets to warm from no airflow or you may get a to warm message. If it pops right up ignore it. But if the quad had be on for more that 5-10 minutes it may be legit and you'll need to shut down and cool her off for half an hour or so. A clean IMU calibration will only take 5-10 minutes at the most. When completed and after going outside do a compass calibration. When these are completed your quad should now have good data. The Roll, Pitch and Yall of the quad,as controlled by the changes in the motor speeds, will have good and proper orentation around the 3-axis as necessary for input from GPS and compass direction to let her know where she is, where she took of from, and how to get back, as long as you acquire the proper number of satellites and get home lock at take off. When you hover you should see a nice stable hovering quad with minimal drift and have good control of your unit. So when in doubt from a crash or hard landing, do your self a favor and re-calibrate. Do it after having shipped your unit or if it has seen rough handling. Reduce the fly a ways! CALIBRATE !!!!
Well, it'll be it ripped-up tomorrow. Bloody useless builder, giving me a wonky floor.. And... I know, I know... Before you ask no.... OK... I didn't use my all-over body earthing onesy either. I could've doomed me aircraft.... 
