Another reason for turning the radio on first - if you start the plane with the transmitter off, the engine may be at full throttle. Without the radio on the plane will take off. I saw this happen at my field once. The guy was in front of it, started it, was holding it down and grabbed the radio and stepped aside to throttle down for taxiing. However, the plane took off before he realized what the problem was! When he finially figured it out and turned the radio on the plane went crazy. Probably like Ken said - the servos then tried to center themselves. If I remember, he had to turn the power off then back on to get control.. Very scary...