This morning and afternoon my flying group and I were flying a Multiplex EasyStar fitted with a Pixhawk complete with all necessary sensors. All the radio and sensor calibrations had been finished successfully, and we launched it with me at the controls and Qgroundcontrol 2.5.0 as the ground station. The plane flew well in manual mode, but when I switched to auto-loiter mode, the plane turned belly up and seemed to fly upside down for a moment before I hastily regained control in manual mode. One person suggested calibrating the gyro upside down, just in case there was a mismatch between the calibration instructions and the true calibration protocol--which had been the case for the compass calibration. We did so, but the plane inverted again immediately upon switching to auto-loiter.
We have had a successful loiter with Qgroundcontrol and a different Pixhawk. This is a recent development.
What suggestions do you have regarding getting to the bottom of this?