pixhawk2.1 has three acclerometer, two on the vibration isolated board, one on the io-board. In ac3.4.4 ,the ardupilot use only one acc,two of them or all? In log bin file, i can not identify which num of acc data is on the vibration isolated board.
By default it only uses two EKF2 cores, which means EKF2 is only doing math in the first two IMUs. But if you lose the first two IMUs for some reason the third one will be used.