The FC(cuav v5+, ArduCopter v4.6.3) was calibrated before. I set INS_ENABLE_MASK=3, INS_USE3=0, in order to reduce CPU load. No other parameters was changed. Then the message “PreArm: 3D Accel calibration needed” came out after reboot. I tried “Force Accel Cal” in the Ctrl-F menu, but it didn’t work.
I also checked the params. INS_ACC3_ID and INS_GYR3_ID was automatically set to 0, but the offsets and scalings remained unchanged. So I can’t figure out what causes the prearm warning.