We're trying to arm our plane with ONLY the PreArm check for INS turned on (16, bitmask 4). However, the Pixhawk fails to arm and returns the error:

"APM: PreArm: "

And:

"Got MAVLink msg: COMMAND_ACK {command : 400, result : 4}" When running on MavProxy

Where there is no error following PreArm. 

Pixhawk is running 3.5.2.

We calibrated the Accelerometer

We have the compass disabled (b/c our launch system is slightly magnetic) with:

        Mag_Enable: 0

        Compass_Use: 0

Gyro's look fine (the graphs are ok)

And the Roll/Pitch look fine too. 

Any ideas on what to check? We'd rather not fly with an INS that's not passing checks.