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It's highly version controlled. Changing software versions on a plane usually means changing a lot of hardware too. After removing the boxes from the a/c, they can be returned to the manufacturer and the new software can be installed.

We test it in a ground-based simulator rig with the exact computers that would be on an a/c. If it gets approved, we put it on an airplane and we go fly the plane around a lot to make sure the software works.

A software upgrade almost always means switching out the hardware, because that's one way we ensure conformity.