Why do aircraft start moving forward when parking brakes set and engines off?

Question

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Not sure why the aircraft starts moving forward and unsure how to set chocks. Cant fly if the aircraft is going to end up 20 miles away from the airport

Answer 1

I could not reproduce this with the default 737. I started a new flight at TXKF with the engines off. By default the parking brake is on in this case, and the plane did not move, even after I toggled the brake off and back on.