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To expand on that, couldn’t you also have a separate power source to warm up the bus before it was taken out on a cold morning? Something like an engine block heater for an ICE vehicle.

The FAA designates 18,000 ft to flight level 600, which is 60,000 ft above sea level, as class A airspace. Any aircraft needs explicit permission to enter class A, so that’s normally only when an airliner is climbing to cruising altitude, but it would also occur when a spacecraft is descending through 60,000 ft.

Other countries, I don’t know exactly, but they all function fairly similarly.