What must occur before opening any of the airplane doors?

The aircraft must be completely depressurized before any doors can be opened. This is crucial for safety reasons. When the aircraft is pressurized, the internal cabin pressure is higher than the ambient air pressure outside the airplane at high altitudes. If doors are opened while the aircraft is still pressurized, it could pose a significant hazard, such as injury to crew and passengers due to the sudden equalization of pressure or difficulty in operating the door due to the forces involved.

While it might seem intuitive to think about engines being turned off or the aircraft needing to be at cruising altitude as important, they do not directly relate to the safety of opening the doors as much as the state of cabin pressure does. Ensuring that the cabin pressure is equalized with the outside atmosphere is essential and is primarily accomplished through complete depressurization before engaging door operations.