I was in a 757 a couple of years ago conducting a maintenance test flight on an airplane that not flown in about a year.
There was a sticky valve that failed to close on takeoff that is used for electronic equipment cooling that is open on the ground.
The airplane never pressurized. Climbing into the low 20s my brand new FO recognized that the airplane was not pressuring (I did not).
We donned our oxygen masks, coordinated with ATC, reduced altitude and returned to base.
Other reasons for loss of pressurization could be a window failure, structural failure, pneumatic plumbing failure. A plumbing failure in the air conditioning/pressurization system or valves not working properly.
For plumbing problems we have checklists that lead us to isolate the problem. Their are two independent halves of the system and isolating the bad half of the system can keep the aircraft pressurized.
For a window, structural failure, or bad valve the failure may not be able to be corrected in flight. In these cases the pilots will descend and divert the flight.
In the case of remote or oceanic flights these are contingencies that are planned for in advance.
It is not permitted for a flight to be planned that can not handle this type of emergency at any time during the course of the flight.
We always have an alternate airport(s) planned to divert to at any point on our course. This includes adequate facilities, approaches, weather and fuel planning if we have to descend due to loss of pressurization, loss of an engine, fire or all of the above.
I routinely fly between the west coast and Hawaii. It is the longest distance in the world with so few alternate airports available. There just simply is no land or islands between them.
We have to carry enough fuel to be able to turn around or proceed to alternate airports at any point on the whole trip.
No. Fortunately it doesn’t happen often
Author – Swanson Dinner (Airline Captain)