Human error

Another odd accident was a Helios flight where the cabin failed to pressurize.

The cause was an engineer had to switch the pressurization settings to manual (or possibly off) while he checked a problem with one of the doors. Apparently he got called to do something else and neglected to return the pressurization controls to the automatic setting. the pilots failed to notice this. I don't know if it's part of the pre-takeoff checklist, but it should be.

The odd part is the pilots did notice a problem, and the oxygen masks did deploy. But the pilots neither activated the cabin pressurization, nor descended to a flight level with a higher oxygen concentration. The latter is the standard response to any incident which requires the emergency oxygen masks to deploy. The masks offer only a limited supply of oxygen. By descending to a thicker area of the atmosphere, the passengers can breathe near normally. The speculation is the pilots were suffering from hypoxia, or lack of oxygen, which does make people sluggish and unfocused; the last thing you want on an airline crew.

The plane kept flying on autopilot until it ran out of fuel and crashed.

Three may be some consolation in the fact that the pilots, crew and passengers were all unconscious, many probably dead, by the time the plane hit the ground. Except the Greek air force sent fighters to look the plane over (it might have been hijacked, after all). The pilots in the fighters reported seeing the emergency oxygen masks deployed, and both pilots slumped over the controls. And one person walking around with a portable oxygen tank and mask, possibly one of the cabin crew. Apparently this person tried to give oxygen to the pilots, but they did not respond.

This is one of the saddest crashes I can think of, because there were so many opportunities to make things right.

I hadn't heard there was a cabin attendant using the mobile oxygen bottle, the cockpit door must have been unlocked then so all he had to do was push the dead or unresponsive pilot out of the way and lower the plane.

Had there been an initial compression and then a sudden decompression the masks would have deployed at a lower altitude.

crew oxygen systems are different than passenger oxygen.

I just accidentally found this today:

A Boeing 377 Stratocruiser and Crew Resource Management

Quote: Mosca

I just accidentally found this today:

I see what you did...

So the door isn't locked, the warning light is on, there is an air leak, they can depressurize the cabin (presumably safely), and presumably return to Rio for repairs and fuel.

How in the world does the captain decide to keep on flying??

I found this handy listing on Wikipedia:

https://en.wikipedia.org/wiki/List_of_accidents_and_incidents_involving_commercial_aircraft