[ventus45]

"k"

All that is true enough, but the fact remains, a crisis forces action first - tell last - if you can.

Moreover, they may have lost consciousness very early in the "return".

Consider this.

The thing that plays on my mind, more than any other possibility, is "shoddy maintenance".

I refer specifically, to the crew oxygen bottle in the MEC.

The fact that it had required a "top up" in a relatively short time / number of flights since it's previous "top up" leads me to think that there may have been a "latent defect", a "pre existing leak - a slow leak" in the system somewhere. It could be a "chaffed hose", or a "incorrectly tightened fitting", anything is possible.

The "refilling" of the cylinder, at high pressure, only a short time prior to take-off, could have "aggravated" that small leak, into a bigger one.

The aircraft takes off, climbs, and the leak is filling the MEC with extra O2.

Something happens in the MEC, an electrical short circuit, which causes a small fire, which in an O2 enriched environment in a "closed space" (the MEC) becomes essentially a short lived flash fire, the heat from which damages a lot of "other equipment" in the MEC (other than the ORIGINAL FAULTY ITEM).

Up on the flight deck, warnings start going off, Z and H are startled, and begin to "work the problem".

Meanwhile, the fire, though small, is now being fed by a now ruptured hose from the O2 bottle, which is rapidly doing much more damage, but worst of all, has cut off the supply of 02 to the cockpit.

Z and H decide to return, (as per my scenario above) but very soon, the fire (which they may not realise is the real problem yet) causes a breach in the skin, and the aircraft suffers rapid decompression.

The O2 masks drop for the pax and Z and H go for their masks, but alas, there is nothing there.

The rest is obvious.