Mount Rakaposhi, Pakistan

At 25,551 feet above sea-level, pilots and cabin crew know that the "Time Of Useful Consciousness" is three to five minutes. That is, the time a normal person has to get an oxygen mask on, and start using-it, before being paralysed by hypoxia, then death.

No wonder that there aren't many people on top of the 27th tallest mountain in the world; as we traverse the highest airway in the world, crossing waypoint Purpa, from Pakistan into China.

If we have a serious depressurisation issue, we can only descend to 28,500 feet until we get a little further north, and can start winding-down within those valleys ahead, and divert to nearby Kashi.

A tricky business at night or in cloud; so for an hour or so everyone on the flight deck is very attentive — while the passengers watch movies.