Pressurisation
I operated from Bogota (8400'amsl) in 757. Make sure APU running for pressurisation if Bleeds Off for T/O (otherwise warnings manifest themselves at 1500', followed by the rubber jungle). When airborne, Cabin Alt starts to decrease in accordance with the auto schedule, to give a cruise Cabin Alt of prob 6000ft. Commencing descent, cabin start to climb. Looks odd if you are staring at the roof. You don't notice it, though, if you would rather watch the shop!