Frozen Outflow Valve

[meatball]

Happened just last week. In descent from 350, passing through FL120 realized the cabin was shooting past 0 feet...over-pressurizing in a big way ! Went to manual DC and toggled the outflowvalve until able to equalize differential. On the ground found that the outflow valve heated seal had failed...this is not an item that is usually checked...even during D checks...but Boeing overlooks it...in winter very very cold operations watch out for this because it can get you.:ok:

.............forgot to mention this was preceeded by AUTOFAIL light on the pressurization panel................

[dartman]

Had the same thing last year! Manual AC and DC had no effect. Ended up 'controlling' the cabin over pressurisation by toggling one of the packs on and off. Gratefully, flying a freighter that day. The outflow was one bit ice cube. Turned out the A/C had been sitting for weeks in blowing snow. Predictable results,

dartman

[BOAC]

A word of caution - if there is a full 'open' demand on a frozen o/v, then if it unfreezes below freezing level the resultant cabin decompression can be extreme, with a large 'bang' below the rear galley(!). Result - mist in rear galley plus noise and c/c assume explosive decompression.

Happened to me a few years back. We also used packs to control cabin.

[meatball]

Oh yes, there was a loud BOOM along with a prolonged and unnatural hisssssing....then BOOM again...girls shaken and fearful of being sucked into and out into minus 20´s with no parachute over snow covered Munich areas...ye gads:{