PPRuNe Forums - View Single Post - Diff between Outflow valves and Positive/Negative Pressure valves?
Old 5th Apr 2018, 14:38
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flyburg
 
Join Date: Nov 2000
Location: Netherlands
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Few years back I had an actual decompression in a B747. About 5 hrs into the flight, somewhere over Canada, the B controller failed. Failed in such a way that both outflow valves were driven to the full open position. For some reason B controller inhibited transfer to the A controller. Anyways, kinda hectic, donned the masks and went to manual control etc,etc. In manual was able to close both outflow valves. Cabin alt went from approx 6000’ and spiked at approx 13.000 in about 35 to 40 seconds before it went down to 10.000. At 10.000 cab alt we removed the oxy mask and were kinda congratulating ourselves for successfully containing the situation when the next warning went off: overpressure! Off course with both outflow valves shut the airplane now started to overpressurize!!

That’s where the overpressure valves kicked in and saved the day!! Had that valve not been there the plane could have possibly looked like the picture above
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