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  • Rapid decompression

    Hey AJ, and other airline pilots,
    The recent AS incident got me wondering how the pilots deal with rapid decompression. What steps do you take to get the aircraft on the ground safely?

    Alec
    You've got to try to find what's right before your eyes-Finger Eleven



  • #2
    First things first is to get your oxygen mask on!

    After that we must do an emergency descent to lower altitudes. We would wind down the altitude knob, and start descending, pulling the speedbrakes to increase the descent rate. At the same time the PNF would manually release the passenger oxygen masks as a back-up in case they did not automatically drop.

    Consideration would be given to turning off the airway (Don't want to descend onto someone!), and the surrounding terrain (if any). If we want to increase descent rate further, we could wind the airspeed up. The aircraft would pitch down more to attain this higher airspeed. However, if the decompression was explosive, then you have to consider possible structural damage (maybe a small bomb?), in which case winding the speed up may not be a good idea.....if you are really desperate you can even put the gear down, although again chances are that it is not a good idea! If the decompression was rapid, then at that moment, the pilot really doesn't know what has happened and should assume that there could well be structural damage.

    Once we are established in the descent, we can inform ATC, make a Mayday call and fine tune the altitude set in the altitude window. If there is time, we can run the checklist to see if there is anything we have missed.

    When we have levelled out, and the checklist is complete, we must assess whether people are ok, and what if any damage has been done. When the integrity of the aircraft is checked, we then begin to think about a diversion...where to go...programming the route in the FMC....telling ATC, telling the company etc...

    That is the brief version! I have missed loads of stuff, but that is the general idea.
    Have a look at my photos, including Kai Tak crazy landings!http://www.jetphotos.net/showphotos.php?userid=460

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    • #3
      This is one thing I've always wondered. If there is decompression, do the pilots actually dive to a lower altitude? Are we talking about a descent where everything in the cabin is floating around?

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      • #4
        Thanks Colin Why might it be bad to put down the gear? Are you exceeding gear extension speed?



        Originally posted by JordanD
        This is one thing I've always wondered. If there is decompression, do the pilots actually dive to a lower altitude? Are we talking about a descent where everything in the cabin is floating around?
        This what I have been wondering also.
        You've got to try to find what's right before your eyes-Finger Eleven


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        • #5
          Rapid decompressions are actually the less dangerous of the decompressions. When a rapid decompression happens, you notice. Because of the decrease in pressure, there is a decrease in temperature and usually a loud noise. Your ears pop, the air in your bowels expands, and it really gets your attention. With a rapid decompression, time of useful conciousness is slightly decreased as compared to a gradual one, but because you notice so quickly, its easy to grab your mask and put it on, which is the most important thing.

          The real dangerous ones are the gradual ones. When I was in the altitude chamber, after you dropped the oxygen mask, you actually start to feel really really good. By the time you realize you are hypoxic, odds are its too late.

          It is possible to make an emergency descent without making things float. If you put the aircraft into a bank while diving, its possible to keep positive G forces on the aircraft. Also banking the aircraft allows less of the lift created by the wings is pointing down so this can increase your descent as well. I highly reccomend any pilots looking for an opportunity to go in an altitude chamber if given the chance, was a great learning experience.

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          • #6
            Colin's pretty much summed it up!

            I did a practice emergency descent in VH-OGM from F430 to A100 with just us and the cabin crew, the manouvre itself is suprisingly tame. That being said we didn't have every loose object on the aircraft flying around, water boiling in galleys, a screaming PA and mist to add to the experience!

            Boeing has two procedures, one which is present speed or up to Mmo/Vmo, idle thrust, full speedbrake and the other is idle thrust, full speedbrake, slow to gear speed, drop the wheels and then go down. Qantas recommends the clean version as it gets you down to more breathable air more quickly.

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            • #7
              What altitude would you decend to after a decompression?
              Also, do the pilots make annoucements to the pax during the decent, for those who might not realize whats going on?

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              • #8
                Descent is made to 10,000' or MSA in most aircraft, although it was 14,000' to the duration of the oxygen, then 10,000' in the 747-400.

                There is an automated PA that comes on with the passenger oxygen, however we try to give a PA from the flightdeck during the descent, mainly so the cabin crew and passengers know we are still in control.

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                • #9
                  Alaska Air Rules,

                  If you have had a small bomb go off or something, or perhaps a part of the aircraft has come loose and punctured the aircraft, the aircraft's structural integrity may be at question. In these cases, you want to limit the amount of stress you put through the aircraft structure. Putting the gear down at the gear limit speed puts unwanted stress on the fuselage. In extreme cases this additional stress could theoretically make the plane fall to bits! Extreme scenario I know, but you never know, it could happen!

                  As for the descent itself, there are a couple of arrivals into Hong Kong where you pass almost overhead the airfield at above FL200, and then get vectored all the way round, halfway down the southern hemisphere (Or so it seems at times! ) and then in to land. However if it is quiet, we get a very short short cut, and this means full speedbrake descents. On the 777 we have a max speed of 330kts, and we normally descend at 300kts, so on these arrivals the descent would not be that different to an emergency descent. It is still enough for the cabin crew to put things away, walk around and do their finals checks. We are not talking luggage flying around, blood splattered everywhere and food carts floating down the aisles. Of course, as AJ mentionned, an emergency descent after rapid decompressionalso includes stress, screaming, cabin crew scrambling all over the place, oxygen masks etc, so it is a bit different, but the manouvre itself is rather tame as AJ says.

                  For the altitude, on long flights we descend down to FL140. This is the altitude at which the passengers are allowed to continue without oxygen. We do this rather than descend straight down to FL100 because we can save fuel by being higher up, and on very long sections between alternates fuel can be very tight. We then remain at FL140 until the crew oxygen begins to run low, and then descend to FL100, which is the altitude at which the crew can continue without oxygen. We then remain at FL100 until we need to descend for a landing.
                  Last edited by Colin Parker; 2006-01-02, 11:10.
                  Have a look at my photos, including Kai Tak crazy landings!http://www.jetphotos.net/showphotos.php?userid=460

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                  • #10
                    Thanks for the info guys Those empty runs sound kind of fun.
                    You've got to try to find what's right before your eyes-Finger Eleven


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                    • #11
                      Why might it be bad to put down the gear? Are you exceeding gear extension speed?
                      Gear Overspeed

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