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  • A Serious Question

    So, there's all sorts of stuff that goes into an aircraft being certified as ETOPS "whatever." but what about this: an aircraft is 1/2 way across the ocean on its way to wherever, but some not real close. a/c loses cabin pressure so crew descends to an oxygenated flight level. how does it make it to its destination without running out of fuel?

    oh, and don't tell me that depressurization is rare. simon posts several instances just about every week, so not so uncommon or rare.

  • #2
    Depressurization descent is part of ETOPS route planning, that is to say, the route must be such that there must be suitable diversion fields for any number of reasons, including depressurization. Thus, the aircraft might NOT make it to destination, but will instead proceed to a diversion airport.

    There is a similar process involved when planning routes over high terrain elevation. Same idea.

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    • #3
      Originally posted by TeeVee View Post
      So, there's all sorts of stuff that goes into an aircraft being certified as ETOPS "whatever." but what about this: an aircraft is 1/2 way across the ocean on its way to wherever, but some not real close. a/c loses cabin pressure so crew descends to an oxygenated flight level. how does it make it to its destination without running out of fuel?
      That's independent of ETOPS. Regulations require that a suitable airport (not necessarily "its destination" as you've said) must be within fuel reach (plus reserves) with such an event happening anywhere along the route.

      That is, if your preferred route does not comply with that with the "standard" fuel calculation, you either add more fuel, plan for a different route that pass closer to some airport, plan for a fuel stop (that you can later skip if you didn't needed it), or something like that.

      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
      --- Defend what you say with arguments, not by imposing your credentials ---

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      • #4
        14 cfr:

        §121.646 en-route fuel supply: Flag and supplemental operations.
        (a) no person may dispatch or release for flight a turbine-engine powered airplane with more than two engines for a flight more than 90 minutes (with all engines operating at cruise power) from an adequate airport unless the following fuel supply requirements are met:

        (1) the airplane has enough fuel to meet the requirements of §121.645(b);

        (2) the airplane has enough fuel to fly to the adequate airport—

        (i) assuming a rapid decompression at the most critical point;

        (ii) assuming a descent to a safe altitude in compliance with the oxygen supply requirements of §121.333; and

        (iii) considering expected wind and other weather conditions.

        (3) the airplane has enough fuel to hold for 15 minutes at 1500 feet above field elevation and conduct a normal approach and landing.

        (b) no person may dispatch or release for flight an etops flight unless, considering wind and other weather conditions expected, it has the fuel otherwise required by this part and enough fuel to satisfy each of the following requirements:

        (1) fuel to fly to an etops alternate airport.

        (i) fuel to account for rapid decompression and engine failure. The airplane must carry the greater of the following amounts of fuel:

        (a) fuel sufficient to fly to an etops alternate airport assuming a rapid decompression at the most critical point followed by descent to a safe altitude in compliance with the oxygen supply requirements of §121.333 of this chapter;

        (b) fuel sufficient to fly to an etops alternate airport (at the one-engine-inoperative cruise speed) assuming a rapid decompression and a simultaneous engine failure at the most critical point followed by descent to a safe altitude in compliance with the oxygen requirements of §121.333 of this chapter;

        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
        --- Defend what you say with arguments, not by imposing your credentials ---

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        • #5
          ok, so given that. how would it be at all possible for a flight to arrive with very low fuel when no depressurization occurred? given that flying at say 10k consumes, what? 2 or 3 times as much fuel?

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          • #6
            Originally posted by TeeVee View Post
            ok, so given that. how would it be at all possible for a flight to arrive with very low fuel when no depressurization occurred? given that flying at say 10k consumes, what? 2 or 3 times as much fuel?
            Because... if it had had a depressurization far away from its intended destination, then it would have never made it to its intended destination but to, as the regulations say, an adequate or alternate airport (very likely well short of its intended destination).

            If you are flying from LAX to Sidney and have a depressurization in the middle of the Pacific Ocean, you will not have enough fuel fly at 10,000 ft to Shanghai but yes to reach Hawaii or Faleolo.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

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            • #7
              i get the part about diverting. i was wondering how that lufthansa a380 flight into MIA a while back was so low on fuel...

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              • #8
                Originally posted by TeeVee View Post
                i get the part about diverting. i was wondering how that lufthansa a380 flight into MIA a while back was so low on fuel...
                If you're referring to this incident:

                http://avherald.com/h?article=475207bb ,

                I'm not real sure what it has to do with your original question.

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                • #9
                  Originally posted by TeeVee View Post
                  i get the part about diverting. i was wondering how that lufthansa a380 flight into MIA a while back was so low on fuel...
                  I don't remember all the details of the case, but make as if what I say is accurate. It's good enough for the point I try to make.

                  This LH flight was already in the final stages of the approach when its destination went below minimums. So it went around and went for the alternate, but on final approach the alternate went below minimums again so it went around for a second alternative, which also went below minimums so they ended making an emergency landing busting minimums at their original intended destination.

                  Now, this kind of situations where the (bad) reality exceeds what is foreseen by the regulations and the flight end up landing below final reserves, and even maybe busting minimums, is rare but not unheard of. Just for the sake of the argument let's say that it happens in one every 10K flights.

                  Just for the sake of the argument, say that a full loss of cabin pressure happens, excluding those that happen early in the flight that are the most common (there was some problem even before take-off and the plane does not pressurize), happen in once every 10K flights too.

                  Since both events are independent one form the other, the chance of both things happening in the same flight would be once everey 10K*10K=100M flights. That's a more-than-acceptable risk. I don't know if there have been 100M flight so far in the history of the pressurized commercial aviation.

                  And even then, LH would have done just what they did in the end, that is, an emergency landing busting minimums, just with fewer attempts of diversion in the middle.

                  By the regulations, at any point, including close to its destination, assuming a depressurization and emergency descent, the plane must have enough fuel to reach a suitable airport (but not an alternate after failing to land in the first one).

                  Put it in another way: How many times it happened that a plane run out of fuel after a depressurization? None that I know.

                  --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                  --- Defend what you say with arguments, not by imposing your credentials ---

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