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  • #16
    Originally posted by juan23 View Post
    The compressors in an aircraft AC system are the engines! Which are part of the system that pump air into the cabin.
    Aircraft use an ACM..(air cycle machine) I think you are thinking of a freon system..... there is no freon.
    The PACS were working just not cooling which would be a fault of the temp control or 35 degree valves.
    Also on the ground the cabin can be cooled by an AC cart, or by an Air Start cart feeding air to the PACS
    ok thanks! my point remains the same though: i would bet that the a/c took off with faulty something. mind you, i do not believe this is a safety matter. rather a very typical disregard for the passengers

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    • #17
      Originally posted by TeeVee View Post
      ok thanks! my point remains the same though: i would bet that the a/c took off with faulty something. mind you, i do not believe this is a safety matter. rather a very typical disregard for the passengers
      I would say it is a safety matter. If it's 100 degrees or so in the cabin and people are passing out and nurse passengers have to respond, it's best to be on the ground ASAP.

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      • #18
        A Colleague of mine was on a UA flight FRA-IAD on July 2nd or around that date and reported the air condition worked intermittently in the forward section - 20 minutes freezing and 20 minutes blazing hot. The rear section was apparently always blazing hot so that the passengers walked to the front to cool off... The flight was normally operated to destination, so the game apparently went on for 8 hours!

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        • #19
          Originally posted by TeeVee View Post
          ok thanks! my point remains the same though: i would bet that the a/c took off with faulty something. mind you, i do not believe this is a safety matter. rather a very typical disregard for the passengers
          Do you really think the cockpit or cabin crew would want to work in the conditions? No, and they have a union to back that if the airline tries to punish the crew. They took off thinking it would kick in like it very well should, it didn't so they returned.
          Tanner Johnson - Owner
          twenty53 Photography

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          • #20
            Originally posted by Evan View Post
            True. And if you decide to depart anyway, the FAA will write your airline an angry letter saying "Hey Bill, you really can't be doing that. And thanks again for the skybox tickets."
            Come on Evan ... I understand you're frustrated with the regulator ... but don't you think this a bit over the top?
            AirRabbit

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            • #21
              Originally posted by Vnav View Post
              Now obviously MELs can vary from airline to airline, so I don't want to make any sweeping statements. However, certainly you can MEL one of the two Packs on a twin engine bird normally with a FL250 restriction. While not an everyday occurence, dispatch under this MEL is fairly common, and I've had single pack ops where it's tough to cool things.....I've also had single pack ops where the remaining pack struggles to maintain pressure at 250.
              Does this mean that if one failed in flight you'd need to descend to below FL250?
              Also if you only had one operational and it failed how easy would it be to descend quickly enough to avoid the cabin oxygen masks being deployed?
              I guess in both situations there would be also be a possible need to divert due to increased fuel consumption.

              However, in the back of my mind I seemed to recall that the MEL allows unpressurized ops (i.e. no packs). Sure enough, a quick peek at the MEL shows 21-00-01B as 2 packs installed/ 0 required. Now I only checked a couple of our fleets as it takes forever to download each MEL, but each plane I checked had that MEL. I can't imagine a situation where you would actaully do it except some extreme IROP, but theoretically, it's allowed.
              Probably comes down to how much the airline would otherwise have to pay in compensation, food/drink, hotel accomodation, etc.
              Also if the flight will get the plane (nearer) to somewhere it can be fixed.

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              • #22
                Originally posted by MCM View Post
                Unrelated but our MEL doesn't allow two pack inop dispatch, and even the one pack inop MEL goes to lengths to explain if the operative pack fails there is significant temperature and humidity issues, and the QRH required landing at the nearest airport. Thats in a twin, but not a 73.
                What is your plane's fuel consumption like below 10,000 ft? I'd guess quite poor, especially in terms of distance travelled per quantity of fuel.

                Out of random interest Evan my airline has cabin temperature guidelines, above which we don't board passengers. I've sat on the gate for an hour with engines running trying to cool the cabin to an acceptable level on occasions. It still feels hot, but at least its comfortable within about 30mins of cruise. I guess thats the "full service" difference .
                Whatever the ambient temperature on the ground even at a fairly moderate altitude the ambient temperature is going to be cold to very cold.

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                • #23
                  Originally posted by mpe View Post
                  Does this mean that if one failed in flight you'd need to descend to below FL250?
                  Valid question, but the answer is No. The MEL as a document applies only to problems that are known about and documented prior to dispatch (dispatch for MEL purposes is the application of Takeoff thrust). For any problems that are discovered in flight, any special AirCrew instructions in the MEL do not apply. You would simply write up the problem, inform MTC and follow the applicable QRH checklist if there is one for the particular malfunction (QRH= Quick Reference Handbook, a compilation of non-normal/emergency checklists). There most certainly would be a QRH procedure for loss of a single Pack, and without grabbing my manuals, I could almost guarantee that it says something along the lines of: "The aircraft can maintain current altitude as long as the remaining Pack can keep the cabin altitude below 10,000ft"

                  Now certainly once the QRH procedure is complete, a good crew practicing CRM might have a discussion about possibly descending, but that would depend on what kind of weather is at the lower altitudes and far more importantly, how much extra gas will be burned. Obviously when an MEL is on the flight plan and known about prior to departure, the Dispatcher will have planned extra fuel to fly below FL250....not so if it's discovered inflight


                  Also if you only had one operational and it failed how easy would it be to descend quickly enough to avoid the cabin oxygen masks being deployed?.
                  In an emergency descent, you can easily get descent rates in excess of 6000fpm. Most PAX oxygen systems that I know about automatically drop at a cabin altitude of 14000ft. In my current plane, flying at FL250 would produce a cabin altitude around 5000 ft. So if you're asking whether I could get from FL250 to below 14000ft before the cabin would climb from 5000 to 14000 and drop the masks, I'd wager that I could do it....but I don't want to have to find out. If the masks did drop, it wouldn't be the end of the world as I would likely be diverting shortly afer that anyway.
                  Last edited by Vnav; 2010-07-28, 19:50.
                  Parlour Talker Extraordinaire

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                  • #24
                    Much more parlour talker than you (re: your signature), but...

                    I could almost guarantee that it says something along the lines of: "The aircraft can maintain current altitude as long as the remaining Pack can keep the cabin altitude below 10,000ft"
                    Wouldn't the cabin altitude warning start bitching at someehow shortly past 8000ft?

                    So if you're asking whether I could get from FL250 to below 14000ft before the cabin would climb from 5000 to 14000 and drop the masks, I'd wager that I could do it....but I don't want to have to find out.
                    I'm very confident, almost sure, that you would be able to do it without any problem (please correct me on the following logic)

                    As soon as the remaining PAC goes west the outflow valves would autmatically close to try to prevent the loss of cabin altitude that would start to develop due to the lack of inflow air to balance the ouflow air. Then, the only loss of air would be through leaks, which are always present but never so severe.

                    At your 6000fpm it would take less than 2 minutes to go from FL250 to 14000ft. For the cabin altitude to go from 5000ft to 14000ft in the same time a cabin climb of some 4900fpm. I cannot see that happen just from standard leaks. More like just 1/10 of 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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                    • #25
                      Gabriel,

                      Far 25.841 for aircraft certification requires that pressurized aircraft maintain their cabins at no greater than 8000ft in normal circumstances. However, the actual warning is not required until 10,000ft (see section b.(6)). I will copy the entire FAR here because it also has some verbiage on how quickly one must be able to descend, but I will concur with the math in your post.

                      Sec. 25.841

                      Pressurized cabins.

                      [(a) Pressurized cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions.
                      (1) If certification for operation above 25,000 feet is requested, the airplane must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system.
                      (2) The airplane must be designed so that occupants will not be exposed to a cabin pressure altitude that exceeds the following after decompression from any failure condition not shown to be extremely improbable:
                      (i) Twenty-five thousand (25,000) feet for more than 2 minutes; or
                      (ii) Forty thousand (40,000) feet for any duration.
                      (3) Fuselage structure, engine and system failures are to be considered in evaluating the cabin decompression.]
                      (b) Pressurized cabins must have at least the following valves, controls, and indicators for controlling cabin pressure:
                      (1) Two pressure relief valves to automatically limit the positive pressure differential to a predetermined value at the maximum rate of flow delivered by the pressure source. The combined capacity of the relief valves must be large enough so that the failure of any one valve would not cause an appreciable rise in the pressure differential. The pressure differential is positive when the internal pressure is greater than the external.
                      (2) Two reverse pressure differential relief valves (or their equivalents) to automatically prevent a negative pressure differential that would damage the structure. One valve is enough, however, if it is of a design that reasonably precludes its malfunctioning.
                      (3) A means by which the pressure differential can be rapidly equalized.
                      (4) An automatic or manual regulator for controlling the intake or exhaust airflow, or both, for maintaining the required internal pressures and airflow rates.
                      (5) Instruments at the pilot or flight engineer station to show the pressure differential, the cabin pressure altitude, and the rate of change of the cabin pressure altitude.
                      (6) Warning indication at the pilot or flight engineer station to indicate when the safe or preset pressure differential and cabin pressure altitude limits are exceeded. Appropriate warning markings on the cabin pressure differential indicator meet the warning requirement for pressure differential limits and an aural or visual signal (in addition to cabin altitude indicating means) meets the warning requirement for cabin pressure altitude limits if it warns the flight crew when the cabin pressure altitude exceeds 10,000 feet.
                      (7) A warning placard at the pilot or flight engineer station if the structure is not designed for pressure differentials up to the maximum relief value setting in combination with landing loads.
                      (8] The pressure sensors necessary to meet the requirements of paragraphs (b)(5) and (b)(6) of this section and Sec. 25.1447(c), must be located and the sensing system designed so that, in the event of loss of cabin pressure in any passenger or crew compartment (including upper and lower lobe galleys), the warning and automatic presentation devices, required by those provisions, will be actuated without any delay that would significantly increase the hazards resulting from decompression.

                      Parlour Talker Extraordinaire

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                      • #26
                        some acft is worse than others when it comes to one pack operation (either cooling on ground or after airborn). if the cabin was hot prior departure and the sector is short then defenatly it will be hot for the PAX. however in this particular flight i am assuming that the fault is in the hot air trim sys. (auto and manual)

                        breif info:
                        when the acft is in flight, the air coming to the cabin is actually heated because of the low ambient temp. acually the bleed air bypass the ACMs which normally cools the air on ground then after it pass the water seperator to the cold plenum, the zone trim valves will regulate the amount of hot air to be mixed with the cool air to heat it up. this is done automatically (by selecting the temp required through the controller and sensors or manually by the crew)

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                        • #27
                          Originally posted by Tanner_J View Post
                          Do you really think the cockpit or cabin crew would want to work in the conditions? No, and they have a union to back that if the airline tries to punish the crew. They took off thinking it would kick in like it very well should, it didn't so they returned.
                          ummmm, perhaps you don't know the unions too well. they are not that strong. and i have personally experienced flight aboard a commercial airline with a non-functioning lavatory. i'm sure you believe that the union would have hooted and hollered about that as well...

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                          • #28
                            Originally posted by old timer View Post
                            some acft is worse than others when it comes to one pack operation (either cooling on ground or after airborn). if the cabin was hot prior departure and the sector is short then defenatly it will be hot for the PAX. however in this particular flight i am assuming that the fault is in the hot air trim sys. (auto and manual)

                            breif info:
                            when the acft is in flight, the air coming to the cabin is actually heated because of the low ambient temp. acually the bleed air bypass the ACMs which normally cools the air on ground then after it pass the water seperator to the cold plenum, the zone trim valves will regulate the amount of hot air to be mixed with the cool air to heat it up. this is done automatically (by selecting the temp required through the controller and sensors or manually by the crew)
                            ok so i was wrong on the compressor thing. i take it from your post that either the thermostat was bad or there was some other problem the "zone trim valves." either way, im still assuming that the aircraft was physically able to maintain normal flight and pressurization, even if temperature regulation was flawed.

                            my point and belief remains the same.

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                            • #29
                              What is your plane's fuel consumption like below 10,000 ft? I'd guess quite poor, especially in terms of distance travelled per quantity of fuel.
                              Yes, its pretty poor. But if it needs to be done, it could be. The 737 doesn't do too well on fuel mileage below 10,000ft either.

                              Whatever the ambient temperature on the ground even at a fairly moderate altitude the ambient temperature is going to be cold to very cold.
                              And your point is? Its not like we can open the doors and windows to cool off. Skin friction will mean that even going at the slowest possible speed the cabin temperature will rise, in some cases significantly.

                              TeeVee, its your right to believe that they departed with something faulty. However, this situation can be explained completely with a departure with everything working properly.

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                              • #30
                                Originally posted by TeeVee View Post
                                ok so i was wrong on the compressor thing. i take it from your post that either the thermostat was bad or there was some other problem the "zone trim valves." either way, im still assuming that the aircraft was physically able to maintain normal flight and pressurization, even if temperature regulation was flawed.

                                my point and belief remains the same.

                                you are correct Tee Vee about the pressurization , further more if the auto and manual operation fail then it is not a GO. (note: acft can be flown unpressurized as maint. ferry fligh to return to base after obtaining the require approvals .

                                general info. : eventhough both aircond& press.sys (ATA 21) uses the same air source however they are completely independant sys with regards to controll and function.
                                pressurization is simply pumping air into the cabin (regardless of the it s temp) by the aircond. sys then it is controlled by the out flow valve/s through the pressurization controller or manually by the crew (depending on the rate of climb or decend)

                                thanks

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