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Guest Tim__757

Mystery real life problem

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On 22 Dec I was on LO281 flying from EPWA to EGLL. Shortly after being told that the cabin crew would soon be passing around refreshments (cardboard sandwiches), the captain came on air to tell us that because of an air-conditioning fault, we'd be returning to Warsaw. Fair enough. But the temp was fine and we'd noticed no change at all. About 20 mins later he came on again to say that they'd fixed the fault and that we'd be carrying on to London as planned. Wierd.Anyone got any idea what could've been happening? Prehaps it was the air-con in our B733, but there was no noticable temperature change.Yours curious,Gavin

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Could have been something as simple as a warning light. An airline wouldn't take chances and would get down as quickly as possible rather than wait for something to really happen.-John

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Well,If it was an AC problem, you most likely would not notice it in the air, but you would on the ground.Remember, air temperature goes down as altitude goes up, so you may not notice a temp difference.Happy New Year,Joe :-outta****************Grab My FREEWARE Cessna 172 Voice recognition Profile here:[a href=http://library.avsim.net/esearch.php?CatID=fs2004misc&DLID=58334]Cessna 172 Voice Profile[/a].You will need the main FREEWARE Flight Assistant program to use it, get it here:[a href=http://library.avsim.net/esearch.php?CatID=genutils&DLID=39661]Flight Assistant 2.2[/a]

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While I haven't worked on the 737, I've worked on quite a few other airliners, and one possibility is the loss of one air conditioning Pack. I don't know of any airliner with less than 2 Packs and they not only provide temperature control, but they supply the pressurizing air to the cabin. The loss of one pack (If it failed for any reason, and there are alot of ways to fail one, temp sensors, pressure sensors, valves, controllers, leak detectors, etc.) would limit the altitude at which the plane could fly, (not enough oomph from one pack to pressurize at a higher altitude)possibly causing enough increase in the fuel burn to keep you from making it to your destination.

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Thanks for those. Interesting. We didn't make any rapid descent or change of heading and we flew all the way to EGLL in about the same time as usual.Happy New Year!Gavin

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Well, actually in my experience the outflow valves in the aft pressure bulkhead do much more to control cabin pressure than the two packs.The "packs" are actually air cycle machines (at least on Boeing products-haven't flown airbus) that take bleed-air off the compressor section(s) of the engine and run it through an "expansion" cycle that decreases the temps and takes out the moisture (cabin air is much dryer than most deserts-drink fluids every 30 minutes). There are intercoolers involved as well but when the packdoors open it creates drag which is a bad thing so they tend to stay closed if we can help it.When run, the packs do put out a LOT of air and also take air off the engines reducing the operational efficiency...which means money. A lot of carriers will run with one pack which decreases the volume(flow) of air flowing into the cabin (but NOT the amount flowing out)maintaining the same pressure even with only one pack is feeding the manifold.If two packs are feeding the cabin the cabin outflow valves would just be open wider changing the cabin air out quicker. I think on the 757 the air in the cabin is changes out every 2 1/2 minutes with 2 packs running where with one the air is changes out every 6 minutes (ok, I might be off on those numbers but you get the drift) The "stale air" is also why you are more likely to catch colds etc..And the air actually is not sent out the "back" but down into the fwd hold to hold pressure/warmth there for animals etc (Why do they call that switch "The Dead Dog Switch???") then sent out.A lot of time a flight crew will "dummy-down" a flightdeck issue since most of the passengers would not know or care if there was a problem with a "13th stage bleed air valve". We know we have it under control and by saying "air conditioning" it sounds a lot less ominus to the under/ill-informed. Why put fear into passengers over a technical issue that is under control yet may require a precautionary landing. Or there was something else.....I won't 2nd guess thatOf course if there are flames shooting out of the engine etc we might "up" our sense of urgency.So that is the quick-n-dirty on aircraft HVAC.Tim__757

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Tim:"A lot of time a flight crew will "dummy-down" a flightdeck issue...""Ladies and Gentlemen, we regret to tell you that one of the engines on this Boeing 727 has failed. Please don't be worried, we can fly perfectly well on two engines, but we will be an hour late to our destination"an hour goes by..."Ladies and Gentlemen, we regret to tell you that a second engine on this Boeing 727 has failed. Please don't be worried, we can fly perfectly well on one engine, but we will now be two hours late to our destination"Paddy turns to Seamus..... "If that third engine goes, we'll be up here all #### day!"

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"Well, actually in my experience the outflow valves in the aft pressure bulkhead do much more to control cabin pressure than the two packs."Yes, the outflow valves are what control the pressurization and rate of the cabin atmosphere, but on the airliners I've worked on, the loss of one pack (in-flight or deferred) limited the altitude at which the plane could be operated. On those aircraft, one pack could not put out enough air to maintain cabin pressure at high altitudes, or perhaps maintain that rate of change in cabin air. I don't think I implied the packs control cabin pressure, just provide it.I've worked on a lot of planes for a few airlines, and rode in all the jumpseats, and NEVER saw an airline turn off packs to save money! That's scary. What if the other pack failed at altitude? Just what airlines are we talking about here?

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OK, I can see where this is going. You are reading way too much into what I'm saying. Your last post, as you said, is "snarky" and "inappropriate". Read it again. I was just trying to give one possible answer from someone who's been in the industry. You couldn't have said something more like, perhaps that's the case on those airliners, oops I meant aircraft, but on these it may have been something like .... ?Also, why, other than possibly being a troll, would you make such assumptions as me saying things like "lets go out to the airliner" just because I was referring in general to a category of aircraft?If it's so important to you, here it is:I give up. You smart....me stupid.....you too smart for me.....Good job!Apologies to all the normal folks around here. I won't bother replying to this guy any longer. However you may want to send him a thank you note for such engaging conversation, and such encouragement from him to others to participate.Well, after a little research, I will be man enough to apologize for the trolling suggestion. Sorry. But I still have to wonder why I got singled out. Do you treat all the A&Ps with such respect?

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>Tim__757>(Who also rides in jumpseats when commuting to work.....)>Wow! Does that mean you are a real airline pilot?

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First let me apologize to all the "normal" readers if I appeared "troll-like". Negative Emotional responses really do not add to the learning experience of this forum and I apologize to the readers. (Though I did enjoy the joke about being left up there-engine failure after my 1st response on this thread....)Yes, I do admit Xmech hit a MAJOR nerve when he suggests in his post the operation is "scary" and I felt he implied that my operation was "unsafe". Normally it is my aim to enhance the knowledge base here by providing real-world answers, concrete examples based on documented experience in an unbeat, non-challenged way (see my 1st post). When "challenged" I do tend to over respond since any good crewmember has to have confidence in his knowledge and abilities. Sorry if I went over-board....diplomacy goes out the window during a check-ride too...so you are not alone Xmech in getting that treatment from me(sorry)...normally I reserve it for the FAA and check-captains. If an inspector gets the feeling I know my "stuff" he goes off in another topic.As far as speaking for other airlines I have a lot of friends at other carriers who share operational issues. One of the issues we did talk about was the exact same issue of pack operation so again I felt confident....sorry if the readers thought I over-reached.(to see about this go to "PILOTS SKIMP ON AIR TO SAVE FUEL" at http://www.flyana.com/full.html ) I respect A&P's, my life is in their hands on a weekly basis. For our mechanics I have a great deal of respect when they operate in a professional way, promoting confidence in the industry. I go out of my way to help them combat the exporting of jobs to contract shops overseas or off-property.It isn't a question of being smarter or stupid....just a question of point of view. My emotional retort didn't aid my efforts to get across what I was trying to explain.Tim__757(Also sorry about the "airliner" shot...uncalled for on my part)

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Just wanted to correct an earlier misleading comment. Engine bleed air ALWAYS has to be cooled significantly, even at cruising altitude. The heat of compression far outweighs the drop in temperature of the ambient air entering the engine. I recall that the bleed air exiting the low-stage compressor is something over 300F, even if the ambient temp is around -50F. If that air wasn't cooled prior to entering the packs and the cabin, the passengers would quickly wind up somewhere between medium rare and well-done. ;-)

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