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B744 Airtight?

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I have another question today :-)In the 747, if you're in level flight and you were to switch off all 3 PACKS, and close both Outflow Valves, the cabin altitude starts to rise indefinetly (ie the plane is slowly depressureising). Does this mean the aircraft is not airtight, and the air is flowing out from somewhere? So are there holes in the airframe where the air leaks out?Subin

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Hi Subin,you can be sure that every pressurised plane will have a small leak rate when you shut off the pressurisation system.Most of the time the air under pressure in the cabin will find its way outside via the doorseals that do not seal completely. Emergency exits can also be the culprit.Regards,S.

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Yep, it's a common Hollywood myth that pressurized airplanes are sealed up airtight or something to that effect. There's a virtually limitless supply of silly action movies for instance where someone causes a near disaster on a plane by shooting a small hole in the skin etc. Pretty much nothing would happen in real life in that kind of situation.

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It's quite common for engineers to carry out pressure runs on aircraft. There is a specified leak-rate allowed, common leaks are at doors, and at drain plugs. When new door sealsare fitted, they do require a bit of "bedding-in" until they seat right :)

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I experience a sudden depressurization at FL220 (in the C414) and the first clue was my ears popping, I had to look at the guage to confirm what happened. The fault was a rubber boot that seals the manual gear extraction rod as it passes through the forward pressure bulkhead, which created a hole of a little over an inch in diameter. I know the effect is greater at higher levels but it still wouldn't satisfy Hollywood writers.

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Used to ride around in C-130s where the cabin pressure warning light never went out. It was my job to wander around in the back with a bottle of water and a roll of paper towels pluggin' as many leaks as I could find. After 'bout half a roll, I could generally get the cabin down to a more tolerable 14000 feet.Yea, they leak . . . all of 'em.

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I wouldn't rely on them being waterproof either (in those disaster movies where the plane goes underwater)..... Some 744's leak like crazy in heavy rain. I can't imagine they would float for very long, either (as the safety card in the seat pockets suggest)... The cargo doors have negative pressure relief doors which would let water in very quickly (if the pressure outside is a few psi greater than the pressure inside the cabin).If you flush the vacuum operated toilets, too, air leaks to atmosphere.Cheers.Q>

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This reminds me, I've heard stories about a pack of sewage ice falling to the ground, like on/near a homes (tend to be in NZ? lol). When you flush the toilet, does it fall out of the airplane, or is it contained within tanks to be emptied at the next stop? If it does just fall out, it'll make my toilet breaks more 'meaningfull' next time ;-)What about "sink drains"? Does the galley sink drains just fall out as well?Out of curiosity, are these toilet/sink drains located at any particular part of the airframe (eg aft, fore, wing/body gear area etc)?And finally, last year I landed at LAX from NZAA and the flight being late, we parked at the remote stands NE of the airport. Being bused to the terminal, I noticed large rectangular, 'stainless steel' type covers on the underside of other B744s between the trailing egde of the wings and the horizontal stab. There are two of them side by side. Are these the actual openings of the outflow valves? I assumed they were round, but these outflow 'structures' seemed rectangular, and I thought these could actually be the outflow valve openings? Does it sound reasonable what I saw was the openings? I'm suspicious, as the airplnes seemed to be powered down, yet the valves we closed - I would imagine they should have been left open?Subin

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Briefly...Differential air pressure provides the suction for the toilets (above a certain altitude), but in theory, the liquid/solid waste is purely sucked into the tanks (although I wouldn't like to stand too close to the fuselage toilet (air) vents... smells pretty bad. Baffles in the tanks should stop liquid/solid waste being ejected from the aircraftToilet handbasin "grey water", however (and galley drain water), does find it's way out of the fuselage (even on the ground), via heated "drain masts", located on the fore and aft sections of the fuselage. The outflow valves are normally rectangular (and the 744/747 valves are kinda side by side (on opposite sides of the rear fuselage). I'm surprised the valves were closed however. They are fully open on the ground and if the aircraft is deadshipped, they should stay that way. I have no idea why they were closed (if indeed they were outflow valves???).Hope this helps.Cheers.Q>

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Ah ha! :-)After looking at photos of the rear of the fueslage, I see what both you and I were refering to are indeed the outflow valve openings. I didn't realise they are hinged, and open like a door downwards. When I saw them directly from the rear at LAX, they appeared closed with stainless steel type covers. Infact they were the inside surface of the valve doors (visible when open) with the outside surface painted in the airline livery (visible when closed).For those interested, the following are examples:http://www.airliners.net/open.file/0405287/L/http://www.airliners.net/open.file/0913987/L/http://www.airliners.net/open.file/0634330/L/While this may seem another trivial qiestion, I notice in the first link above (BAW 747) the paintwork (BAW paintwork in general) has a massive amount of detail to acheive the design. I've seen videos from youtube showing airframes being painted, and they all seem to be done with varying layers of masking tape. The BAW livery showing in the above link seems very extensive to be done with masking tape! Is every line, every edge (shown particularly in the blue and red parts) done via manually covered masking tape?Subin

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Actually, each outflow valve consists of two rectangular flaps. One hinges inwards and one hinges outwards (the part that you can see). They overlap each other to a certain extent when the doors almost fully close for cruise. A tie rod links the doors together, and they are normally driven by a single AC motor.Re: painting.... Complex colour schemes are often now achieved with plastic decals. I'm sure BA use these, or a combination of techniques.Cheers.Q>

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Those outflow valves are also the 747's tail skid. If there is an over rotation, those get clipped first. Generally, it grinds off just a little bit from the very lower inboard edges, nice and even. Almost looks machined. Had one where it had NoT just happened on last flight. Seems the valves got clipped several flights prior. Finally an observant FE (yea, one of those) brought me over there said, "It that right?" "Ahh, . . . no." I said. Leaky airplanes you ask? Sure, they're supposed to leak. The outflow valves never fully close. They're not supposed to. They need to stay at least a bit open to provide fresh air circulation. Well, as fresh as engine compressor bleed air gets, anyway. Remember, there are 4 massive (and I mean humungus!) air compressors blowing air into the airplane . . . the engine's compressor sections. (And then there was the time we had a skydrol leak into the compressor section of a JT9. You can probably imagine. It wasn't pretty.) In this case, since there was just a bit missing off the outflow valves, ol' auto-pressurization simply closed its valves a bit further to get the differential. "No need to bother the crew with this little detail," it decided. The next tail skid "component" is the belly skin about 20 feet aft of the body gear well. Gives a great show from the tower I hear. Lots of sparks. If you ever see a big sheetmetal belly patch a ways aft of the body gear well, just smile.

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"The outflow valves never fully close. They're not supposed to."Even if the cabin loses pressure?Partially open is normal, but are the valves physically unable to run to full closed with either AC or DC control? (Can't say I've been next to one, looking for light leakage, when it has been tested from the flight deck. Remind me to check it one day ;)Cheers.Q>

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Ahhh, so its another story ya want?STL (St Louis, Mo for you non-US-ers) mid-winter, 0-dark-thirty, wickedly cold . . . like -40F windchill . . . and snowing.Two 742s on the remote ramp. That was our departure gate. We were a real poor airline . . . and what the heck, it was just MAC charters anyway.Both were quick turning. In reverse directions. One in from Seoul, Korea, >> STL, >> Frankfurt, Germany. The other in the reverse direction. They were never scheduled to be in STL at the same time, but stuff happens. Lucky us. Both airplanes were fully lit. APUs running and full of activity, so I thought. One of the guys left a door open and I scolded, "Hey, Born in a barn. Close the darn door." Big mistake.I was tromping 'round on one of them and decided to go over and see how guys were doing on the other airplane. Man, it was cold. There seemed to be a break in the action on this one. No catering trucks. Nothing around. Just a glowing airplane. I scampered on up the stairs to find the door closed. "That's better." I muttered. I popped the handle out and leaned on it, but it didn't budge. "What the . . " All my weight on it, the handle wouldn't budge. The door would not open. This is very strange. I pounded on the door. "Hey, let me in. I'm freezing to death out here!" I shouted.About that time I heard a famaliar honk from bottom of the stairs. One of our Mx trucks had pulled up and a little pink face was poking out the window. "Hey chief, what cha doing?""What am I doing? What are YOU doing?""We're taking a break.""Hummm, great idea. I'm with ya!"So down I went and clambered into the truck. "Thanks for closing the door. The rugs get all wet than the ops guys whine at me." I commented, "but the door won't open. Are you guys working on it? "Nope, It's fine," they're all looking at each other. "We think."A faint dawn begins to glow. "Drive back to the tail." I asked. Back we went . . . and guess what we spied? The outflow valves were ComPletely closed. The airplane was pressurizing. That's why I couldn't get the door open. Holy smokes. Well head for the gear well and shut down the APU. Sure hope that emergency panel works. Never tried it before. BoooVVVvvvvvv. Down shuts the APU. Well that's better. Back to the door. But it still wouldn't open. Now I'm thinking I should have just let it go 'till those pressure relief valves blew. Evidently the airplane had accumulated quite a "head pressure." As luck would have it, a catering truck rumbeled up. This was a quick turn. The airplane would eventually depressrize on its own (remember they all leak), but we're gonna be late."What are we gonna do, guys?" Well, I about got trampelled as I shouted at their heals. "Hey, Where ya goof balls going." The next thing I know they are building a human pyramid under the E&E door(back of the nose WW). I just stayed out of the way. This was (mainly) the cabin crew and a bunch of big St.Louis boys they were. As they pushed, first there was a whisp, then a tornado blowing out of that hole. Finally, the door collapsed into the recess and we were in!And the departure was saved. As we later discovered, the auto pressure controller had a bad habit of failing to auto-close the outflow valves when left in auto. That's what had happened. When I finally managed to find the cockpit that dark night (and got the lights back on), I found the switch in auto and - of course - the outflow valves closed. We later changed the crew's cockpit post-flight shutdown procedure to leave the auto pressure controller in Manual. Sometimes the easiest fixes are the best . . . . so yea, they'll close all the way.Back then, we always kept a couple of 6-ers in the Mx shop fridge. Once we finally got all that tonnage launched over the fence, we may have cracked a - well deserved - few. Those were the days when we just got the job done. And from then, on we always left a door cracked. Let 'em whine.

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Hey, I was at one airline where the security check of the aircraft had the controller in manual, and outflow valves closed (B737).

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Yea, we currently have a line item to manually close the outflow valves. Keeps the birds out, so the story goes. But it's just asking for trouble, if you ask me . . . like this . . . Had a 767 freighter departure tbe other day. The crew was onboard and had closed the door. I wanted in but couldn't open the door. I had to scribble a note and hold it up to the window. "You're pressurized. Open the door!"

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Why not contact them on the ground comm?

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The stairs run right next to the Capt's side window. It was more fun to bang on the window and give 'em ThE eYe.

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