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The upper atmosphere is not a friendly place. It's cold up there, real cold, -40C cold. The air is also much too thin to breath. In order to fly at high altitude we need to keep the aircraft cabin heated and pressurized. So let's take a look at what's keeping us alive up at 40,000 feet.

An airliner is not completely sealed off like a submarine. There is always air going in and there's always air going out. By controlling how much air we let out at a time, we can keep the insides under pressure.

So where does the air come in from? It originates at the engines. We "bleed" some air off the compressor stage of the engine. Since this is at the front of the engine, this is just normal air - it doesn't have any exhaust in it. It's quite hot, however, because it is under considerable pressure.

757 Bleed Air System
The hot bleed air travels through ducts into the fuselage into the air conditioning "packs". Pack is either shorthand for "package" or an acronym for Pneumatic Air Cycle Kit depending on who you ask.

The packs work a little differently than the air conditioner in your house or your car. An air conditioning pack uses what's called an Air Cycle Machine. It's a pretty slick little device that uses ambient air rather than freon for cooling.

Basically, we take the hot bleed air and run it through a heat exchanger with ambient air. This cools it a bit. Then we run it through a compressor, which heats it back up again (but it's still cooler than it was starting out). Then we pipe it through a second heat exchanger. Finally it goes through what's called an expansion turbine which cools the air even more by making it expand. The turbine turns the compressor so the whole unit is self contained. It's almost like a jet engine in reverse. Hot air goes in one end and cold air comes out the other.

In fact, the air coming out is so cold that we actually need to warm it up a bit before we can use it. So we save a little bit of that hot bleed air to mix in with the super cold air.

Cooling the air causes water to condense. This should mostly be trapped in the "coalescer" but sometimes on humid days you'll see mist coming out of the vents. It's just excess moisture. That little vent above your seat? It's called a "gasper". I have no idea why it's called that.

Here's a link to the Wiki article with more information than you ever wanted to know about an Air Cycle Machine:

Air Cycle Machine

Here's a diagram of a 727 air conditioner:

727 Air Conditioning Pack
Depending on the size, an airliner will likely have two, maybe three packs. By controlling the temperature coming out of the packs, we can control the temperature inside the aircraft. By mixing in a little bit of hot bleed air at various points, we can even have different temperatures in different parts of the aircraft. We call that "trim" air. That way I can keep the chickens nice and cool without freezing my butt off in the cockpit. I was once asked what temperature we should keep the chickens at - I said "350 for about 40 minutes ought to do it".
757 Air Conditioning (3 Zones)
You've probably heard about the air on airliners being recirculated. On newer planes like the 757 we do recirculate a little bit of the air. This lets us take in a little less "ram air" from the outside with a corresponding decrease in drag. I can't tell you exactly how much of the air is recirculated but I'm told it's not a large percentage. You're still getting mostly fresh air.

Now that we have all this air going into the plane, we have to let it out somewhere. After flowing through the cabin and the cargo compartments it exits out one or more outflow valves. The outflow valves are moved electrically and controlled by the cabin pressure controller. By metering how much air we let out of the plane we can keep the cabin under pressure.

I'm told that back in the days when people smoked on airliners the area behind the outflow valve would be streaked yellow with nicotine. Hope you weren't eating anything right now.

Since pressurization is rather important we need some redundancy. The 757 is pretty typical. We have three sources of bleed air, two engines plus the Auxiliary Power Unit. We have two air conditioning packs. We also have two automatic pressure controllers plus a manual backup. The A300 had two outflow valves. The 757 has only one but it has, if memory serves me, more than one electric motor to operate it.

We also have pressure relief valves. In case something goes drastically wrong these will blow out to keep the cabin from pressurizing too much and popping like a balloon. There are also negative pressure relief valves that blow inwards. We're not a submarine and we're not stressed for negative pressure.

Let's take a look at how this all works. Depending on the aircraft, the automatic pressure controller should pressurize the plane shortly before takeoff and depressurize it shortly after landing. It does this because having a little bit of positive pressure actually strengthens the fuselage.

Pressurization Schedule (Airbus 300)
Once airborne the cabin altitude is programmed to go up at a slower rate than the aircraft is actually climbing. If our cruise altitude is up around 40,000 feet, the altitude inside the cabin will get up to somewhere around 7 or 8 thousand feet.

Why can't we keep the plane pressurized to sea level? To do that we'd need much greater pressure than the 8 psi we normally use. The skin of the plane would have to be much thicker and therefore much heavier.

Once the aircraft starts descending, the automatic controller will start to bring the cabin altitude down to the elevation of the airport we're landing at. It does this at a relatively slow rate to hopefully keep everybody's sinuses and eardrums happy.

Allow me to digress a little bit here and talk about aviation physiology. If you can avoid it, I would suggest not flying with a bad head cold. You'll be OK on the ascent, but on the descent the pressure may not be able to equalize in your sinuses or ear cavity and the results can be excruciating. It feels like someone is trying to drive a hot knife through your face.

Remember when you were a little kid and you tried to pop your eyeballs out by holding your nose and blowing with your mouth closed? You didn't? You must have been a boring kid. Anyway, that's actually the best way to equalize the pressure in your eardrums and sinuses. Much better than chewing gum. Teach your kid to do this and they'll annoy the heck out of you with it but at least they won't be screaming because their ears are hurting them.

So what can go wrong with all this?

Bleed air leak - Hot bleed air leaking from a duct is as hot as a blowtorch and has about the same effect on things it comes in contact with. Once we figure out which system it is (left or right) we'll shut down that whole system by closing the bleed valves. No big deal. One source of bleed air is sufficient to pressurize the jet.

Pack failure - If one of the air conditioning packs starts to run too hot it will "trip off" automatically. We've got another one so not a problem. I have had one of these fail where it filled the plane with the smell of dirty gym socks. Icky but not dangerous.

Pressure controller failure - We've got another one. You see why we like to have at least two of anything important. If both fail we can still regulate the pressure manually but I've never had to do it.

Rapid decompression - This is the bad one. We all saw Goldfinger. This could be caused by both packs failing or by a structural failure of the aircraft (bad).

Aloha Airlines 243

This is thankfully an extremely rare occurrence. Still, best to keep your seat belt on all the time. I certainly do.

Let's suppose it's not our day. We're cruising along fat, dumb and happy at 40,000 feet when we hear a loud "bang" and a "whoosh", the cabin fills with mist (water vapor) and the oxygen masks drop.

I got to experience this in the Air Force altitude chamber years ago. They don't do that any more because they figured out it's bad for you. Even knowing it was coming - it was an event. In real life you won't know it's coming.

In a rapid depressurization at 40,000 feet you have maybe 20 seconds of useful consciousness before you're off in la-la land. I can't stress this enough, when the flight attendant gives her little speech that everyone ignores and says to put your oxygen mask on before helping your kids - PUT YOUR MASK ON FIRST!

Maybe you didn't hear me. PUT YOUR MASK ON FIRST!!!! We get it. You're a wonderful parent. You love your kids. That's why we need to keep you in the game - so you can help your kids.

The problem we have up front is - we need to get down below 10,000 feet in a big hurry. However, if we think we just blew a big hole in the side of the plane, we have to be gentle with the plane and not make the damage any worse. That means we may not be able to come down as fast as we'd like. It's likely to be rather noisy and probably very cold in the back until we get down and slow down.

OK, that's enough horror theater for one day. This is a very unlikely occurrence and now that you know what to do you're better equipped in case it ever happened.

That's about it. It's a reliable system with plenty of backups, just the way we like it. Other than fiddling with the temperature we usually set it and forget it. And now you can impress your seatmates by knowing that little vent is called a "gasper".

Originally posted to Major Kong on Thu Jun 06, 2013 at 07:55 AM PDT.

Also republished by Aviation & Pilots, Kossack Air Force, and Central Ohio Kossacks.

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Comment Preferences

  •  Tip Jar (37+ / 0-)

    If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

    by Major Kong on Thu Jun 06, 2013 at 07:55:38 AM PDT

  •  I have flown a lot (a lot) and experienced... (5+ / 0-)
    Recommended by:
    MKinTN, blueyedace2, Bronx59, eyesoars, ER Doc

    ...cabin pressure loss only once flying between Tel Aviv and Athens on EL AL.

    The masks came down and the plane dove down below 10,000 feet.

    Everybody stayed cool and we enjoyed an interesting ride with a great view of the Mediterranean.

    Thanks for the info Major.

    Daily Kos an oasis of truth. Truth that leads to action.

    by Shockwave on Thu Jun 06, 2013 at 08:14:43 AM PDT

  •  So if the cabin depressurizes at (3+ / 0-)
    Recommended by:
    Shockwave, blueyedace2, ER Doc

    40,000 ft and you do not get a mask on, you pass out. But you are still breathing some oxygen in that thin air, so do you stay alive for a while? If you are in the restroom, say, are you likely going to wake up when the plane gets down to 10,000 (with a big headache), or are you never going to wake up?

    •  There's actually a mask in the bathroom (8+ / 0-)

      Just for that occasion.

      If you don't get the mask on you'll be delirious within about 20-30 seconds and pass out shortly after. You would eventually die if not returned to more breathable air.

      Not sure how long that would take but it's probably measured in minutes.

      Normally, once down in thicker air you would wake up at some point.

      I'm not an MD, but I would guess that if the brain were deprived of oxygen long enough there could be damage.

      When I was a pilot trainee in the Air Force they took us up to 42,000 feet in the altitude chamber and had us take our masks off to see what it felt like.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu Jun 06, 2013 at 09:40:58 AM PDT

      [ Parent ]

      •  The book "Mayday" (1+ / 0-)
        Recommended by:

        By Thomas Block and Nelson Demille features a supersonic airliner over the Pacific bound for Tokyo, when a Navy pilot in an incredible foul up mistakenly targets it, thinking it's a practice drone he's supposed to be intercepting - what else would be that high and supersonic, right?

        The airliner isn't destroyed, but with a gaping hole punched through the cabin, it decompresses. And here's the thing: above a certain altitude, those drop down masks aren't going to be enough. You need a pressurized space suit to deliver oxygen to you at a high enough pressure to get enough into your blood stream to stay conscious.

        The pilots barely have enough time to put the autopilot into a programmed descent before they and everyone else lose consciousness. By the time the plane levels itself off at an altitude where the air is breathable, everyone on board is either dead, or unconscious with brain damage and other injuries. Well, almost everyone.

        A handful of people (including one with some piloting skills) by chance happened to be in compartments that held enough pressure long enough that they only passed out, but weren't otherwise harmed. The rest of the tale is a struggle to get the plane back on the ground against cascading complications, including corporations hoping for a crash with no survivors to minimize publicity of the horrible details and lawsuits.

        It was first published in 1979, updated, and eventually adapted into a movie for TV in 2005. Thomas Block has a bit more here.

        Mind you, to the best of my knowledge airliners can not reach such extreme altitudes; pilots of the U2 do, as did the crews of SR-71 Blackbird and they had to wear full pressure suits. When Felix Baumgartner made his record breaking sky dive from over 24 miles up, he absolutely had to be wearing a full pressure suit.

        There's a lot more here about air pressure and altitude, and at the bottom of the link, an account of what happened to an SR-71 pilot who didn't have his visor sealed...

        One more detail. Back in the days when people smoked on planes, after a while the cabins were alleged to get just a bit more air tight - from all the tar, etc. in the smoke sealing up micro holes and gaps.

        "No special skill, no standard attitude, no technology, and no organization - no matter how valuable - can safely replace thought itself."

        by xaxnar on Fri Jun 07, 2013 at 04:29:30 PM PDT

        [ Parent ]

        •  ... (1+ / 0-)
          Recommended by:

          This is something covered in the chamber rides. Above about 40,000', oxygen masks (NOT the little masks that drop down for the pax) switch from demand regulation to 'pressure breathing'. In this mode, masks force air, under pressure, into your lungs, and the wearer must forcibly exhale.

          This only works up to 45,000-48,000' or so, and a full pressure suit is required above that.

          The concern above is the major reason that the SST has such small windows -- the pressurization system must be able to cope with a blown out window enough to allow the passengers to live while the SST dives from 60,000' (where it cruises) to the 30,000' or so where people can function with the non-military-style O2 masks. (IIRC, it can maintain pressurization with one window blown out.)

          I presume the SST has proper masks for the pilots, but there is still considerable question in my mind regarding the consequences of a cockpit depressurization incident -- it isn't immediately obvious how the pilots could be kept conscious for the time required to descend 20,000' or more.

  •  Another good read in this series (4+ / 0-)


    "Please proceed, Governor" -- you know who, and when

    by lotac on Thu Jun 06, 2013 at 08:23:07 AM PDT

  •  This diary's a breathe of fresh air (5+ / 0-)


    What we call god is merely a living creature with superior technology & understanding. If their fragile egos demand prayer, they lose that superiority.

    by agnostic on Thu Jun 06, 2013 at 08:40:16 AM PDT

  •  Can our eardrums really be damaged when landing (2+ / 0-)
    Recommended by:
    blueyedace2, ER Doc

    if we have a cold or nasal congestion?

    Sometimes I've had pretty intense pain, but I don't know how close I've come to being permanently injured.

    •  Oh yes (5+ / 0-)
      Recommended by:
      VTCC73, rbaillie, blueyedace2, eyesoars, ER Doc

      I was flying combat missions with pneumonia because I didn't want anyone to think I was trying to get out of it (yeah, stupid, I know).

      I got a terrible sinus/ear block, over bad-guy territory no less. When we landed I was certain my left eardrum was going to burst. The pressure got so bad I actually lost hearing in that ear for a time.

      I finally managed to get my ear to clear - all I could think about was how bad it was going to hurt when that eardrum went.

      Your sinuses can also be damaged. Long story short - don't fly with a bad cold.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu Jun 06, 2013 at 09:44:41 AM PDT

      [ Parent ]

      •  Actually it would feel a lot better after (2+ / 0-)
        Recommended by:
        eyesoars, RiveroftheWest

        the eardrum went... That takes the pressure off. The drum usually heals ok. Doesn't help with sinus pressure though, if one of those is plugged. Putting a hole in the drum to take the pressure off was one of the ways doctors used to treat ear infections in the pre-antibiotic days. One of the instruments that the old-time docs used to carry in their "doctor bags" was a cute, skinny little probe with a sharp blade on the end, called a "myringotomy knife." (Myringes = eardrums, & otomy = make a hole.) ENT docs use modern versions with disposable blades when they put tubes in kids' ears.

        -7.25, -6.26

        We are men of action; lies do not become us.

        by ER Doc on Thu Jun 06, 2013 at 09:04:54 PM PDT

        [ Parent ]

      •  If you still have doubts, check with an AF ENT doc (1+ / 0-)
        Recommended by:

        when I was enlisted, back in the 1970s, if you busted an eardrum in flight, that was not a service-connected disability (you were supposed to tell the FS if you had a cold, allergies, etc. -- it's literally a "no-go" situation for a refueling boomer or loadmaster).

        LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

        by BlackSheep1 on Fri Jun 07, 2013 at 05:14:08 PM PDT

        [ Parent ]

  •  Is the cargo compartment pressurized and heated? (3+ / 0-)
    Recommended by:
    grover, blueyedace2, ER Doc

    And why do airlines kill so many pets?

    •  Good question (5+ / 0-)
      Recommended by:
      grover, rbaillie, blueyedace2, eyesoars, ER Doc

      We have the same problem in the freight world when flying live animals (mostly birds).

      I think it sometimes happens when they leave the critters sitting out on a very hot or very cold ramp while waiting to load them.

      Plus the cargo compartment is normally only heated/cooled when the engines or APU are running. They could be sitting in there baking while the plane is sitting on the ramp.

      On the 727 there was a switch that routed heat to the forward cargo compartment - where the critters normally rode. That's the infamous "dead dog" switch. Forget to turn it on and it's going to get very cold at altitude.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu Jun 06, 2013 at 09:48:45 AM PDT

      [ Parent ]

    •  They are, (4+ / 0-)
      Recommended by:
      rbaillie, blueyedace2, Tinfoil Hat, ER Doc

      But no little doggie masks drop in the unlikely event of loss of cabin pressure.

      I've flown my dog (when he was a puppy)  once because he had to travel almost 3000 miles, and I didn't send him unaccompanied. I flew there, picked him up, took him to the airport. The flight was delayed, took him to a hotel for several hours, then took a taxi back to the airport and we checked back in.

      I have few doubts that the airline staff would have treated him with care. But they are busy with other tasks. My task was to get him on my flight, ensure the crew double checked that he was loaded, and then if anything changed, I could be his voice.

      But normally, we'll drive as much as 1500+ miles to avoid shipping the dogs.

      I really respect airlines in whole. They move millions of passengers safely every single day, including getting  (almost) all their baggage to their destinations on time with them.

      But transporting live animals isn't their main business. Some airlines do it better than others. But it's always going to be their sideline.

      So I don't risk it unless I absolutely have to, and then, when I do, I'm very careful.  I have a lot of dog industry friends who ship dogs. Following airline rules and recommendations exactly helps a lot because these rules are set up to avoid injury and illness. Shipping midday in summer or in the middle if the night in winter are bad ideas even if the airline may permit it, for example.

      © grover

      So if you get hit by a bus tonight, would you be satisfied with how you spent today, your last day on earth? Live like tomorrow is never guaranteed, because it's not. -- Me.

      by grover on Thu Jun 06, 2013 at 10:34:34 AM PDT

      [ Parent ]

  •  this raises a curious question... (2+ / 0-)
    Recommended by:
    northsylvania, ER Doc

    that Helios Airlines jet that crashed a few years back apparently decompressed and everyone passed out. Did no one wake back up when the jet descended below 10,000 feet? Would someone wake back up?

    •  Depends on how long. (2+ / 0-)
      Recommended by:
      grover, ER Doc

      Brains can't go long without sufficient oxygen.
      Looking up the Wikipedia article on the crash you mentioned, it seemed that masks deployed and the passengers were awake but the crew were not.

      You..ought to be out raising hell. This is the fighting age. Put on your fighting clothes. -Mother Jones

      by northsylvania on Thu Jun 06, 2013 at 09:45:57 AM PDT

      [ Parent ]

    •  Not enough time (3+ / 0-)
      Recommended by:
      grover, Bronx59, ER Doc

      They were up at 34,000 feet in a holding pattern.

      They came down through 10,000 with both engines flamed out just before hitting a mountain.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu Jun 06, 2013 at 09:51:32 AM PDT

      [ Parent ]

    •  Depends... (3+ / 0-)
      Recommended by:
      ER Doc, terrypinder, lazybum

      I also took the 'chamber ride' that Major Kong mentioned, back in 1992. We also got the 'explosive decompression', in our case to 20,000' (we were told the military folks got it to 30,000'). Instant pea-soup fog.

      Along with that, they gave us a bunch of medical information and played some tapes of presssurization problems, including one of an F-4 augering in after the pilot took off his oxygen mask. (It was uncomfortable, you see, because his cabin pressure had failed, and at 40,000' the mask required "positive pressure breathing". The pilot had not made the connection that it was uncomfortable because it was working correctly.)

      But the F-4 flew in big circles all the way down, with his wingman yelling at him to wake up. He didn't.

      FWIW, my basic ground school instructor flew jets out of Las Vegas for years. Lots of rowdy and often difficult passengers. It made the flight crews' jobs much more pleasant if they took the cabin pressure up above 10,000' for a little while before bringing it back down to a more normal 6-7,000'. The high cabin altitude put the drunks to sleep within a few minutes, and then the more normal altitude let everyone else function while the drunks slept.

  •  Another great diary. (1+ / 0-)
    Recommended by:
    ER Doc

    One thing that I learned years ago that really surprised me is how the oxygen in the passenger masks (but not crew) results from chemical reaction. I guess I never really thought about it, that you can't have an aircraft with sufficient O2 for every passenger in it.

    Pressurization, oxygen and airflow could probably be their own diary series...

    © grover

    So if you get hit by a bus tonight, would you be satisfied with how you spent today, your last day on earth? Live like tomorrow is never guaranteed, because it's not. -- Me.

    by grover on Thu Jun 06, 2013 at 10:42:29 AM PDT

    •  I learned that by (1+ / 0-)
      Recommended by:
      ER Doc

      reading about what happened to that ValuJet plane.

      (I realize Air Tran is a different company and safety is way better but I just can't bring myself to fly with that airline as it's the successor to ValuJet...)

    •  ??? (2+ / 0-)
      Recommended by:
      ER Doc, RiveroftheWest

      I believe it is usually generated by an oxygen generator, but there's not any reason I'm aware of that it can't be done with an oxygen tank. (Since I've used those many times in private aircraft.)

      On point, the flight crew's oxygen masks are generally supplied from an oxygen tank.

      However, the oxygen generators are lighter than an equivalent high-pressure oxygen system, and probably rather safer and cheaper. High pressure oxygen tanks need to be checked and tested regularly, where oxygen generators would simply be replaced periodically.

      The oxygen generators can only generate oxygen for 10-20 minutes.

  •  thanks again (2+ / 0-)
    Recommended by:
    eyesoars, ER Doc

    Your articles are unfailingly interesting, informative and entertaining.
    All the best.

  •  This is a little grim (1+ / 0-)
    Recommended by:
    ER Doc

    In a hijacking, could you depressurize the cabin so that the hijackers could either go back to their seats for oxygen or else pass out?

    Freedom isn't free. Patriots pay taxes.

    by Dogs are fuzzy on Thu Jun 06, 2013 at 11:14:52 AM PDT

    •  You might (3+ / 0-)
      Recommended by:
      eyesoars, LeftyAce, ER Doc

      Although the FAA doesn't recommend it.

      The philosophy today is to protect the cockpit at all costs and get the plane on the ground as soon as possible.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu Jun 06, 2013 at 11:54:53 AM PDT

      [ Parent ]

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