Pressurization of cargo/transport aircraft and others
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  1. #1
    Moderator Emeritus Crusader's Avatar
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    Off topic Pressurization of cargo/transport aircraft and others

    I have recently been looking into the history of pressurization and was intrigued to learn that the effects of high altitde (quite apart from oxygen flow) can be quite severe.

    So these are probably stupid question to those closely involved with flying, but here they are anyway:

    - Are military transport planes pressurized (since they can transport paratroopers, litter patients etc.), and can they be depressurized in flight for e.g. high-altitude-low-opening jumps?

    - Are (civilian) cargo aircraft and the cargo holds of passenger airplanes pressurized or not?

    - Are helicopters pressurized at all?
    Cheers,

    Martin ("Crusader")

  2. #2
    Member lysanderxiii's Avatar
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    Re: Pressurization of cargo/transport aircraft and others

    All cargo aircraft, military and civilian, are pressurized, at least in the passenger/cargo compartment and flight deck.

    Pressurization is accomplished by taking bleed air (around 1000F and 70-80 psi) from the compressor stages of the jet engine(s)* running it through a heat exchanger (primary, to cool it), compressing it again (which heats it up slightly), running it through another heat exchanger (secondary, to cool it again), expanding it through a turbine**, this extracts almost all the energy from the air, so it comes out nice and cold (it can be as low as -20 F. depending on the bleed air temp and the efficiency of the H/Xs) and at about 20 psi. This is mixed with bleed air straight from the primary heat exchanger (so called warm air, it runs about 150 F) to get the proper cabin temperature, and this then runs through a flow regulator to keep the pressure at the cabin outlets near the cabin pressure. Somewhere in the cabin there is the cabin pressure relief valve the opens to the outside and this maintains the cabin pressure at around a maximum of 7.5 to 8 psi above the outside pressure. So, as the aircraft descends, the cabin pressure relief valve open to depressurizes the cabin automatically. There is also a cabin safety valve that will open if the cabin pressure exceeds a maximum limit, this keeps the cabin from popping like a balloon.

    In your average Boeing 7X7 type aircraft the entire cylindrical portion of the fuselage is pressurized. The floor of the passenger compartment is not a pressure bulkhead.

    Helicopters run out of ceiling before they get to where they need pressurization, most helicopters stay below 10,000 feet. Pressurization is useful at 20,000 feet but not required (B-17s in WW2 flew at 20-25,000 feet and were unpressurized), I know of no helicopter that can operate above 18 to 19,000 feet. For example, the CH-47 has a service ceiling of 18,000 feet. The V-22 does crack new ground and probably should have had a pressurized cabin, as it can operate around 25,000 feet

    At 25K 100% O2 is required, higher altitudes will require positive pressure masks (forcing air into the lungs to maintain required oxygen saturation), and above 40k you have to be in a pressurized cabin or pressure suit.

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    *Older piston-engined aircraft used bleed from the super-chargers. Some turbo-props use engine-driven compressors or bleed-air depending on the available engine bleed, and size of the cabin.

    ** the work gained by the expansion of the air over the turbine is used by the compressor earlier in the sequence. This Compressor/Turbine unit, the two heat exchangers and the collection of valves are collectively referred to as the ECS PACKS in civilian and cargo aircraft.
    Last edited by lysanderxiii; February 5th, 2008 at 22:33.
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  3. #3
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    Re: Pressurization of cargo/transport aircraft and others

    What he said....

    Pretty much nailed everything. There is a French helo that can get on up there (around 30k if I remember right) but the crew use supplemental ox to get up there.

    In the helo world, and fix wing for that matter too, we have rules on how high you can be for how long etc. Really don't start to see restrictions until around 13.5k, on up to 15k where ox is required. However, getting above much more than 12k is pushing it as far as the chopper goes.

    hypoxia is an amazing thing, that's why we "ride" the alt chamber and get all jacked up at 35k, so that if we start to notice symptoms, we correct the issue.
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    Re: Pressurization of cargo/transport aircraft and others

    Yup, darn near 30k. Incredible!

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    Member lysanderxiii's Avatar
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    Re: Pressurization of cargo/transport aircraft and others

    The standard AS350 B3 has a service ceiling* of 20,000 feet at 3500 lbs, at least according to Eurocopter's sales literature.

    Although,
    Aérospatiale (now part of Eurocopter) is known for very good high altitude performance. The Alouette II was renown for its hot-high performance, the French said it performed better in the highlands of Algeria during the war than any other helicopter.

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    * Service ceiling is defined as the altitude at which the maximum rate of climb is 100 fpm, obviously, the lighter you can get the aircraft, the higher the service ceiling, also the denser the air, (ie colder,) the higher the service ceiling. The absolute ceiling is the point at which the maximum rate of climb is zero. For a helicopter, the worst conditions are high ambient temperature and high altitude, in these conditions the rotors generate the least lift.

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    Re: Pressurization of cargo/transport aircraft and others

    The world's highest rescue was performed at about 22,000 feet by in a Nepalese Colonel if my memory serves me. He landed and picked up the victim after stair stepping up the mountain refueling at different logistical points on the mountain. He operated with as little as 5 minutes of fuel. Those guys are pure mountain goat!!

    I have executed numerous rescues above 10,000, many of which were in out of ground effect hovers, in a Blackhawk. The highest OGE hoist rescue that I did was on Mt Shasta at 13,500 feet with about a 4% power available margin. It was on the back side of Shasta on the glacier.
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    Re: Pressurization of cargo/transport aircraft and others

    This should go over to the AOPA forums. They could use a breath of fresh air at times.
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  9. #8
    Moderator Emeritus Crusader's Avatar
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    Re: Pressurization of cargo/transport aircraft and others

    Thank you all for your replies!

    So am I right to assume that before a HALO parachute drop is initiated from a military aircraft, the entire cargo compartment of a military transport aircraft is depressurized, and those not jumping will have to be on oxygen (assuming that the jumpers are on oxygen for the jump anyway) ?
    Cheers,

    Martin ("Crusader")

  10. #9
    Member MaxStatic's Avatar
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    Re: Pressurization of cargo/transport aircraft and others

    Quote Originally Posted by Crusader View Post
    Thank you all for your replies!

    So am I right to assume that before a HALO parachute drop is initiated from a military aircraft, the entire cargo compartment of a military transport aircraft is de-pressurized, and those not jumping will have to be on oxygen (assuming that the jumpers are on oxygen for the jump anyway) ?

    Yes, that is correct. See more HAHO these days vs HALO...different discussion.

    Another example is the AC-130. Un-pressurized in the back so everyone has to be on O2.

    Yet another would be the mighty, now retired, T-37 tweet. Of course both the IP and Stud wear O2 masks. Heck even some of the pressurized aircraft you have to wear mask. For instance the replacement of the T-37 tweet, the T-6 Texan II, works like most fighter aircraft in that it is pressurized on a gradient scale i.e. it maintains a px differential with the outside atmosphere. So without going into detail, let's say you are like 6kft, it will maintain cockpit px at like 4k or something. So if you are at 25k, inside px would be like 17k or something. Make sense? Obviously needing to be a little vague on the numbers but you get the idea.

    High alt really isn't that bad, especially if you are just sitting in a chair wiggling a stick. That's why pilots do the hypoxia training because it can have an insidious on-set if you don't know what to look for. If you aren't under any real physical strain, you might not notice it, especially in the realm of 16k~22k.

    Since I've been flying chops though I haven't seen altitudes any greater than like 12 or 13k. Google "high altitude tests" and read up on some of the crazy stuff that has been done. Very eye opening.
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  11. #10
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    Re: Pressurization of cargo/transport aircraft and others

    A few more remarks that may be interesting to the passenger.

    A modern airliner has enough pressure differential (up to about 9 psi) to keep the cabin altitude at a max. of about 8000 foot, even at altitudes of 40000'+. More likely the 'cabin altitude' is around 6000'. On that altitude you will be out of breath a bit sooner, but since we don't want you running around anyhow, this should not be a problem . As you may have noticed, the effects of alcohol are more pronounced, and the taste of your food is less (honestly!!). It is generally accepted that people will suffer no ill effects of staying at 10.000' for a longer period of time. That's why we consider 10.000' the max altitude we can fly with an unpressurized aircraft. We will never start a flight without pressurization, but due to system failure, or a leaking hull, an aircraft may become unpressurized. In 26 years of commercial flying, I've never had that happen, so it's pretty rare. However we do need to take that possibility in to account, and that's why we practice the 'rapid descent'. Should the cabin pressure drop, (I guess you guys always pay attention to the safety demonstration so you already know this ) an oxygen mask will be presented to you, and we will start the rapid descent, to get down to 10.000' as quickly as possible. In area's where that's not possible (Himalaya's, Greenland, Andes and several others) you may need to keep the mask on for half an hour or longer. For every route we fly, we have sufficient oxygen for the crew and passengers, to leave the area at a safe altitude, and descend to 10.000' once clear of terrain. You need only a little oxygen mixed with cabin air to make 'breathable air'. The system automatically adds the right amount of oxygen, more at higher altitudes, less at lower.
    Last edited by Ron Engels; February 14th, 2008 at 15:54.
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