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Discussion Starter #1
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?
 

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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.
 
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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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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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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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Discussion Starter #8
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) ?
 

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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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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.
 

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More likely the 'cabin altitude' is around 6000'.
Last few times I flew commercial I brought my Suunto along and never saw any higher than like 6.5k or something. I think the average was right around 6k. A pretty comfy ride! :-!
 

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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
I haven't looked at the pertinent references in years, but military helicopters operate at altitudes which require pressurised O2 delivery. IIRC, it's 12,000 feet but I'd have to check to be sure. I've flown at 14,000+ feet in South and Central America and we used an add-on pressurised O2 system which was similar to the BIBs used in hypobaric/hyperbaric chambers.
 

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I haven't looked at the pertinent references in years, but military helicopters operate at altitudes which require pressurised O2 delivery. IIRC, it's 12,000 feet but I'd have to check to be sure. I've flown at 14,000+ feet in South and Central America and we used an add-on pressurised O2 system which was similar to the BIBs used in hypobaric/hyperbaric chambers.
Requiring oxygen and requiring a pressurized cabin are two very different things. For an aircraft with an unpressurized cabin AR 95-1 states:

For aircraft crew, on flights above 10,000 feet pressure altitude for more than 1 hour and on flights above 12,000 feet pressure altitude for more than 30 minutes, oxygen will be used.

For aircraft crews and all other occupants, on flights above 14,000 feet pressure altitude for any period of time, oxygen must be used.

For flights above 18,000 feet pressure altitude, oxygen pre-breathing using 100% for not less than 30 minutes at ground level and will continue while on route to altitude.

Aircraft with Pressurization (AR 95-1):

In flight, cabin pressure altitude will be maintained at or below 10,000 feet.
( Note: Most commercial aircraft will pressurize to an altitude of 7,000 to 8,000 feet.)

As a minimum, a 10-minute emergency supply of oxygen will be available to all occupants when the aircraft is above 14,000 feet pressure altitude.

Above 25,000 feet pressure altitude, oxygen masks will be connected and readily available.

If pressurization is lost in flight above 14,000 feet pressure altitude, descent will be made immediately to a cabin pressure altitude of 10,000 feet or below.

Beginning at 50,000 feet, 100% oxygen supplied under pressure will not protect from hypoxia; a sealed, pressurized cabin or pressure suits required. When at 63,500 feet, boiling of body fluids will occur when the total barometric pressure is less than the vapor pressure of water at 98.6 F (body temperature).
 

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Discussion Starter #14
Very interesting, and the scenario of boiling body fluids is positively scary!

May I ask what AR 95-1 is? A military flight manual?
 

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So the amount of emergency oxygen can vary? According to Discovery, the Helios flight that crashed in Grecce 2 years ago, the oxygen supply for the passengers ran out after 10-15 minutes...

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. We carry about 3000 liters of 100% oxygen, and 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.
 

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So the amount of emergency oxygen can vary? According to Discovery, the Helios flight that crashed in Grecce 2 years ago, the oxygen supply for the passengers ran out after 10-15 minutes...
FAA regulations:


Sec. 121.333 - Supplemental oxygen for emergency descent and for first aid; turbine engine powered airplanes with pressurized cabins.
(a) General. When operating a turbine engine powered airplane with a pressurized cabin, the certificate holder shall furnish oxygen and dispensing equipment to comply with paragraphs (b) through (e) of this section in the event of cabin pressurization failure.
(b) Crewmembers. When operating at flight altitudes above 10,000 feet, the certificate holder shall supply enough oxygen to comply with §121.329, but not less than a two-hour supply for each flight crewmember on flight deck duty. The required two hours supply is that quantity of oxygen necessary for a constant rate of descent from the airplane's maximum certificated operating altitude to 10,000 feet in ten minutes and followed by 110 minutes at 10,000 feet. The oxygen required in the event of cabin pressurization failure by §121.337 may be included in determining the supply required for flight crewmembers on flight deck duty.
(c) Use of oxygen masks by flight crewmembers. (1) When operating at flight altitudes above flight level 250, each flight crewmember on flight deck duty must be provided with an oxygen mask so designed that it can be rapidly placed on his face from its ready position, properly secured, sealed, and supplying oxygen upon demand; and so designed that after being placed on the face it does not prevent immediate communication between the flight crewmember and other crewmembers over the airplane intercommunication system. When it is not being used at flight altitudes above flight level 250, the oxygen mask must be kept in condition for ready use and located so as to be within the immediate reach of the flight crewmember while at his duty station.
(2) When operating at flight altitudes above flight level 250, one pilot at the controls of the airplane shall at all times wear and use an oxygen mask secured, sealed, and supplying oxygen, in accordance with the following:
(i) The one pilot need not wear and use an oxygen mask at or below the following flight levels if each flight crewmember on flight deck duty has a quick-donning type of oxygen mask that the certificate holder has shown can be placed on the face from its ready position, properly secured, sealed, and supplying oxygen upon demand, with one hand and within five seconds:
(A) For airplanes having a passenger seat configuration of more than 30 seats, excluding any required crewmember seat, or a payload capacity of more than 7,500 pounds, at or below flight level 410.
(B) For airplanes having a passenger seat configuration of less than 31 seats, excluding any required crewmember seat, and a payload capacity of 7,500 pounds or less, at or below flight level 350.
(ii) Whenever a quick-donning type of oxygen mask is to be used under this section, the certificate holder shall also show that the mask can be put on without disturbing eye glasses and without delaying the flight crewmember from proceeding with his assigned emergency duties. The oxygen mask after being put on must not prevent immediate communication between the flight crewmember and other crewmembers over the airplane intercommunication system.
(3) Notwithstanding paragraph (c)(2) of this section, if for any reason at any time it is necessary for one pilot to leave his station at the controls of the airplane when operating at flight altitudes above flight level 250, the remaining pilot at the controls shall put on and use his oxygen mask until the other pilot has returned to his duty station.
(4) Before the takeoff of a flight, each flight crewmember shall personally preflight his oxygen equipment to insure that the oxygen mask is functioning, fitted properly, and connected to appropriate supply terminals, and that the oxygen supply and pressure are adequate for use.
(d) Use of portable oxygen equipment by cabin attendants. Each attendant shall, during flight above flight level 250 flight altitude, carry portable oxygen equipment with at least a 15-minute supply of oxygen unless it is shown that enough portable oxygen units with masks or spare outlets and masks are distributed throughout the cabin to insure immediate availability of oxygen to each cabin attendant, regardless of his location at the time of cabin depressurization.
(e) Passenger cabin occupants. When the airplane is operating at flight altitudes above 10,000 feet, the following supply of oxygen must be provided for the use of passenger cabin occupants:
(1) When an airplane certificated to operate at flight altitudes up to and including flight level 250, can at any point along the route to be flown, descend safely to a flight altitude of 14,000 feet or less within four minutes, oxygen must be available at the rate prescribed by this part for a 30-minute period for at least 10 percent of the passenger cabin occupants.
(2) When an airplane is operated at flight altitudes up to and including flight level 250 and cannot descend safely to a flight altitude of 14,000 feet within four minutes, or when an airplane is operated at flight altitudes above flight level 250, oxygen must be available at the rate prescribed by this part for not less than 10 percent of the passenger cabin occupants for the entire flight after cabin depressurization, at cabin pressure altitudes above 10,000 feet up to and including 14,000 feet and, as applicable, to allow compliance with §121.329(c) (2) and (3), except that there must be not less than a 10-minute supply for the passenger cabin occupants.
(3) For first-aid treatment of occupants who for physiological reasons might require undiluted oxygen following descent from cabin pressure altitudes above flight level 250, a supply of oxygen in accordance with the requirements of §25.1443(d) must be provided for two percent of the occupants for the entire flight after cabin depressurization at cabin pressure altitudes above 8,000 feet, but in no case to less than one person. An appropriate number of acceptable dispensing units, but in no case less than two, must be provided, with a means for the cabin attendants to use this supply. (f) Passenger briefing. Before flight is conducted above flight level 250, a crewmember shall instruct the passengers on the necessity of using oxygen in the event of cabin depressurization and shall point out to them the location and demonstrate the use of the oxygen-dispensing equipment.
 

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AR 95-1 = ARMY Regulation "Flight Rules and Regulations"

It is our primary regulatory source.

I think the Navy calls theirs NAVTOPS
 

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AR 95-1 = ARMY Regulation "Flight Rules and Regulations"

It is our primary regulatory source.

I think the Navy calls theirs NAVTOPS
NATOPS - Naval Air Training and Operating Procedures Standardization (Program)

(Or, Not Applicable To Our Present Situation.)

NATOPS is a collection of OPNAVINST (Office of the Chief of Naval Operations Instructions) and NAVAIR (Naval Air Systems Command Manual) publications that covers more than the Army's 95-1, NATOPS is not only general flight instructions and rules, it also includes aircraft specific flight manuals (what the Army has in the -10) and clecklists (the -CL), as well as special aircraft related operations.
 

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A bit off topic but just finished the book
Masters of the Air
Donald L. Miller
Fascinating read about the American bomber crews of WW2 over Europe, talk about pressure issues, how about flying in a B17 full of flak and holes.
Not to mention the temperature extremes at that altitude, amazing what the human condition can endure.
eds
 
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