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Fighter Jet Pressurization And Oxygen Mask

Wed Nov 09, 2005 5:41 pm

How is the aircraft pressurized and what kind oxygen content is forced through the mask. I have heard 100% oxygen, but that would cause the pilot to hyperventilate wouldn't it? How does that work?
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RE: Fighter Jet Pressurization And Oxygen Mask

Thu Nov 10, 2005 9:10 am

I don't think most fighter jets are pressurized, hence the reason for the mask. I'm not sure what the % of O2 goes through the mask but I do believe its constant pressure.
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RE: Fighter Jet Pressurization And Oxygen Mask

Thu Nov 10, 2005 9:43 am

All fighter jets are pressurized. You can't really survive at 35K without it.

I can speak for the F/A-18. The A & B models and early lot C & D's have a LOX (liquid oxygen) system which is 100% pure oxygen. Newer models have a system called OBOGS (On Board Oxygen Generation System). It takes bleed air from the engine and through a minor miracle, oxygen is born. Its not 100% O2, but its close enough. It has some drawbacks though, but that's for a later discussion.
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RE: Fighter Jet Pressurization And Oxygen Mask

Thu Nov 10, 2005 1:54 pm

Quoting Ftrguy (Reply 2):
The A & B models and early lot C & D's have a LOX (liquid oxygen) system which is 100% pure oxygen.

As did most Navy Tacair jets from years on back. Oftentimes the system would go haywire, causing the REDAR oxygen hose (series of hoses that go from the ejection seat to the pilot's mask) to freeze up with liquid O2...made for a cold left-arse cheek apparently. Happened to my dad once or twice, that NATOPS quick-ref booklet was a good shield!

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RE: Fighter Jet Pressurization And Oxygen Mask

Thu Nov 10, 2005 9:26 pm

Extracted from (For a "condensed version", just read items 12-14.)

Oxygen Pressure Systems
7. High Pressure Gaseous (Slide 7)
- In this system, oxygen is stored under extremely high pressure in cylinders painted green for identification. The cylinders must be heavily constructed to contain the high pressure. Reinforced cylinders reduce the danger of explosion, but increase weight and reduce the performance capabilities of the aircraft. The T-1A, T-43A, KC-10, C-12, C-20, C-21, C-25 and C-130 A,B all use a high pressure gaseous system. The emergency oxygen cylinder found in the parachute pack or on ejection seats is a modified example of this system (with a pressure range of 1,800 psi to 2,200 psi).
a. Full---1800 psi to 2000 psi
b. Operational Empty---200 psi
c. Maintenance Empty---100 psi
d. Color Code---Green
8. Low Pressure Gaseous (Slide 8)
- This oxygen is stored in lightweight yellow cylinders. A low pressure gaseous system reduces the possibility of explosions, requires very little maintenance, and limits the volume of oxygen. This limited volume dictates immediate descent to altitudes not requiring supplemental oxygen any time the pressure drops below 100 psi (operationally empty). When a system drops below 50 psi, it must be refilled within two hours to prevent contamination or water formation. If this requirement is not met, the system must be purged (cleaned) to rid the system of the contaminants and water vapor. An example of an aircraft that uses this type of storage system is the T-37 Primary Jet Trainer. These systems are also used in some multiplace aircraft.
a. Full--- 400 psi to 450 psi
b. Operational Empty---100 psi
c. Maintenance Empty---50 psi
d. Color Code---Yellow
e. Purge
f. T-37, MA-1 portable assembly
9. Liquid Oxygen (LOX) (Slide 9, 10, 11) (Slide 10 OPTIONAL)
- The oxygen is measured in liters by a quantity gauge. Both the pressure and quantity gauge should be checked during preflight inspection of the oxygen system if LOX is being used. The amount of liquid oxygen required to be in the converter subsequent to flight varies depending of aircraft, mission, and number of personnel on board. For example, the T-38 Talon uses a 10 liter, low pressure LOX system as the primary source of oxygen, and the converter is considered full at 9.5 liters and empty at 1 liter. The LOX system is also used in the C-141, AC/MC-130, C-17, B-52, E-3, E-4, F-117, F-15, F-16, and several other aircraft.
a. Advantages
(1) One unit of liquid oxygen yields about 860 units of gaseous oxygen
(2) Storing the oxygen in a liquid state saves about 70 percent in space and weight
b. Disadvantages
- Because of the extremely low temperature of LOX (a boiling point of -182.8°C), handling and servicing the system are its only disadvantages.
c. Low pressure LOX systems - Single/Duel - Place Aircraft---70 psi
d. High pressure LOX systems - Multi-place aircraft, normally maintain a line pressure of approximately 300 psi.
10. Solid State (Slides 12)
a. Produce oxygen from a solid state. Found on C-5 military airframes.
b. It provides 30 minutes of emergency 100 percent oxygen to passengers in aircraft via a chemical reaction.
c. System is contained in a canister and is activated by removing a continuous flow mask from the canister or by pulling an activation ring.
d. The user may detect a harmless amount of chlorine for about the first 10-15 seconds.
e. The amount of oxygen supplied depends on the size and chemical reaction rate of the candle.
11. OBOGS/MSOGS (Slides 13 and 14)
- The OBOGS concept of producing oxygen in-flight provides the potential for increased operational safety and reduced logistical support of the system. The Molecular Sieve Oxygen Generation System (MSOGS), a type of OBOGS filters oxygen from engine bleed air and stores it in a small container before delivery to the crew member. The container is constantly being replenished with oxygen. Basically, as long as the aircraft has operating engines, oxygen is provided. Failure of the engine or electrical power will result cessation of the primary oxygen supply to the crew. A back-up oxygen system (BOS) may be activated automatically or manually when necessary.
a. On Board Oxygen Generation System (OBOGS)
b. Molecular Sieve Oxygen Generation System (MSOGS)
c. No Storage Required
d. Various Pressures
e. B-1B / F-15E
12. Oxygen Delivery Systems (Slide 15)
- Regardless of the basic configuration, all oxygen delivery systems serve the same purposes. Oxygen progresses from storage containers through distribution lines, regulated at a suitable flow rate or pressure, and is delivered to the mask. The two types are continuous flow and pressure demand.
13. Continuous Flow System (Slides 16, 17, 18)
- Some continuous flow systems are modified for use at higher altitudes and are normally installed in transport, medical evacuation and commercial aircraft. The emergency oxygen cylinder in the parachute pack or the ejection seat is another modified continuous flow system rated to provide emergency use up to FL500, when used with a pressure demand capable mask.
a. Regulator continuously delivers oxygen to you, whether you are inhaling or not.
b. System wastes oxygen.
c. Poor oxygen mask fit.
d. The operational ceiling for this system is FL250 and, in emergencies, it can be used to FL300.
14. Pressure Demand System (Slide 19)
- At FL430, 100 percent oxygen alone is insufficient to maintain normal levels of blood oxygen saturation. Positive pressure breathing is necessary to increase the lung pressure above the atmospheric pressure. This increase is mandatory for survival at altitudes above FL400. The pressure demand oxygen system has been developed to protect you from hypoxia at the higher altitudes.
a. Most widely used automatic pressure demand regulators in modern aircraft.
b. Operational ceiling of FL430 and an emergency ceiling of FL500 for very short durations.
15. Pressure Demand Regulators (Slide 20)
a. Altitude Chamber
b. Aircraft
16. Pressure Demand Regulator (Slide 21, 22, 23, 24)
(All four different types of narrow panel regulators are shown for all configurations)
a. Pressure gauge - is connected directly to the oxygen storage system and calibrated in pounds per square inch pressure. You must know your aircraft oxygen storage system to determine the correct pressure reading of this gauge. This gauge should be monitored periodically during flight and any rapid or excessive drop in pressure should be considered cause for immediate descent.
NOTE--Low pressure gaseous storage systems will generally reflect a constant drop in pressure during ascent as a result of the atmospheric temperature decrease. (Charles’ Law)
b. Oxygen supply lever - is the green lever on the right side of the regulator. In the up (ON) position, oxygen is permitted to enter the regulator from the storage supply. In the down (OFF) position, oxygen will not enter the regulator. Some narrow panel regulators do not contain an OFF WARNING feature, which means you can still breath ambient air through the regulator with the supply lever turned off and the diluter lever in the NORMAL OXYGEN position.
c. Diluter lever - is the white lever in the middle of the regulator. It’s a two-position lever used for selecting normal or 100 percent oxygen. During most flight conditions it is placed in the down (NORMAL OXYGEN) position to permit the regulator diluter mechanisms to function automatically. When cabin altitude increases, more 100 percent oxygen is added to the mixture until FL320 is reached. Then the ambient air port closes automatically and only 100 percent oxygen is delivered to you. The lever should be placed in the up (100% OXYGEN) position below FL320 for situations requiring 100 percent oxygen. When 100 percent oxygen is no longer required, the lever should be returned to the NORMAL OXYGEN position. When the regulator is not in use, place the diluter lever in the 100% OXYGEN position to close the ambient air port and prevent the introduction of contaminants.
d. Emergency lever - is the red lever on the left side of the regulator. This three-positioned lever permits you to select oxygen under various pressures. Normal flight conditions call for the emergency lever to remain in the middle (NORMAL) position. The up (EMERGENCY) position delivers oxygen under pressure (3.5 inches of water pressure). If the lever is in the spring loaded down (TEST MASK) position, the regulator delivers 6-16 inches of water pressure (three to four times the pressure delivered by the EMERGENCY position).
e. Flow indicator - is a small window in the upper left-hand corner of the regulator. This indicator is a blinker-type instrument indicating white when a gas is flowing through the regulator. The window will be black when a flow is absent; as you inhale, the white indicator reappears. The indicator should be monitored frequently to ensure flow continues. This flow indicator will let you monitor three things (1) gasous flow, (2) rate and depth of breathing, and (3) mask seal.
NOTE -- Some regulators have prisms installed to present the flow indicator at eye level. This feature enables you to see the flow indicator without lowering your head.
f. Emergency Procedures - Placing the supply lever ON, the diluter lever to 100% OXYGEN and the emergency lever to EMERGENCY ensures you are receiving 100 percent oxygen under a slight safety pressure. This technique is referred to as gangloading the regulator. It’s used to check the integrity of the mask and equipment, during suspected hypoxia/hyperventilation incidents or during smoke and fumes in the cockpit.
g. Altitudes
- Positive pressure delivery
(1) FL280-FL320
(2) FL390-FL410
h. 100 percent O2 delivered by FL320
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RE: Fighter Jet Pressurization And Oxygen Mask

Thu Nov 10, 2005 10:56 pm

The F-4 would maintain a pressure differential, approx 5%, so as to preclude a rapid decompression. Air Force systems used an diluter system versus Navy that had 100% oxygen. It was not uncommon to fly a joint formation with Navy fighters and to see them with their masks hanging off to the side.

One of the biggest reasons to wear the mask is that is where the microphone is located.
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RE: Fighter Jet Pressurization And Oxygen Mask

Mon Nov 21, 2005 8:29 pm

In the good old days that oxygen mask with a good flow of 100% o2 was also the perfect way to get things straight again after a night out in the town.
Obviously it made for some interesting questions from crew chiefs why the safety wire on the Max o2 was broken. Even though they knew damn well why it was broken, they just went with it, as long as you brought back their baby in one piece.
Slightly off topic, but hey... just went down memory lane

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