Pressure Mask outlets High Low 70 ° • o Cylinder Charging
- Pressure gage connection Pressure Check reducing valve Charging valve valve Line valve Filter Filter Shutoff valve FIGURE 14-45. Continuous-fow oxygen system.
Depending on the type of equipment used, it is normally possible to select at least two rates of flow, normal or high. With some equipment three flow rate selections are possible, i.e., normal, high, and emergency, which would correspond to 2, 4, and 10 liters per minute. With these flow rates a 120-liter cylinder would last for 60, 30, and 12 min., respectively.
SMOKE PROTECTION EQUIPMENT In some instances there is a requirement to carry smoke protection equipment for use by a member of the crew in a smoke or fume-laden atmosphere.
Smoke protection equipment consists of a special smoke protection facial mask with eye protection in the form of a clear-vision visor, together with the necessary oxygen supply hose and head straps.
Some are designed for use with oxygen from the aircraft oxygen system, and others are self-contained portable equipment.
OXYGEN CYLINDERS The oxygen supply is contained in either highpressure or low-pressure oxygen cylinders. The high-pressure cylinders are manufactured from heat-treated alloy, or are wire wrapped on the outside surface, to provide resistance to shattering. All high-pressure cylinders are identified by their green color and have the words AVIATORS' BREATH- ING OXYGEN" stenciled lengthwise in white, 1-in.
letters.
High-pressure cylinders are manufactured in a variety of capacities and shapes. These cylinders can carry a maximum charge of 2,000 p.s.l., but are normally filled to a pressure of 1,800 to 1,850 p.s.i.
There are two basic types of low-pressure oxygen cylinders. One is made of stainless steel; the other, of heat-treated, low-alloy steel. Stainless steel cylinders are made nonshatterable by the addition of narrow stainless-steel bands that are seam-welded to the body of the cylinder. Low-alloy steel cylinders do not have the reinforcing bands but are subjected to a heat treatment process to make them nonshatterable. They have a smooth body with the word "NONSHATTERABLE" stenciled on them.
Both types of low-pressure cylinders come in different sizes and are painted light yellow. This color indicates that they are used for low-pressure oxygen only. The cylinders may carry a maximum charge of 450 p.s.i., but are normally filled to a pressure of 588
regulator Copilot's regulator Engineer's Pressure gage Shutoff valve Pressure gage Cylinder, Pressure reducing valve Pressure reducing valve Check valve Line valve Filter - Filter Pilot's Observer's regulator regulator Pressure „Charging valve Charging connection Low C FIcURE 14-46. Typical pressure-demand oxygen system.
from 400 to 425 p.s.i. When the pressure drops to 50 p.s.i., the cylinders are considered empty.
The cylinders may be equipped with either of two types of valves. One type used is a self-opening valve which is automatically opened when the selfopening valve coupling assembly attached to the oxygen tubing is connected to the valve outlet. This coupling unseats a check valve, allowing oxygen from the cylinder to fill the oxygen system under high pressure. the other type is a hand-wheel, manually operated valve. This valve should be safety wired in the "full on" position when the cylinder is installed in the aircraft. This valve should be closed when removing or replacing parts of the oxygen system and when the cylinder is to be removed from the aircraft. Cylinders are often provided with a disk designed to rupture if cylinder pressure rises to an unsafe value. The disk is usually fitted in the valve body and vents the cylinder contents to the outside of the aircraft in the event of a dangerous pressure rise.
SOLID STATE OXYGEN SYSTEMS Emergency supplemented oxygen is a necessity in any pressurized aircraft flying above 25,000 ft.
Chemical oxygen generators can be used to fulfill the new requirements. The chemical oxygen generator differs from the compressed oxygen cylinder and the liquid oxygen conveter in that the oxygen is actually produced at the time of delivery.
Solid-state oxygen generators have been in use for a number of years, dating back to 1920, when it was first used in mine rescues. During World War Il the Japanese, British and Americans, all worked to develop oxygen generators for aircratt and submarines.
In figure 14-47, 120 standard cubic feet of oxy.
gen (10 Ibs.) is shown schematically in the number of cubic inches of space it would occupy as a gas, a liquid or a solid. In figure 14-48, the necessary hardware to install and operate the system has been included in the size and weight measurements. A close comparison of these values makes it apparent that the solid state oxygen generator system is the most efficient space wise. Likewise less equipment and maintenance is required for solid state oxygen converters. Integrity inspection is the only requirement until actual use is implemented.
Solid state describes the chemical source, sodium chlorate, formula NaCIO,. When heated to 478° F., sodium chlorate releases up to 45% of its weight as gaseous oxygen. The necessary heat for de- 589
→
44.5 Ibs GAS DIA 2160 in
- 10"
10. Ibs LIQUID 1000 in* LIQUID 295°F
6.9~ CUBE SOLID SOLID · 400 in ¡" 28 lbs
SODIUM CHLORATE CANDLE DENSITY OF 22 GRM/CC 26 LB.
FIGURE 14-48. Weight and volume comparison-gas, liquid FIGURE 14-47. Volume comparison. and solid oxygen storage.
composition of the sodium chlorate is supplied by iron which is mixed with the chlorate.
The Oxygen Generator Figure 14-49 illustrates a schematic representation of a basic oxygen generator. The center axial one end of the core is larger than the other. The purpose of this is to program a high oxygen evolu.
tion rate during the initial few minutes of burning such as is required for an emergency descent supply. Burning continues until the core is expended.
position is occupied by a core of sodium chlorate, BREATHING TUBE CONNECTION iron, and some other ingredients mixed together HERMETIC SEAI.
and either pressed or cast into a cylindrical shape.
This item has been popularly referred to as an oxygen candle, because when it is ignited at one end, it burns progressively in much the same manner - HAXDGRENADE as a candle or flare. Surrounding the core is por- BOLCHON ous packing. It supports the core and filters salt - GLASS WOOL particles from the gas as it flows toward the outlet.
A chemical filter and particulate filter at the outlet IGNITION CONE end of the container provide final clean up of the gas so that the oxygen delivered is medically pure 010" COPPER breathing oxygen. An initiation device is an in- I/I6 TK. MICARTA tegral part of the package. This may be either a LOCK FINISH mechanical percussion device or an electric squib. The choice depends on the application. The entire GLASS CLOTH NSULATION GLASS WOOL assembly is housed in a thin shelled vessel. Often CANDLE included is a layer of thermal insulation on the inside shell; a check valve seal on the outlet, and a KC10, pressure condition. relief valve to protect against an inadvertent over- •GLASS WOOL HOPCALITE In operation, the burning is initiated at one end of the core by activating the squib or percussion device. Oxygen evolution rate is proportional to the cross sectional area of the core and the burn OXYGEN OUTLET rate. The burn rate is determined by the concentration of fuel in the chlorate. In certain cases, FiGURE 14-49. Apparatus for burning chlorate candles.
The simplicity of the process should be readily apparent; likewise, the limitations. There are no on/off valves and no mechanical controllers. Refill is accomplished by simply replacing, in total, the entire device. A limitation is that once the generator is initiated, flow is delivered at a predetermined rate, thus demand use is not very efficient.
In order to keep the process from consuming a great quantity of the oxygen, the quantity of iron is kept to a minimum. There is a tendency toward liberation of small amounts of chlorine. Barium peroxide, or barium dioxide, may be added by the manufacturer to provide an alkaline medium for removing the trace amounts of chlorine that may On a volume basis, which is extremely important in the aircraft installation, the storage capacity of oxygen in candles is about three times that of compressed gas.
A typical three outlet module for a 15 minute decompression emergency descent supply for a supersonic transport (25000 foot max. cabin altitude) weighs less than 0.9 pound and consists simply of a 2.1 inch diameter by 3.55 inch long stainless steel cylinder attached to three manifolded hose nipples. The cylinder contains the generator, initiator, salt, fume filter, enough insulation to keep the cylinder surface below 250° F. during burning, a pressure relief plug, and a temperature indicating paint spot for generator status visual inspection.
The nipples contain orifices just small enough to assure essentially equal flow to all three masks.
The generators are inert below 400° F. even under severe impact. While reaction temperature is high and considerable heat is produced, the generators are insulated so that the outer surface of the cylinder is cool enough to avoid any fire hazard. The portable units may be held comfortably throughout the entire operation, as the 'heat generated is dissipated steadily over a long period of time. The same insulation works in reverse to delay initiation should a unit be subjected to an external fire. If such a fire is sufficiently prolonged to ignite the chlorate generator, oxygen production will be at a relatively low and continuous rate. In the simple continuous flow systems no pressure would be generated as all outlets would permit unrestricted flow of the oxygen, eliminating the intense jet torch effect of pressurized oxygen
• Elimination of distribution and regulation components-saves weight and maintenance.
• Simplification of individual distribution manifolds and drop-out mechanisms by the use of modular chlorate candle units.
• Improved reliability, hence safety by design of initiation circuitry, such that, an individual malfunction would not make other units inoperative (comparison here would be to ruptured lines, or high leakage in gaseous distribution systems).
• Simple, visual surveillance of each unit for condition of chlorate candle within the sealed container, by use of inspection window.
• Simple replacement of any unit, should it show any sign of deterioration, by plug-in cartridge, by relatively unskilled services crew; easily checked for installation and readiness for functioning from flight deck.
• Programmed oxygen release rates irrespective of the type of emergency.
OXYGEN PLUMBING Tubing and fittings make up most of the oxygen system plumbing and connect the various components. All lines are metal except where flexibility is required. Where flexibility is needea, rubber hose is used.
There are several different sizes and types of oxygen tubing. The one most frequently used in low-pressure gaseous systems is made of aluminum alloy. Tubing made of this material resists corrosion and fatigue, is light in weight, and is easily formed. High-pressure gaseous supply lines are Installed oxygen tubing is usually identified with color-coded tape applied to each end of the tubing, and at specified intervals along its length. The tape coding consists of a green band overprinted with the words "BREATHING OXYGEN" and a black rectangular symbol overprinted on a white background.
Solid State vs. High Pressure Gaseous Oxygen
• Elimination of high pressure storage containers-saves weight.
Oxygen System Fittings Tube segments are interconnected or connected to system components by fittings. Tubing-to-tubing fit- 591
tings are designed with straight threads to receive Tube flared tube connections. Tubing-to-component (cylinder, regulator, and indicator) fittings have straight threads on the tubing end and external pipe threads (tapered) on the other end for attachment Sleeve to the component, as shown in figure 14-50.
Nut hex
• Tube Recessed tube seat Fitting hex
- Sleeve Double tube flare External pipe thread • Fitting nut • Nut hex Nipple FIcURE 14-51. A typical flareless fitting.
Fitting hex External pipe thread FIGURE 14-50. Sectional view of a typical oxygen system hitting.
Oxygen system fittings may be made of aluminum alloy, steel, or brass. These fittings may be either of two types, flared or flareless. A typical flared fitting is shown in figure 14-50. A flareless fitting is shown in figure 14-51.
The sleeve in a flareless fitting must be preset before final installation in a flareless seat. Presetting causes the cutting edge of the sleeve to grip the tube sufficiently to form a seal between the sleeve and the tubing. The end of the tubing bottoms on the seat of the flareless fitting to provide tube end support after installation.
To seal oxygen system tapered pipe thread connections and to prevent thread seizure, use only an approved hread compound. Never use a mixture containing oil, grease, or any other hydrocarbon on any fittings used in oxygen systems.
Replacement Lines The same cutting and bending methods described in Chapter 5, "Fluid Lines and Fittings," of AC 65-9A, Airframe and Powerplant Mechanics General Handbook, also apply to oxygen lines. As a general rule oxygen lines are double flared. The double flare makes the connection stronger and able to withstand more torque.
When installing a line, make sure there is proper clearance. The minimum clearance between oxygen plumbing and all moving parts should be 2 in. It is desirable to maintain a 6-in. clearance between oxygen tubing and electrical wires. When this is not possible, fasten all electrical wires securely with clips so that they cannot come to within 2 in. of the oxygen tubing.
OXYGEN VALVES Five types of valves are commonly found in high-pressure gaseous oxygen systems. These are filler valves, check valves, shutoff valves, pressure reducer valves, and pressure relief valves. Low-pressure systems will normally contain only a filler valve and check valves.
Filler Valves The oxygen system filler valve is located on most aircraft close to the edge of an access hatch or directly behind a cover plate in the skin. In either location, the valve is readily accessible for servicing. It is usually marked by a placard or a sign stenciled on the exterior, reading: &OXYGEN FILLER VALVE.' There are two types of oxygen filler valves in use, a low-pressure filler valve and a high-pressure filler valve.
The low-pressure filler valve, figure 14-52, is used on systems equipped with low-pressure cylinders. When servicing a low-pressure oxygen system, push the recharging adapter into the filler valve casing. This unseats the filler valve and permits oxygen to flow from the servicing cart into the aircraft oxygen cylinders. The filler valve contains a spring-loaded locking device which holds the recharging adapter in place until it is released. When the adapter is removed from the filler valve, reverse flow of oxygen is automatically stopped by a check valve. A cap is provided to cover the filler opening and prevent contamination.
The high-pressure valve has a threaded fitting to receive the oxygen supply connector and a manual valve to control the flow of oxygen. To service an oxygen system that uses a high-pressure filler valve, screw the recharging adapter onto the aircraft filler valve. Open the manual valve on the filler valve and the servicing bottle. When recharging is completed, close the valves, remove the recharging adapter, and screw a valve cap on the valve to prevent contamination.
Check Valves Check valves are installed in the lines between cylinders in all aircraft that have more than one storage cylinder. They are provided to prevent a reverse flow of oxygen or to prevent the loss of all oxygen as the result of a leak in one of the storage cylinders. Check valves permit the rapid flow of oxygen in only one direction. The direction of unrestricted flow is indicated by an arrow on the valves.
Of the two basic types of check valves commonly used, one type consists of a housing containing a spring-loaded ball. When pressure is applied to the Valve cap Locking device Low-pressure oxygen.
Fill to 425 Ibs. pressure.
Do not exceed 450 Ibs.
pressure.
CAUTION Keep clean, dry and a free of all oils.
Oxygen system filler valve Access door FicuRE 14-52. Low-pressure gaseous oxygen filler valve.
inlet side, the ball is forced against the spring, thus breaking the seal and allowing oxygen to flow.
When pressure is equalized, the spring re-seats the ball, preventing any reverse flow of oxygen. The other type is a bell-mouthed hollow cylinder fitted with a captive ball in its bore. When pressure is applied at the bell-mouthed end (inlet), the ball will permit oxygen to flow. Any tendency of reverse flow causes the vall to move onto its seat, covering the inlet and preventing a reverse flow.
Shutoff Valves Manually controlled two-position (on, off) shutoff valves are installed to control the flow of oxygen being emitted from a cylinder or & bank of cylinders. For normal operation, the knobs which control the valves are safetied in the "on" position.
When necessary, such as changing a component, the appropriate valve can be closed. As a precaution, when opening a valve, the knob should be turned slowly to the "on" position. Otherwise, the sudden 593
rush of highly pressurized oxygen into a depleted system could rupture a line.
Pressure-Reducer Valves In high-pressure oxygen systems, pressure-reducing valves are installed between the supply cylinders and the cockpit and cabin equipment. These valves reduce the high pressure of the oxygen supply cylinders down to approximately 300 to 400 p.s.i. required in the low-pressure part of the system.
Pressure-Relief Valves A pressure-relief valve is incorporated in the main supply line of a high-pressure system. The relief valve prevents high-pressure oxygen from entering the system downstream of the pressure reducers if the reducer fails. The relief valve is vented to a blowout plug in the fuselage skin.
REGULATORS Diluter-Demand Regulators The diluter-demand regulator gets its name from the fact that it delivers oxygen to the user's lungs in response to the suction of his own breath. To prolong the duration of the oxygen supply, the oxygen is automatically diluted in the regulator with suitable amounts of atmospheric air. This dilution takes place at all altitudes below 34,000 ft.
The essential feature of a diluter-demand regulator is a diaphragm-operated valve called the demand valve (figure 14-53), which opens by slight suction on the diaphragm during inhalation and which closes during exhalation. A reducing valve upstream from the demand valve provides a controlled working pressure. Downstream from the demand valve is the diluter control closing mechanism. This consists of an aneroid assembly (a sealed, evacuated bellows) which controls the air inlet valve. When the diluter lever is set in the position marked "normal oxygen," atmospheric air at ground level is supplied with very little oxygen added. As altitude increases, the air inlet is grad ually closed by the bellows to give a higher concen- Emergency metering control Pressure reducing valve Diaphragm Demand valve Aneroid Oxygen - Diluter control metering closing mechanism port Air metering port lever Air iniet check valve FIGURE 14-53. Schematic of a diluter-demand regulator.
tration of oxygen until at about 34,000 ft. the air inlet is completely closed and 100% oxygen is supplied. As altitude decreases, this process is reversed.
The diluter control as shown in figure 14-54, can be set by turning the lever to give 100% oxygen at any altitude. At moderate altitudes, however, this causes the oxygen supply to be consumed much more rapidly than normal. The diluter control should be set at "normal oxygen" for all routine operations. It can be set at "100 percent oxygen" for the following purposes: (1) Protection against exhaust gases or other poisonous or harmful gases in the aircraft, (2) to avoid the bends and chokes, and (3) to correct a feeling of lack of oxygen.
Emergency knob (red)
OILUTER DE Diluter lever FIcURE 14-54. Diluter-demand regulator control.
The diluter-demand regulator is provided with an emergency valve, operated by a red knob (figure 14-54) on the front of the regulator. Opening this valve directs a steady stream of pure oxygen to the The following paragraphs illustrate a typical procedure for checking the operation of a diluter-demand regulator. First, check the oxygen system pressure gage, which should indicate between 425 and 450 p.s.i.; then check out the system using the following steps:
(1) Connect an oxygen mask to each diluterdemand regulator.
(2) Turn the auto-mix lever on the diluter-demand regulator to the 100 percent oxygen" position and listen carefully to make certain that no oxygen is escaping.
(3) Breathe oxygen normally from the mask.
The oxygen flowmeter should blink once for each breath. (Figure 14-55 shows a 250
BLINKER 500
OXYGEN FLOW
INDICATOR FICURE 14-55. Flow indicator and pressure gage.
typical oxygen fowmeter and pressure gage.)
(4) With auto-mix lever in "100 percent oxygen" position, place the open end of the mask-to-regulator hose against the mouth and blow gently into the hose. Do not blow hard, as the relief valve in the regulator will vent. There should be positive and continued resistance, if not, the diaphragm or some part of the air-metering system may be leaking.
(5) Turn the auto-mix lever to normal oxy.
gen" position.
(6) Turn the emergency valve on the diluterdemand regulator to the "on" position for a few seconds. A steady flow of oxygen should result, ceasing when the emergency valve is turned off.
(7) Safety wire the emergency valve in "off" position with Federal Specification QQ-W-341, or equal, annealed copper wire, 0.0179-in. diameter.
Another type of diluter-demand regulator is the narrow panel type. This type regulator face (figure 14-56) displays a float-type flow indicator which signals oxygen flow through the regulator to the mask.
The regulator face also displays three manual control levers. A supply lever opens or closes the oxygen supply valve. An emergency lever is used to obtain oxygen under pressure. An oxygen selector lever is used for selecting an air/oxygen mixture or oxygen only.
Figure 14-57 illustrates how the narrow panel oxygen regulator operates. With the supply lever in the "on" position, the oxygen selection lever in the "normal" position, and the emergency lever in the "off" position, oxygen enters the regulator inlet.
The addition of air may be cut by turning the oxygen selection lever to "100%." When this lever is in "normal," air enters through the air inlet port, and the required amount is added to the oxygen to form the correct air/oxygen mixture.
OXYGEN REGULATOR GILUTER DEMAMO obtained by turning the emergency lever to "on." Positive pressure at the regulator: outlet may be This mechanically loads the demand diaphragm to provide positive outlet pressure.
Continuous-Flow Regulator Continuous-flow regulators of the hand-adjustable and the automatic type are installed for the Emergency lever crew and passenger oxygen supply respectively.
Flowmeter The hand-adjustable, continuous-flow regulator Oxygen lever delivers to the user's mask a continuous stream of oxygen at a rate that can be controlled. The system Supply lever usually contains a pressure gage, a flow indicator, and a manual control knob for adjusting the oxy- FicURE 14-56. Typical narrow panel oxygen regulator. gen flow. The pressure gage indicates the p.s.i. of oxygen in the cylinder. The flow indicator is calibrated in terms of altitude. The manual control Demand diaphragm knob adjusts the oxygen flow. The user adjusts the manual control knob until the altitude of the flow Relief valve indicator corresponds to the cabin altimeter read- On ing- Emergency lever Off • Demand valve The automatic continuous-flow regulator is used Inlet air cover in transport aircraft to supply oxygen automatically 100% Supplementary to each passenger when cabin pressure is equivalent Oxygen selection lever oxygen valve to an altitude of approximately 15,000 ft. Operation Normal of the system is initiated automatically by means of On • Flow indicator an electrically actuated device. The system can also Supply lever be actuated electrically or manually should the au- Off tomatic regulator malfunction.
Oxygen inlet Air inlet valve Upon actuation, oxygen flows from the supply
- Air metering valve cylinders to the service units. A typical passenger FIcURE 14-57. Schematic of a narrow panel: service unit is shown in figure 14-58. During the oxygen regulator.
When there is sufficient differential pressure across the demand diaphragm, the demand valve Mask stowage box opens to supply oxygen to the mask. This pressure Mask stowage box door differential exists during the user's inhalation cycle.
After passing through the demand valve, the oxygen is mixed with air that enters through the air inlet port. The mixture ratio is determined by an aneroid-controlled air metering valve. A high oxygen ratio is provided at high altitudes and a high air ratio at lower altitudes. The air inlet valve is set to permit the airflow to begin at the same time as the oxygen flow. FIGURE 14-58. Typical passenger service unit.
first few seconds of oxygen flow, a pressure surge of 50 to 100 p.s.i. causes the oxygen mask box doors to open.
Each mask assembly then falls out and is suspended by the actuating attachment on the flexible tubing. The action of pulling the mask down to a usable position withdraws the outlet valve actuation pin, opening the rotary valve, allowing oxygen to flow to the mask.
OXYGEN SYSTEM FLOW INDICATORS Flow indicators are used in oxygen systems to give visual indications that oxygen is flowing through the regulator. They do not show how much oxygen is flowing. Furthermore, their operation does not indicate that the user is getting enough oxygen.
In the blinker type indicator, figure 14-59, the eye opens and closes each time the user inhales and exhales. To check the flow indicator, set the diluter lever to "100% oxygen" and take several normal breaths from the mask-to-regulator hose. If the blinker opens and closes easily with each breath, it is in operating condition.
BLINKER
OXYGEN FLOW
INDICATOR FICURE 14-59. Oxygen flow indicator.
PRESSURE GAGES Pressure gages are usually of the Bourdon tube type. Figure 14-60 shows the faces of two oxygen gages: (1) A low-pressure gage and (2) a highpressure gage.
200 300 1000 100 400 FULL LBS. PER $0.500 • 2000 ÖXYGEN O OXYGEN e CYLINDER
PRESSURE PRESSURE Low pressure gage High-pressure gage FIGURE 14-60. Oxygen pressure gages.
Because of their connection into a system, the gages do not necessarily show the pressure in each cylinder. If the system contains only one supply cylinder, the pressure gage will indicate cylinder pressure. In systems where several cylinders are interconnected through check valves, the gage will indicate the pressure of the cylinder having the highest pressure.
Immediately after the system has been filled, pressure gage accuracy can be checked by comparing the aircraft pressure gage with the gage on the servicing cart. On low-pressure systems, the aircraft gage should read within 35 p.s.i. of the 425 p.s.i.
servicing cart pressure. The same check can be used for high-pressure systems, but servicing pressure is 1,850 p.s.i. and a tolerance of 100 p.s.i. is allowed.
The tolerances shown for pressure gage accuracy are typical and should not be construed as applying to all oxygen systems. Always consult the applicable aircraft maintenance manual for the tolerances of a particular system.
OXYGEN MASKS There are numerous types of oxygen masks in use which vary widely in design detail. It would be impractical to discuss all of the types in this hand book. It is important that the masks used be compatible with the particular oxygen system involved.
In general, crew masks are fitted to the user's face with a minimum of leakage. Crew masks usually contain a microphone. Most masks are the oronasal type, which covers only the mouth and nose.
Large transport aircraft are usually fitted with smoke masks for each crew position. The smoke masks are installed in stowage containers within easy grasp of the individual. These masks provide crew protection in an emergency and are not used frequently like the demand and continuous-flow masks. Smoke mask equipment consists of a full- 597
tace mask, a flexible breathing tube, and a coupling. The coupling connects to a demand regulator.
A microphone is permanently installed in the mask.
Passenger masks, figure 14-61, may be simple, cup-shaped rubber moldings sufficiently flexible to obviate individual fitting. They may have a simple elastic head strap or they may be held to the face by the passenger.
Valve actuating attachment Base plate Head strap Reservoir bag Face piece FicURE 14-61. Passenger oxygen mask.
All oxygen masks must be kept clean. This reduces the danger of infection and prolongs the life of the mask. To clean the mask, wash it with a mild soap and water solution and rinse it with clear water. If a microphone is installed, use a clean swab, instead of running water, to wipe off the soapy solution.
The mask must also be disinfected. A gauze pao, which has been soaked in a water solution of merthiolate can be used to swab out the mask. This solution should contain one-fifth teaspoon of merthiolate per quart of water. Wipe the mask with a clean cloth and air dry.
SERVICING GASEOUS OXYGEN SYSTEMS The servicing procedures for a gaseous oxygen system depend upon the type of system. Before charging an aircraft system, consult the aircraft manufacturer's maintenance manual. Precautions such as purging the connecting hose before coupling to the aircraft filler valve, avoiding overheat.
ing caused by too rapid filling, opening cylinder valves slowly, and checking pressures frequently during charging should be considered.
The type of oxygen to be used, the safety precau tions, the equipment to be used, and the procedures for filling and testing the system must be observed.
Gaseous breathing oxygen used in aircraft is a special type of oxygen containing practically no water vapor and is at least 99.5% pure. While other types of oxygen (welder, hospital) may be pure enough, they usually contain water, which might freeze and block the oxygen system plumbing especially at high altitudes.
Gaseous breathing oxygen is generally supplied in 220- to 250-cu. ft. high-pressure cylinders. The cylinders are identified by their dark green color with a white band painted around the upper part of the cylinder. The words &OXYGEN AVIATORS' BREATHING" are also stenciled in white letters, lengthwise along the cylinders.
Oxygen Service Safety Gaseous oxygen is dangerous and must be handled properly. It causes flammable materials to burn violently or even to explode. Listed below are several precautionary measures to follow:
(1) Tag all reparable cylinders that have leaky valves or plugs.
(2) Don't use gaseous oxygen to dust off clothing, etc.
(3) Keep oil and grease away from oxygen equipment.
(4) Don't service oxygen systems in a hangar because of the increased chances for fire.
(5) Valves of an oxygen system or cylinder should not be opened when a flame, electrical arc, or any other source of ignition is in the immediate area.
(6) Properly secure all oxygen cylinders when they are in use.
Gaseous Oxygen Servicing Trailers Even though several types of servicing trailers are in use, each recharging system contains supply cylinders, various types of valves, and a manifold that connects the high-pressure cylinders to a purifier assembly. In the purifier assembly, moisture is removed from the oxygen. Coarse particles are trapped in the filter before reaching a reducing valve. The reducing valve has two gages which are used to monitor inlet and outlet pressures respectively. The reducing valve also has an adjusting screw for regulating the outlet pressure. This pres- 598
sure is discharged into a flexible hose which connects to the charging valve and the adapter. The charging valve controls oxygen flowing away from the servicing trailer, and the adapter connects the recharging equipment to the aircraft filler valve.
On many aircraft a chart is located adjacent to the filler valve which shows the safe maximum charging pressure for the ambient temperature. This must be observed when charging the system.
It is common practice to have a warning placard cautioning against using oil or grease on the filler connections. Oxygen ground equipment should be maintained to a standard of cleanliness comparable to that of the aircraft system.
Leak Testing Gaseous Oxygen Systems This test is performed at different times, depending on the inspection requirements for the particular type of aircraft. The system is allowed to cool, usually 1 hr., after filling before the pressures and temperatures are recorded. After several hours have elapsed, they are recorded again. Some manufacturers recommend a 6-hr. wait, others a 24-hr. wait.
The recorded pressures are then corrected for any change in temperature since filling. Figure 14-62 is typical of the graphs provided in the aircraft main-
• 50 - 25 45
- 20 CHANGE IN TEMPERATURE-°C +15.
- 10 5 10 15 20 25 30 35 40
CHANGE IN PRESSURE-PSI
CORRECTION OF PRESSURE DURING LEAKAGE
TEST FOR CHANGE IN TEMPERATURE
ADD PRESSURE CHANGE IF TEMP.
ERATURE RISES:
SUBTRACT PRESSURE CHANGE IF TEMP.
ERATURE FALLS FicURE 14-62. Pressure/temperature correction chart.
tenance manual to aid in making pressure/temperature corrections.
As an example for using the chart, assume that the oxygen system was recently charged. An hour later, the oxygen pressure gage read 425 p.s.i. at a temperature of 72° F. When the pressure gage was read 6 hrs. later, the pressure was 430 p.s.i. with a temperature of 79° F. By referring to figure 14-57, we see that the 7° temperature rise should have caused the pressure to increase 5 p.s.i., making the pressure gage read 430 p.s.i.
When oxygen is being lost from a system through leakage, the gage reading will be less than shown on the pressure/temperature correction chart. Leakage can often be detected by listening for the distinct hissing sound of escaping gas. If the leak cannot be located by listening, it will be necessary to soap-test all lines and connections with a castile soap and water solution or specially compounded leak-test material. To make this check, apply the test solution to areas suspected of leakage. Watch for bubbles. Make the solution thick enough to adhere to the contours of the fittings.
Any leak no matter how small must be found and repaired. A small leak may not cause trouble but if leak continues over a period of time, the surroundings and atmosphere become saturated Such conditions are especially dangerous because personnel may not be aware that oxygen-enrichment exists. Oxygen-enriched conditions are almost always present in poorly ventilated areas.
NO ATTEMPT SHOULD BE MADE TO
TIGHTEN A LEAKING FITTING WHILE THE SYSTEM IS CHARGED.
Draining the Oxygen System When it is necessary to drain the system, it can be done by inserting a filler adapter into the filler valve and opening the shutoff valves. Do not drain the system too rapidly as this will cause condensa tion within the system. An alternate method of draining the system is opening the emergency valve on the demand oxygen regulator. Perform this job in a well-ventilated area and observe all fire precautions.
Cleaning the Oxygen System Always keep the external surfaces of the components of the oxygen system, such as lines, connections, and mounting brackets, clean and free of corrosion and contamination with oil or grease.
As a cleaning agent, use anhydrous (waterless) ethyl alcohol, isopropyl alcohol (anti-icing fluid), 599