11.15 Oxygen

• Aircraft need oxygen systems to ensure the oxygen supply of everyone on board in case of an emergency.
• EU-OPS 1.760 highlights the importance of oxygen supply on aircraft.
• Engineers should be aware of safety precautions when working with oxygen and understand the layout of both the flight deck and cabin oxygen systems.

EU-OPS 1.760 Requirements

• Pressurized aircraft operating above 25,000 ft (7620 m) and carrying cabin crew must have a supply of undiluted oxygen for passengers who may need it after cabin depressurization.
• Cabin pressurization systems should maintain cabin altitude below 2440 m (8000 ft) at the aircraft's maximum altitude.

Breathing Air at Altitude

• Modern aircraft fly at high altitudes for economic reasons.
• Air composition: 78% nitrogen, 21% oxygen, 1% inert gases.
• As altitude increases, air pressure and density decrease, reducing the oxygen content per breath.
• Reduced oxygen availability lowers blood oxygen saturation, making it harder for the body to deliver oxygen to tissues, muscles, and the brain.
• Examples of effective oxygen percentage at different altitudes:
  • Sea Level: 20.9%
  • 10,000 ft (3048 m): 14.3%
  • 20,000 ft (6096 m): 9.7%
  • 30,000 ft (9144 m): 6.3%

Human Reaction to Cabin Pressure Loss

• At sea level, oxygen pressure in the lungs is approximately 19.6 hPa (3 psi), which is sufficient for normal function.
• Higher altitudes reduce this pressure, leading to hypoxia, characterized by headache and fatigue.
• Hypoxia is caused by insufficient oxygen.
• The negative effects of high altitude can be overcome by increasing the pressure or quantity of oxygen.
• Aircraft cabins are pressurized to increase oxygen pressure due to modern flight levels.
• Cabin pressure is maintained at 2500 m (8000 ft) during cruising altitude, but loss of cabin pressure may still occur.

Characteristics of Oxygen

• Pure gaseous oxygen is stored in high-pressure cylinders, usually painted green.
• Moisture in oxygen can freeze at very low temperatures.
• Only aviator's breathing oxygen, which is very dry, should be used in aircraft.
• Medical oxygen contains water droplets and can freeze.
• Technical oxygen is contaminated and unsuitable for breathing.

Oxygen Safety Rules

• Oxygen is an invisible, odorless, non-flammable gas under normal conditions.
• Pure oxygen readily combines with other substances, sometimes explosively.
• Keep pure oxygen away from petroleum products to prevent explosions.
• Follow inspection and maintenance practices to ensure safety.
• Maintenance is recommended to be done outside when possible.
• Observe Aircraft Maintenance Manual (AMM) warnings and precautions.
• Have adequate fire extinguishers available before starting work.
• Cordon off the area and place "NO SMOKING" placards.
• Ensure all tools and servicing equipment are clean.
• Avoid power checks and using the aircraft’s electrical system.
• Use clean, grease-free hands, clothes, and tools.
• Use only tools dedicated to oxygen systems.
• No smoking or open flames within 15 m (50 ft) of the work area.
• Always use protective caps and plugs on oxygen cylinders and components.
• Store oxygen cylinders in a cool, ventilated area away from petroleum products or heat sources.

Flight Deck System Layout

• Flight crew uses diluter-demand and pressure-demand flow systems in high-performance aircraft.
• High-pressure (1850 psi) oxygen bottles and servicing panels are remotely mounted.
• Large aircraft typically have separate storage bottles for flight crew and passengers.
• A Pressure Reducing Valve (PRV), or pressure regulator, is located near the bottles.
• Oxygen is distributed to the flight deck through tubing, valves, and individual demand regulators at each crew station.
• Each demand regulator is user-controlled.
• A Shut-off Valve (SOV) is located upstream of the station regulators and masks.
• An electric switch, often on the flight deck overhead panel, controls the shutoff valve.
• Bottle pressure indicators can be located near the switch.
• The crew oxygen system supplies only the cockpit crew with oxygen and is always available.
• The crew oxygen system consists of one or more oxygen bottles, which are stored on the lower deck.
• Each bottle has a pressure regulator attached, which feeds oxygen into the distribution components.
• The oxygen passes through a diluter demand regulator and then into the mask of the flight crew member.

Cabin System Layout

• The passenger oxygen system is only used if the cabin depressurizes.
• Normal flight cabin pressure equals a cabin altitude of 2500 m (8000 ft).
• The passenger oxygen system activates automatically if the cabin altitude is above 4300 m (14,000 ft), and oxygen masks are deployed.
• Oxygen masks must be available for every person in the cabin, including at every seat, in all lavatories, and at every attendant station.

Oxygen Sources and Storage

• Breathing oxygen is stored and transported in high-pressure cylinders, typically painted green.
• Engineers must keep pure oxygen away from oils and greases.

Oxygen Bottles

• Oxygen is stored in high-pressure cylinders known as oxygen bottles.
• There are oxygen bottles for the flight crew and multiple bottles for the passengers.
• Oxygen bottles are stored in the cargo compartment or the Main Equipment Centre (MEC).
• Traditional oxygen bottles were heavy steel tanks rated for 1800–1850 psi.
• Newer, lighter bottles have been developed:
  • Lightweight aluminum shell wrapped by Kevlar™.
  • Heavy-walled all-aluminum bottles.
• Most oxygen storage bottles are painted green, but yellow and other colors can be used.
• Bottles are certified to Department of Transportation (DOT) specifications.
• Bottles must be hydrostatically tested periodically.

Oxygen Bottles – Cylinder Head

• Each oxygen bottle has a slow-opening Shut-off Valve (SOV).
• Oxygen bottles have a safety device (burst disc) that bursts before pressure damages the cylinder or components.
• Oxygen bottles have a direct-reading pressure gauge.

Bottle Pressure

• Bottle pressure and temperature are used to determine how full the bottle is.
• A chart near the bottles is used to determine how full the bottle is.
• The chart is also used to determine the maximum pressure for refilling the bottles.

Overpressure Discharge

• If temperature or pressure is too high, the safety disc bursts at about 2600 psi.
• The safety outlet is connected to an overboard discharge line to prevent oxygen buildup.
• The overboard discharge line ends in a port at the aircraft's skin.
• A green plastic disc ruptures if a bottle’s safety device opens.
• Multiple oxygen bottles are connected to the same discharge line.
• If the green disc fractures, each bottle gauge must be read to find the one that triggered the discharge.

Pressure Indicator Gauge

• A pressure indicator gauge is found near the filler port.

Thermal Compensator

• Some aircraft are equipped with a thermal compensator to distribute heat during refilling.
• Do not disconnect the compensator B-nuts or the unit must be replaced.

Oxygen Bottle Summary

The bottle contains:

• Manual slow opening Shut-off Valve (SOV)
• Direct reading gauge
• Thermal compensator connected to the filling line
• Overpressure burst disc and overboard line, which ends at the aircraft skin

Distribution

• Oxygen is transported to the flight deck via lines and manifolds.
• A pressure regulator is located near the oxygen bottles to reduce the length of high-pressure lines.
• Most oxygen lines are metal in permanent installations.
  • High-pressure lines are usually stainless steel.
  • Low-pressure parts of the oxygen system are typically made of aluminum.
• Flexible plastic hosing is used near the masks, plus its use is increasing in permanent installations.
• Oxygen tubing is usually identified with color-coded tape:
  • A green band overprinted with the words “BREATHING OXYGEN”
  • A black rectangular symbol overprinted on a white background border strip
• Tubing-to-tubing fittings often use straight threads for flared tube connections.
• Tubing-to-component fittings usually have straight threads on the tubing end and external pipe threads (tapered) on the other end for attachment to the component.
• Fittings are typically made of the same material as the tubing.
• Flared and flareless fittings are both used.

Valves

Five types of valves are commonly found in high-pressure gaseous oxygen systems:

• Filler valve
• Check valve
• Shut-off Valve (SOV)
• Pressure Reducer Valve (PRV)
• Pressure relief valve
• Oxygen system SOVs are designed to open slowly.

Portable Oxygen

• Portable oxygen equipment is normally stored in the cockpit and near the exits in the passenger compartment.
• Portable oxygen cylinders (bottles) provide a portable supply of breathing oxygen independent of the fixed systems for emergency and first aid use.
• They also provide eye protection for the user in the form of smoke hoods.

Portable Oxygen System Components

• Portable oxygen cylinders (bottles)
• Smoke hoods
• Oxygen masks
• The oxygen cylinder (bottle) assembly includes the oxygen cylinder (bottle) and the cylinder head.

Portable Oxygen System Features

• Typical portable gaseous oxygen cylinder includes valve, pressure gauge, regulator/reducer, hose, adjustable flow indicator, and rebreather cannula.
• A padded carrying case/bag can be strapped to the back of a seat for certification/testing.
• The cylinder (bottle) head consists of:
  • Manual Shut-off Valve (SOV)
  • Pressure gauge
  • Frangible disc
  • Pressure regulator
  • Built-in relief valve
  • Filling valve

Portable Oxygen Masks

• Two types of masks are used:
  • Full-face smoke mask (demand regulator which only allows oxygen to flow when the user breathes for protection from smoke and dangerous gases).
  • Continuous-flow oxygen mask (first aid purposes).
• Each mask has its own connection to the oxygen cylinder.

Smoke Hoods

• Smoke hoods protect the eyes and respiratory system for crew members.
• They are used when fighting a fire, or to prevent breathing in smoke or noxious gases.
• Smoke hoods are vacuum packed in storage containers with a serviceability indicator.
• The indicator reacts to humidity. If the color has changed, the smoke hood and container must be replaced.
• The smoke hood consists of the hood itself, the inner mask, and an oxygen generator.
• A key issue is the assumed respiration rate, which varies according to physical activity.
• Minimum use for cabin crew smoke hoods is usually 20 minutes when “physically active”.
• Oxygen is kept in a closed circuit with a tight neck seal.
• A scrubber system may be present to reduce carbon dioxide levels.
• When the oxygen supply ends, the hood deflates and must be removed.
• Positive-pressure respirators prevent the ingress of smoke or toxic gases by maintaining a higher air pressure inside the mask than outside.

Oxygen Charging

• High performance and air transport aircraft have built-in oxygen systems for refilling gaseous oxygen cylinders.
• Consult the Aircraft Maintenance Manual (AMM) before charging.
• Use the correct type of oxygen, follow safety precautions, and use the correct equipment.
• Open oxygen valves slowly and fill slowly to avoid overheating.
• Purge the hose from the refilling source to the oxygen fill valve on the aircraft of air before transferring oxygen.
• Check pressures frequently while refilling.
• Oxygen filler carts are used to service oxygen systems in maintenance shops.
• Ensure all cylinders on the cart are aviator's breathing oxygen and contain at least 50 psi of pressure.
• Do not refill bottles with less than 50 psi as moisture may have entered.
• Each cylinder must also be within its hydrostatic test date interval.
• Record the remaining pressure on the cylinder after dispensing oxygen.
• Connect the cart to the aircraft and open cylinders with increasingly higher pressure to manage oxygen flow.

Leak Testing

• Leaks in a continuous-flow oxygen system may be difficult to detect because the system is open at the user's end.
• Blocking the flow of oxygen allows pressure to build.
• Use oxygen-safe leak check fluid to detect leaks.
• Apply leak check fluid to fittings and mating surfaces; bubbles indicate a leak.

Supply Regulation

• Oxygen is commonly stored as a gas at atmospheric temperature in high-pressure cylinders (bottles).
• The oxygen system is designed based on aircraft type, operational needs, and presence of a pressurization system.
• Systems are often characterized by the type of regulator used:
  • Continuous-flow
  • Demand-flow
• Some aircraft use continuous-flow systems for both passengers and crew.
• The pressure demand system is widely used as a crew system, especially on larger transport aircraft.
• Many aircraft have a combination of both systems, augmented by portable equipment.

Continuous-flow Systems

• A continuous-flow oxygen system allows oxygen to exit the storage tank through a valve and regulator/reducer.
• A pre-set flow of oxygen continues until the tank valve is closed.
• Fine adjustments to the flow can be made with an adjustable flow indicator.

System Layout (Continuous-flow)

• Systems include a fixed location for the oxygen cylinders (bottles) and permanent delivery plumbing to stations in the cabin.
• Integrated oxygen systems usually have separate, remotely mounted components to reduce pressure and regulate flow.
• A pressure relief valve, filter, and gauge are typically installed.
• By capturing air that is exhaled still contains usable oxygen in a bag, or in a cannula with oxygen-absorbing reservoirs, it can be inhaled with the next breath, reducing waste.

Supply Regulation – Demand-flow Systems

• Demand-flow systems deliver oxygen only when the user inhales.
• Oxygen supply is stopped during hold and exhalation periods, prolonging the supply.
• When the user inhales (demands) oxygen through the mask, this valve unseats and allows oxygen to flow through the regulator.
• Pressure reduction occurs at the inlet of each individual demand regulator by limiting the size of the inlet orifice at the pressure-reducing inlet valve.
• There are two types of individual regulators: the diluter-demand-type and the pressure demand-type.

Regulator Types

• Diluter-demand-type regulator: holds back the flow of oxygen until the user inhales with a demand-type oxygen mask.
  • The user can select 100% oxygen delivery at any time or use a built-in emergency switch for continuous-flow 100% oxygen.
• Pressure-demand oxygen systems: deliver oxygen under higher pressure, used on aircraft that regularly fly at approximately 12 000 m (40 000 ft) and above.
  • Forcing oxygen into the lungs under pressure ensures saturation of the blood, regardless of altitude or cabin altitude.
  • Both diluter-demand and pressure-demand regulators also come in mask-mounted versions.

Two-step Process

• First, a pressure reducer mounted on each bottle reduces the pressure to an intermediate level. Then a pressure regulator reduces the pressure even further to a usable pressure.

One-step Process

• High oxygen pressure can also be reduced in one step. Here, the pressure regulator is mounted directly on the oxygen bottle.

Electrical Switch

• On some aircraft types, a switch must be operated on the flight deck for the flight crew to receive oxygen.
• The switch actuates a solenoid-controlled supply valve.

Flight Crew Mask Storage

• Diluter-demand and pressure-demand flow systems are used most frequently by the crew on high-performance and air transport category aircraft.
• The masks are stored in stowage boxes, each with two flap doors.
• When the mask is removed, a supply valve in the box opens and oxygen flows to the mask.
• A pressure switch connected to the supply valve allows a microphone to be used while the oxygen system is in use.
• After opening the left-hand door, the supply valve in the stowage box must be reset.
• The “OXY ON” flag indicates that the supply valve is still open.

Flight Crew Full-face Oxygen Mask

• The mask itself is a full-face quick donning mask.
• The flight crew can put on their masks with one hand in less than 5 seconds.
• The mask covers the eyes, nose, and mouth, even when the user is wearing glasses.
• The mask consists of a harness, a face piece, a diluter demand regulator and a microphone.
• When the mask is removed from the stowage box the user pulls and presses the two red grips.
• When the grips are pressed the harness is inflated with oxygen, allowing the mask to be put on. When the grips are released, the harness deflates to give the mask a tight fit.

Passenger Oxygen Masks

• On pressurised passenger aircraft, masks are stowed overhead in the Passenger Service Unit (PSU).
• Depressurization triggers the deployment of oxygen masks for each passenger seat.
• A lanyard attached to the mask turns on the flow when pulled.
• A PSU is hinged over each row of seats.
• There will always be an additional mask for infants sharing a seat.
• Deployment may also be controlled by the flight crew with a switch.
• Continuous-flow oxygen masks are simple devices made to direct flow to the nose and mouth of the wearer.
• They fit snugly but are not airtight.
• Vent holes allow cabin air to mix with the oxygen and provide an escape for exhalation.
• Rebreather masks allow the exhaled mixture that is not trapped in the rebreather bag to escape.
• When the passenger oxygen system is activated, the oxygen masks drop down from the overhead stowage boxes or Emergency Oxygen Containers (EOC).
• There is always at least one more mask than there are seats in a row.

Passenger Oxygen System Activation

• For maintenance, there is a restraint/release tool for opening a single stowage box without activating the whole system.
• The tool tests each individual PSU door latch/actuator.
• The restraint tool will let the door open sufficiently to check that the latch/actuator operates correctly without allowing the masks to completely drop.
• Passenger oxygen switch in the flight deck can be selected to release and open all the passenger oxygen doors on the PSUs.

Supply System

• The passenger oxygen system is activated similarly on most aircraft, but the oxygen supply can differ.

Chemical System

• Two primary types:
  • Portable (like a portable carry-on gaseous oxygen cylinder)
  • Fully integrated supplementary oxygen system (backup on pressurised aircraft)
• Solid chemical oxygen generators are common on airliners.
• Generators are stored in the overhead PSU attached to hoses and masks.
• Depressurization or a switch from the flight crew opens a compartment door.
• Pulling the mask down actuates an electric current or ignition hammer that ignites the oxygen candle and initiates the flow of oxygen.
• Stowage boxes are called Emergency Oxygen Containers (EOC).
• Each EOC door has its own solenoid activated automatically by a pressure switch or manually from the flight deck.
• Every EOC contains oxygen masks and a chemical generator.
• To start oxygen supply, tighten the lanyards on the masks, this releases a pin which fires a striker.
• The striker triggers a chemical thermal reaction in the oxygen generator.
• The chemical oxygen generator is filled with sodium chlorate (NaClO2).
• Oxygen (O2) is generated by the high-temperature decomposition of sodium chlorate (NaClO2) (about 350 °C or 660 °F) that is generated by the oxidation of a small amount of iron powder mixed with sodium chlorate (NaClO2).
• BaO2 is used to absorb chlorine. This oxygen supplies all attached masks.
• An activated generator produces a steady flow of oxygen for ten to twenty minutes.
• Once lit, a sodium chlorate (NaClO2) oxygen generator cannot be extinguished.
• When a generator has been used it must be replaced.
• The heat generated, changes the colour of an indicator on the side of the canister to show that it needs changing.

On Board Oxygen Generating Systems (OBOGS)

• OBOGS are generally used on military aircraft.
• Bleed air from turbine engines passes through a molecular sieve that separates the oxygen for breathing.
• Some separated oxygen purges the sieve of nitrogen and other gases.

Gaseous System

• The other passenger oxygen supply system is known as a gaseous system.
• Doors to these boxes are opened by the same oxygen pressure that feeds the masks.
• The oxygen supply is stored in oxygen bottles.

Flow Control Units (FCU)

• The gaseous oxygen system uses Flow Control Units (FCU) to reduce oxygen pressure.
• The FCUs have four main purposes:
  • Shut-off function
  • System activation/pressure switch
  • Generate the pressure push to open the PSU doors
  • Reduce the intermediate oxygen pressure to a usable level

Ring Line

• There is a ring line in the cabin connecting every PSU to the flow control units.
• It transports the pressure push from the FCUs to release the PSU doors.

Activation

• During activation, higher-pressure oxygen flows from the FCU moving a diaphragm.
• The latch plate with the hooks moves to the left. The PSU door opens, and the masks drop down.
• Afterwards, the FCU regulates oxygen flow.

Flow Control

• There is no oxygen flow to the masks until the masks are pulled and at least one lanyard is tightened, which pulls a pin.
• Now the oxygen pressure can move a flow control plunger and oxygen can flow to the mask.

Electronic Pulse Demand (EPD) Systems

• EPD systems deliver timed pulses of oxygen on demand, saving oxygen lost during the breathing cycle.
• A small, portable EPD is connected between the oxygen source and the mask.

Indications and Warnings

• Oxygen systems also have components that monitor and guarantee the correct operation of the system.
• The oxygen system also has components that indicate when it is not intact and must be repaired.

Relief Valve

• Oxygen bottle valves and high-pressure systems have a relief valve if the desired pressure is exceeded.
• The valve is ported to an indicating or blowout disc located in a visible place, such as the fuselage skin.
• Most blowout discs are green.

Flow Indicators

• Flow indicators, or flow meters, are common in all oxygen systems.
• When flow exists, this movement signals to the user.
• Flow indicators verify that an oxygen system is functioning.
• Flight crew flying in smaller aircraft can use a portable pulse oximeter to measure blood oxygen saturation. With those measurements, adjustments to the oxygen flow rates of onboard equipment can be made.