assignment 3 - oxygen systems

8/2/2019 Assignment 3 - Oxygen Systems

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Assignment 3 - Oxygen Systems Robert Henderson

On most jet airliners each crew station holds quick-donning diluter demandmask/regulators that the aircrew oxygen system uses. Below is a schematic of the

oxygen system in an executive jet. Most executive jets that operate at high altitudeare equipped with diluter demand or pressure demand oxygen regulators for the flightcrew and a continuous flow system for the occupants of the cabin.

On a Boeing 737 the oxygen in the cabin is supplied by a single cylinder and the flowis controlled by a pressure-reducing regulator, allowing low pressure oxygen to besupplied. Normal pressure is 1850 psi.The mask/regulator on a 737 is stored in a box that is adjacent to each crew station.

The mask is placed over the head and levers are released to allow the mask harness tocontract and properly fit the mask to the head and face. A regulator is mounted oneach oxygen mask to control the flow of oxygen. Communication must be maintainedat all times while using the mask so each mask is fitted with a microphone.The type of oxygen system on an aircraft is determined by the oxygen regulator.There are two basic types of regulators in use, and in each type there are variations.For almost all pressurized aircraft, the aircrew use diluter demand systems and thepassengers use continuous flow systems.


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Oxygen System Boeing 737


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Crew member oxygen station - Boeing 737

Passenger Oxygen Compartment Boeing 737


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Demand regulators

More efficient systems use the demand-type regulators. These allow a flow of oxygen

only when the user inhales and shuts it off during exhale. There are several types of

demand regulators. Below is an example of a simple demand-type oxygen regulator.

Diluter Demand RegulatorsDiluter demand type regulators are used by most commercial jets. Whenever thesystem is operated below 34,000ft it automatically dilutes the oxygen in the regulatorwith approximate amounts of atmospheric air.

Typical Diluter-Demand Regulator


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FAR Part 91 requires that under certain flight conditions, the pilot operating thecontrols wear and use an oxygen mask. Because of this, passenger aircraft thatoperate at high altitude are equipped with diluter demand or pressure demandoxygen regulators for the flight crew.Oxygen flows from the supply into the regulator when the supply lever is turned on.

The pressure is decreased by a pressure reducer at the inlet of the regulator to avalue that is useable by the regulator. All flow of oxygen to the mask is shut off bythe demand valve until the wearer inhales and decreases the pressure inside theregulator. This decreased pressure opens the demand valve by pulling the demanddiaphragm over so oxygen can flow through the regulator to the mask.A diluter demand regulator dilutes the oxygen supplied to the mask with air from thecabin. This air enters the regulator through the inlet air valve and passes around theair metering valve. The air inlet passage is open at low altitude and the passage tothe oxygen demand valve is restricted so the user will get mostly air from the cabin.As the aircraft climbs in altitude, the barometric control bellow expands and opensthe oxygen passage while shutting off the air passage. The air passage completely

closes off as the aircraft reaches an altitude of around 34,000ft and as the userinhales, only pure oxygen is metered to the mask. This flow of pure oxygen can alsobe controlled manually, if for any reason the flight crew needs pure oxygen, theoxygen selector on the face of the regulator can be moved from the NORMAL positionto the 100% position. This closes the outside air passage and opens a supplementaloxygen valve inside the regulator, allowing pure oxygen to flow to the mask.An additional safety feature is incorporated that bypasses the regulator. When theemergency lever is place in the EMERGENCY position, the demand valve is held openand oxygen flows continuously from the supply system to the mask as long as thesupply lever is in the ON position.


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Pressure Demand RegulatorsAircraft that fly at altitudes of more than 40,000ft, such as military aircraft, musthave provision to supply 100% oxygen under a positive pressure to the mask. As webreathe we expand our lungs and the atmospheric pressure forces air into them, but

at altitudes above 40,000ft we cannot get enough oxygen into our lungs, even withthe regulator on 100%.Pressure demand regulators also create airtight and oxygen-tight seals, but they alsoprovide a positive pressure application of oxygen to the mask face piece that allowsthe users lungs to be pressurized with oxygen. This feature makes pressure demandregulators safe at altitudes above 40,000 feet.Some systems may have a pressure demand mask with the regulator attached directlyto the mask, rather than mounted on the instrument panel or other area within theflight deck. The mask-mounted regulator eliminates the problem of a long hose thatmust be purged of air before 100 percent oxygen begins flowing into the mask. Individual pressure demand regulators allow the user to adjust the airflow to their

own requirements.A different technique is required with this system when breathing at high altitudes.This technique is the exact opposite of normal breathing because effort is notrequired by the user as the air flows into the lungs but is required to force the usedair out of the lungs.

Typical Pressure-Demand Oxygen System


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Continuous Flow Regulators

Smaller piston-engine powered aircraft normally use a continuous flow regulator thatallows the oxygen to flow from the storage cylinder regardless of whether the user is

inhaling or exhaling. These systems, although not economical of the oxygen, aredesirable when the demand for oxygen is low because of their low cost and simplicity.For this reason, the emergency oxygen systems that drop the mask to the passengersof large jet transport aircraft in the event of cabin depressurisation are of thecontinuous flow type.Because the oxygen for this system is carried in a high pressure bottle, a pressure

reducing valve regulates the pressure down to about 400psi. Before being delivered to

the mask, the oxygen is metered by a pressure regulator to around 70psi. In the event

of a reducing valve failure, a pressure relief valve is incorporated into the system to

prevent damage.

Manual Continuous Flow RegulatorThe picture shows a typical manually adjusted continuous flow oxygen regulator. Thepressure of the oxygen in the system is shown on the gauge on the right and indicatesindirectly the amount of oxygen available. The flow indicator is the gauge shown onthe left and is adjusted by the knob in the lower center of the regulator. As theaircraft ascends into less dense air, more oxygen is needed to breathe. With this typeof regulator the user is able to adjust the flow of oxygen to correspond with thealtitude being flown and the regulator will meter the correct amount of oxygen.


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Automatic Continuous Flow RegulatorAn automatic regulator, such as the one shown below, has a barometric control valvethat automatically adjusts the flow of oxygen to correspond to the altitude beingflown. When the valve on the front of the regulator is opened the correct amount of

oxygen will be metered into the system for the altitude being flown.

Over-Pressurisation disc

There is a blow-out disc located on the aircrafts skin to allow excess pressure to be

evacuated overboard. The disc blows out overboard giving a green visual indicator

ring on the skin surface. Some aircraft deploy a red streamer, in an over pressure

condition, through the blow out orifice.


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Masks

There are numerous types of oxygen masks in use that vary in design detail. It isimportant that the masks used be compatible with the particular oxygen systeminvolved. Crew masks are fitted to the users face with a minimum of leakage. Crew

masks usually contain a microphone. Most masks are the oronasal-type, which coversonly the mouth and nose.Passenger masks may be simple, cup-shaped rubber mouldings sufficiently flexible toobviate individual fitting. They may have a simple elastic head strap or the passengermay hold them to the face.

Continuous Flow MasksFor continuous flow systems, all masks are usually re-breather type and are eitherdisposable bag type masks or rubber bag type masks. The passenger oxygen systemand some of the portable systems use the disposable type while the rubber bag type

masks are used for some of the flight crew systems.The passenger mask typically has a reservoir bag, which collects oxygen from thecontinuous flow oxygen system during the time when the mask user is exhaling. Theoxygen collected in the reservoir bag allows a higher aspiratory flow rate during theinhalation cycle, which reduces the amount of air dilution. Ambient air is added tothe supplied oxygen during inhalation after the reservoir bag oxygen supply isdepleted. The exhaled air is released to the cabin.


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Oxygen enters a re-breather mask (shown in picture) at the bottom of the bag and themask fits to the face of the user very loosely so air can escape around it. The lungs fillwith oxygen if the re-breather bag is full of oxygen when the user inhales. Oxygenflows continuously into the bag, filling it from the bottom at the same time the userexhales used air into the bag from the top. When the bag fills, the air that was in the

lungs longest will spill out of the bag into the outside air, and when the user inhales,the first air to enter the lungs is that which was first exhaled and still has someoxygen in it. This air is mixed with pure oxygen, and so oxygen rich air is alwaysbreathed with this type of mask.More elaborate re-breather type masks have a closefitting cup over the nose andmouth with a built-in check valve which allows the air to escape, but prevents theuser breathing air from the cabin. The oxygen masks that automatically drop from theoverhead compartment of a jet transport aircraft in the event of cabindepressurisation are of the re-breather type. The plastic cup that fits over the mouthand nose has a check valve in it and the plastic bag attached to the cup is the re-breather bag.

Demand-type MasksAll demand-type masks must fit tightly to the face so no outside air can enter todisturb the metering of the regulator, as shown below. Demand masks all connect tothe regulator with a large diameter corrugated hose, whereas the continuous flowmasks all use a small diameter tube to carry the oxygen to the mask.

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A full-face mask is available for use in case the cockpit should ever be filled withsmoke. These masks cover the eyes as well as the mouth and nose, and the positivepressure inside the mask prevents any smoke entering


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Portable Oxygen

Flight Crew Portable OxygenThe flight crew portable oxygen unit provides both demand type and constant flow

capabilities. The example below consists of a portable oxygen cylinder, a pressureregulator (constant flow), an on-off valve, a pressure gauge to show oxygen supply, ademand regulator and a sling type carrying strap.The equipment is located in the cockpit and has a full face smoke mask attached to

it. The demand regulator has a connection for a demand-type mask and delivers 100%oxygen when a supply valve is opened and the mask is donned with the mask strapstightened.A bayonet-type fitting accommodates a disposable continuous flow mask for constantflow oxygen. Using the three litre constant flow outlet, the cylinder provides oxygenfor duration of approximately 103 minutes.

Flight Crew Portable Oxygen System Boeing 737


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Passenger Portable OxygenInstalled at suitable locations throughout the cabin are first aid and supplementalportable oxygen cylinders. They are equipped with a pressure gauge, pressureregulator and an on/off valve. At a pressure of 1800psi and temperature of 21C the

cylinders have a capacity of 311 litres of free oxygen.On each cylinder two continuous flow outlets are provided, one regulates flow at twolitres per minute for walk-around and the other provides flow for first aid at fourlitres per minute.Duration can be determined by dividing the capacity by the outflow (311 litres

divided by 4 litres/min = 77 minutes).

Passenger portable oxygen equipment Boeing 737


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Usually there are two independent oxygen systems on passenger jets. There are anumber of differences between these two oxygen systems. There are differences inthe two systems because of a lower demand for passenger oxygen and a higherpriority for the crew members because they are in control of the plane.

Below are the main points and differences of each system.

Flight Crew Oxygen System

Each crew member station has a diluter demand mask regulator. Have availability of 100% oxygen The flight compartment crew receives their oxygen from a high pressure

storage cylinder.

Demand masks all connect to the regulator with a large diameter corrugatedhose

The demand valve shuts off all flow of oxygen to the mask until the wearerinhales

A diluter demand regulator dilutes the oxygen supplied to the mask with airfrom the cabin

As the aircraft goes up in altitude, the barometric control allows more oxygento flow

Emergency bypass (demand valve is held open and oxygen flows continuouslyfrom the supply system to the mask)

Passenger Oxygen System

Provided from chemical oxygen generators or a single cylinder

Continuous flow system because of its low cost and simplicity 100% oxygen flows from each mask Oxygen flows for approximately 12 minutes and cannot be shut off Can be deployed electrically either by a switch on the pilots overhead panel The chemical oxygen generators are manually activated by pulling on the mask

assignment 3 - oxygen systems

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