Aircraft oxygen and breathig system definition, explanation & limitations.

021 Aircraft General Knowledge theory known as airframe and systems, electrics, powerplant & emergency equipment.
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Aircraft oxygen and breathig system definition, explanation & limitations.

Postby CaptainMFT » 17 Jun 2016 01:04

Aircraft oxygen system & functionality:-
Supplemental oxygen for the deck is usually gaseous and kept in cylinders. Supplemental passenger oxygen is most often chemically generated (with exceptions on some airplane that use cylinders of gas). 
For medical purposes the primary first aid oxygen is in transportable bottles. Oxygen controlling valves must be operated slowly and contact with grease should be avoided because it leads to a chemical process that may cause a spontaneous fire or explosion. 

What is gaseous oxygen and it's use in aircraft:-
It is kept in steel cylinders (European cylinders are painted black with a white shoulder and neck. American area unit all green). Pressure of 1800 psi within the cylinder is reduced to an intermediate pressure of 80-100 psi. Mask pressure is 8-10 psi. 
A thermal compensator adjusts the charging pressure to even out the temperature of the gas flowing into a cylinder. If pressure exceeds within the cylinder a security disc (green) can rupture permitting the contents to flee to atmosphere. A red bowl is visible once green safety disc isn't in its place. 

Oxygen may be filled from outside the pressure hull. 
Flow of oxygen will be regulated, it will be turned off. 
It has a larger capability. 
Disadvantage is that the extra weight. 

What is continuous flow systems, usage and limitations:-
Normally utilized in lightweight airplane for flights higher than (ten) 10,000 ft. 
When oxygen supply is on there's a continuous flow into every blocked in mask. 

Diluter demand regulators:-
Fitted at flight crew stations of larger airplane. 
Diluter - because they combine air with oxygen in differing proportions, counting on cabin altitude. 
Demand - because they solely provide oxygen on demand i.e. once the user of the mask breathes in. 
The system will provide a mixture of air and oxygen up to a cabin altitude of (thirty two) 32,000ft. when this 100% oxygen is supplied. The system will provide 100% oxygen on demand. The system will provide 100% oxygen under continuous flow. 

Mode of operation: 
The main unit is the pressure regulator. It is made of a chamber and a demand valve. The chamber is split in two (2) sections by a diaphragm. One section is ventilated to cabin pressure. 
The other section is where the oxygen comes from (via a demand valve). This section is additionally coupled to the user's mask. 
During inhalation a partial vacuum is made within the chamber. Differential pressure (due to creation of vacuum on one side) deflects the diaphragm. This lifts the demand valve off its seat. oxygen flows in. As pressure rises within the chamber, diaphragm is moved back to its original position. Demand valve closes. Oxygen flow stops. 

Normal Operation - 100% selected Off: 
Inhalation during respiration can cause a partial vacuum within the chamber. This will permit oxygen into the chamber (as mentioned above). This will conjointly attract cabin ambient air via the air metering valve. Thus breathing air is cabin air enriched with oxygen. As cabin altitude will increase, the barometric capsule progressively closes the air inlet and opens the oxygen port, increasing the oxygen supply. At 32,000ft the air-metering valve can fully shut the air supply. Note: At (thirty three) 33,000 ft, sea level conditions will be maintained by respiration 100% oxygen. 

Normal Operation - 100% selected On: 
Normal operation below (thirty two) 32,000 could be a mixture of cabin air and oxygen. Incase of smoke contamination the user can get smoke contaminated air and oxygen. To avoid this the air inlet port is closed by choosing 100% oxygen. 
A supplementary oxygen valve opens to fulfill the demand as a result of the air metering valve can still be reacting to the lower cabin altitude. 
Emergency Selection: 
Emergency selection at any altitude forces the diaphragm to move which displaces the demand valve. Thus supply is going to be 100% continuous flow of oxygen (higher pressure than normal). 

Mask Test: 
It operates at a higher pressure than the emergency selection (painful to breathe). 

Transportable Oxygen: 
These cylinders contain one hundred and twenty 120 litres of oxygen at 1800 PSI. They can have two or three flow rates. 
The flow rates will be: 
- 2 Litres per minute for 60 minutes. 
- 4 Litres per minute for 30 minutes.
- 10 Litres per minute for 12 minutes.

Chemical oxygen generators: 
Located within the personal service units (PSU) on top of the passengers heads. They contain a solid charge block made from sodium chlorate and iron powder mixed together. Pulling the mask down from a half-hung position pulls a lanyard which electrically fires an igniter. Mask release and igniter use 28 volts DC from the battery vital services busbar. Once ignited the charge block can burn continuously for 15 minutes. It will produce additional oxygen than it needs for its own combustion. This excess of oxygen is provided to the user's mask once its filtered (at a temperature no more than 10 deg C higher than ambient). 
To save wasting oxygen and permit for cooling, a reservoir bag is fitted before the mask. A white heat sensitive painted strip round the generator turns black once the unit is ignited. 
The generators have a shelf life of 5 years. 
JAA needs 100% more masks than seats fitted. 
Advantages of this method are:
- Cheap 
- Lightweight 
- Needs no maintenance 
- Lower risk of explosion than gaseous oxygen. 

Disadvantages are: 
- because of mixture of oxygen and cabin air, there's no smoke protection. 

Toilet Masks: There are 2 masks in every toilet provided by chemical generators. 

Gaseous passenger oxygen System: 
Masks are released by oxygen pressure once it's turned on, either automatically or by the pilot, and not 28V DC. 

First aid oxygen: 
Approx discharge rate is 3 litres a minute. 
JAA requirement: 
At least a pair of bottles, enough oxygen|chemical element|element|gas} for 24 of the passengers for the time higher than 8,000 feet. 

Protective breathing equipment: 
Minimum endurance specified by JAR Ops for cabin crew protective breathing equipment is 15 minutes. A typical smoke hood oxygen supply lasts for fifteen minutes (15).
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Good luck,
CaptainMFT.

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