Everything You Need to Know About Aircraft Oxygen Supply Systems
To know about Aircraft oxygen supply systems you need to understand that Aircraft fly at very high altitudes. A typical passenger aircraft cruises at an altitude of 35,000 ft – that’s 6,000 ft higher than the highest mountain in the world.In the region of the atmosphere where aircraft fly called the troposphere, the air pressure, and density decrease as altitude increases. This is a two-fold problem; density decrease means there are fewer oxygen molecules in the air while pressure decrease makes it harder for the human body to take in oxygen.
The low pressure of oxygen can result in hypoxia as oxygen can no longer be absorbed into the bloodstream easily. As the altitude increases and oxygen pressure becomes less adequate, the symptoms of the hypoxia progress from headaches, fatigues, blue lips and fingernails, increased pulse and respiration and vision impairment to even unconsciousness.
This problem can be overcome by either increasing the amount of oxygen present or increasing the oxygen pressure. Most commercial aircraft do the latter by pressurizing the air in the cabin.
Aircraft oxygen systems are responsible for providing enough breathing oxygen to the flight crew and passengers if the aircraft becomes depressurized or in the presence of smoke and toxic gases.
In this article, I will talk about:
- The components of an aircraft oxygen supply system.
- The operation of an aircraft oxygen supply system.
NB: I will be using a typical passenger aircraft such as the Airbus A320 as a reference for most of my explanations. The workings, components, and arrangement of other aircraft oxygen systems may slightly differ.
Types of Aircraft Oxygen Supply Systems
There are two main types of oxygen systems used in commercial aircraft. They are:
- Gaseous Oxygen Systems:
This is the more common type of the two. The major characteristic here is that the oxygen is carried in a gaseous form in high-pressure cylinders. The cylinders feed an oxygen distribution network. The oxygen in the cylinders is stored at atmospheric temperature. In passenger aircraft, such as the A320, built-in gaseous oxygen systems are used as a backup for aircraft pressurization systems.
- Solid Oxygen Systems:
They are so-called because they involve the use of chemical generators which use other solid chemicals to produce oxygen onboard the aircraft. Some aircraft, including the A320, have a combination of gaseous oxygen systems and solid oxygen systems. This type is especially used in portable oxygen systems.
Components of an Aircraft Oxygen Supply System
The oxygen supply system of commercial aircraft is usually split into three: two separate stationary systems for the cockpit and cabin, and one portable system for first aid purposes.
Below are some of the components that make up a typical aircraft oxygen system and their functions:
- High-pressure cylinders: are used for pressurizing oxygen. Each oxygen cylinder in the A320 aircraft is rated at 1850 psi but is capable of pressuring the oxygen up to 2, 400 psi. The cylinders are said to be empty when the pressure in them is below 50 psi. This is to prevent air with water vapor traces from entering them and corroding the insides of the tank. Any tank below this pressure is removed from service and replaced.
- Chemical generators: they are used in the cabin to generate oxygen which is supplied to the masks. They do not produce oxygen unless the masks are activated. The generator makes use of ignited sodium chlorate which produces oxygen when burnt. When activated, the generator produces a steady flow of pure oxygen until the sodium chlorate is exhausted. The chemical reaction has a by-product of heat thus, the smell of burning or smoke and temperature rise may accompany the oxygen generators’ normal operation. Chemical generators are fitted above the passenger seats, lavatories, and each cabin flight attendant’s station. A standard generator will produce oxygen for approximately 15 minutes.
- Pressure regulator: it ensures that the pressure in the cylinders is suitable for the flight crew and passengers. It is directly connected to the cylinders and regulates the oxygen pressure before it is fed into the masks. The percentage of oxygen the regulator supplies can be selected by the crew. The regulators reduce the oxygen pressure from the cylinders to about 60-85 psi which is suitable for use by individuals. A metering valve within the regulators is what is adjusted to control the proportion of cabin air and pure oxygen supplied to the masks. When the cabin altitude is above 35 000 ft, pure oxygen is automatically supplied.
- Pressure gauge: it measures the pressure of the gas.
- Over-pressure safety systems: if the pressure of the oxygen becomes too high, it allows the flight crew to vent off some oxygen into the atmosphere through a safety port. The safety valve port opens and causes the green oxygen blowout disc on the skin of the fuselage to remove, thereby letting some of the oxygen escape.
- Supply solenoid valve: it allows the crew to shut off the distribution of the oxygen. The solenoid, when it receives an electrical signal, opens and closes as required. When open, the oxygen flows from the pressure regulator to the distribution manifold (and then the masks).
- Masks: they can be put on very quickly and supply enough oxygen to the user. There are typically 3 to 4 full face masks in the cockpit. The ones in the cabin are not full face. The oxygen from the regulators is supplied to the masks. The cabin oxygen systems have 2 to 4 masks at each passenger station which are connected to chemical generators.
- Portable oxygen system: it has a smoke hood that protects the eyes and respiratory system of a flight crew member while s/he fights a fire, or if smoke or toxic gases enter the cabin. The smoke hood uses a chemical air regeneration system. The oronasal mask allows the person to breathe in regenerated air, returning the exhaled air to the regeneration system. The portable oxygen system provides at least 20 minutes of breathing time.
- Flow indicators/meters: this is a component that is moved by the oxygen stream in the system. When oxygen is flowing, it signals the user, allowing them to be sure that the system is working. Some flow indicators blink when oxygen is delivered to the user, while some move a colored pith object.
- Tubes through which the oxygen moves throughout the distribution network.
- Cockpit displays and computers help monitor the status of the system and give the pilot’s commands to the system.
Operation of Aircraft Oxygen Systems
The A320 aircraft is used for these explanations.
The oxygen panel on the overhead panel in the cockpit is used to control the fixed oxygen systems.
If the high-pressure indication on the ECAM (cockpit display) is below 1500 psi, it will be green and boxed in amber. The flight crew will then check the manual to see if the remaining oxygen is enough for the flight.
If the high-pressure indication is below a certain threshold (depending on the model of the A320), the ECAM will display advisory directives and the handbook will have to be referred to for guidance. If the high-pressure indication continues to drop, it will turn to amber when below 400 psi. The OXY indication on the ECAM also turns amber when the pressure is below 400 psi.
The red EMERGENCY pressure selector on the cockpit mask stowage provides oxygen flow for a few seconds when pressed. Tuning the knob in the direction of the arrow makes the overpressure permanent. The overpressure generated is useful for the elimination of condensation and the prevention of smoke, odors, or ashes from entering the mask. The overpressure is automatically activated when the cabin pressure is above the corresponding 30 000 ft altitude
To use the cockpit mask:
- A grip is squeezed so the mask can be taken.
- The mask is removed so the harness can inflate.
- The mask can then be worn once the harness inflates.
- Releasing the red hand side grip will cause the harness to deflate so the mask can be held in place.
To activate the cockpit masks:
- The blinker flow meter becomes yellow when the oxygen is flowing then turns black because of how tight the storage box is.
- The N100 % selector allows the pilot to make a selection between a mixture of cabin air and oxygen or just oxygen. The TEST and RESET control allow the pilot to test the oxygen flow.
- Pressing the RESET control after the oxygen mask has been used disconnects the oxygen microphone.
To test the emergency oxygen pressure on the cockpit masks:
- Press and hold the TEST and RESET control and the EMERGENCY pressure selector at the same time.
- The yellow blinker shows that the oxygen is flowing continuously and stays on as long as the EMERGENCY pressure selector is pressed. The oxygen flow can be heard through the loudspeakers.
The following should be checked at the beginning of the flight:
- Full-face oxygen masks should be stowed and a portable oxygen assembly should be in its place.
- Oxygen bay on the external of the aircraft should be closed.
- Green oxygen discharge disc should be present. If the disk is missing, it means the cockpit oxygen system is over-pressurized.
In the event that the pressurization systems in the aircraft have failed, the oxygen pressure level in the aircraft will be fatal as it can cause the aircraft’s inhabitants to develop hypoxia.
The oxygen systems ensure there is adequate pressurized oxygen until an emergency landing can be made by the pilot. In that time, the lives of all the inhabitants of the aircraft will be preserved. The overpressure safety port of the oxygen system ensures that the pressure of the oxygen is not at too high a level as this can be harmful to the system components. The pressure regulator ensures that the oxygen supply is not too pressurized for individual use.
References:
Oxygen. (n.d.). [ebook] Airbus training. Available at: http://www.smartcockpit.com/docs/A320-Oxygen.pdf
Youtube (2017). A320 CBT Oxygen 1 System Presentation. Available at: https://www.youtube.com/watch?v=wT2NMGsewyI&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=37
Youtube (2017). A320 CBT Oxygen 2 Normal Operation. Available at: https://www.youtube.com/watch?v=wT2NMGsewyI&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=37
Federal Aviation Administration (FAA) (2018). Aviation Maintenance Technician Handbook-Airframe, Volume 2. Federal aviation administration.
Aeronautics Guide. n.d. Aircraft Oxygen Systems and Components. [online] Available at: <https://www.aircraftsystemstech.com/2017/05/aircraft-oxygen-systems-and-components.html>
