Supplemental Oxygen


  • Supplemental Oxygen used at high altitudes to counteract the effect of decreasing pressure (i.e., hypoxia)
    • Percentage of oxygen in the atmosphere does not decrease, pressure for the body to absorb it does
  • Aircraft designed to operate at high altitudes will normally have an oxygen generation system
  • Aircraft designed to operate at low altitudes will normally have an portable oxygen system
  • Portable equipment usually consists of:
    • Container
    • Regulator
    • Mask outlet
    • Pressure gauge
  • Effects of oxygen deprivation:
    • Above 10,000' the crew may begin to make errors in judgment, mental alertness diminishes
    • Above 15,000' collapse and unconsciousness are not uncommon (hypoxia)
    • At 40,000' death will occur in approximately 8 to 12 seconds


  • Containers (tanks) are green in color
  • Aircraft oxygen is usually stored in high pressure system containers of 1,800-2,200 psi
  • When the ambient temperature surrounding an oxygen cylinder decreases, pressure within that cylinder decreases because pressure varies directly with temperature if the volume of a gas remains constant
  • If a drop in indicated pressure on a supplemental oxygen cylinder is noted, there is no reason to suspect depletion of the oxygen supply, which has simply been compacted due to storage of the containers in an unheated area of the aircraft
  • High pressure oxygen containers should be marked with the psi tolerance (i.e., 1,800 psi) before filling the container to that pressure
  • The containers should be supplied with aviation oxygen only, which is 100% pure oxygen
  • Industrial oxygen is not intended for breathing and may contain impurities, and medical oxygen contains water vapor that can freeze in the regulator when exposed to cold temperatures
  • To assure safety, periodic inspection and servicing of the oxygen system should be done

Types of Aircraft Oxygen System:

  • Portable Gaseous Oxygen System:
    • The portable gaseous oxygen system is used when flight altitudes require the use of oxygen and the aircraft is not equipped with an oxygen system
    • This systems is used for passengers or crew members when the aircraft oxygen systems services only the pilot and copilot, or it is used when the duties of the crew require them to move about the aircraft
    • A portable unit weighs approximately 20 pounds and typically consists of: lightweight steel oxygen cylinder (usually 1800 psi, capacity vary), associated plumbing, combined flow control/reducing valve pressure gauge, and breathing mask and connecting hose
  • On-Board Oxygen Generating System (OBOGS):
    • Supplies oxygen from engine bleed air
    • Components consist of a heat exchanger, concentrator, regulator, and associated plumbing
  • Liquid Oxygen System (LOX):
    • This system is used in modern jet aircraft because LOX can be stored in containers, which occupy less space and weigh less than those used for gaseous oxygen
    • LOX systems typically consists of: LOX converters, check valves and manifolds, oxygen heat exchanger, oxygen/vent airflow control panel, liquid quantity indicator, and a breathing mask with connecting hoses

Safety Precautions:

  • Oxygen is flammable and an extremely dangerous material that reacts violently with many oils, fuels, and other chemicals common in aviation
  • Connecting houses must be purged before coupling to aircraft filler valves to avoid contamination
  • Containers must be filled in a controlled manner to avoid overheating
  • Keep oil and grease away from oxygen equipment including tools and protective clothing to avoid contamination
  • Use spark free tools (brass) when servicing oxygen systems
  • 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
  • Extreme caution should be taken not to touch implements containing liquid oxygen without gloves due to the extremely low temperature
  • Protective clothing shall be used when working with LOX including: gloves, coveralls, face shields, and LOX boots
  • Never seal or cap the vent port of a LOX system
    • vents are designed to have sufficient flow capacity to carry away LOX that may escape
    • Expansion ratio of liquid oxygen is 862 to 11-liquid oxygen at atmospheric pressure will generate pressure of up to 12,000 PSI
    • If allowed to evaporate in a sealed container or system which has no relief provisions, LOX could cause an explosion
  • Never saturate cloth, wood, grease, oil paint, or tar with LOX
    • LOX by itself will not burn, but mixing it with any material will cause the LOX to boil and splash violently with possible combustion
    • Combustible material saturated with liquid oxygen at low temperature will urn with explosive violence when subjected to a very mild shock or impact


  • Regulators approved for use up to 40,000' are designed to provide zero percent cylinder oxygen and 100% cabin air at cabin altitudes of 8,000' or less, with the ratio changing to 100% oxygen and zero percent cabin air at approximately 34,000' cabin altitude [Figure 1]
  • Regulators approved up to 45,000' are designed to provide 40% cylinder oxygen and 60% cabin air at lower altitudes, with the ratio changing to 100% at the higher altitude
  • Pilots should avoid flying above 10,000' without oxygen during the day and above 8,000' at night
  • Smoking during any kind of oxygen equipment use is prohibited
  • Before each flight, the pilot should thoroughly inspect and test all oxygen equipment
  • When inspecting, make sure your hands are clean of oils and greases which may ignite if exposed to oxygen
  • After any oxygen use, verify that all components and valves are shut off

Pilot Handbook of Aeronautical Knowledge, Oxygen Regulator
Figure 1: Pilot Handbook of Aeronautical Knowledge, Oxygen Regulator

Oxygen Masks:

  • There are numerous types and designs of oxygen masks in use
    • May be a mask or cannula setup
  • The most important factor in oxygen mask use is to insure the masks and oxygen system are compatible
  • Crew masks are fitted to the user's face with a minimum of leakage and usually contain a microphone
  • Most masks are the oronasal type, which covers only the mouth and nose
  • A passenger mask may be a simple, cup-shaped rubber molding sufficiently flexible to obviate individual fitting
  • It may have a simple elastic head strap or the passenger may hold it to his or her face
  • All oxygen masks should be kept clean to reduce the danger of infection and prolong 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 should also be disinfected
  • A gauze pad that has been soaked in a water solution of Merthiolate can be used to swab out the mask
  • This solution used should contain one-fifth teaspoon of Merthiolate per quart of water
  • Wipe the mask with a clean cloth and air dry
  • Both pressure and diluter-demand systems use a diaphragm operated demand valve, which opens when suction from inhalation is present
    • This makes the system more efficient

Pilot Handbook of Aeronautical Knowledge, Cannula With Green Flow Indicator
Figure 2: Pilot Handbook of Aeronautical Knowledge, Cannula With Green Flow Indicator


  • A cannula is an ergonomic piece of plastic tubing which runs under the nose and is often used to administer oxygen in non-pressurized aircraft [Figure 2]
  • Cannulas are typically more comfortable then masks and can be used up to 18,000'
  • Not authorized above 18,000', therefore altitudes greater than 18,000' require the use of an oxygen mask
  • Many cannulas have a flow meter in the line and if equipped, a periodic check of the green flow detector should be part of a pilot's regular scan

Continuous-Flow Oxygen System:

  • Continuous-flow oxygen systems are usually provided for passengers
  • The passenger mask typically has a reservoir bag, which collects oxygen from the continuous-flow oxygen system during the time when the mask user is exhaling
  • The oxygen collected in the reservoir bag allows a higher aspiratory flow rate during the inhalation cycle, which reduces the amount of air dilution
  • Ambient air is added to the supplied oxygen during inhalation after the reservoir bag oxygen supply is depleted
  • The exhaled air is released to the cabin [Figure 3]

Electrical Pulse-Demand Oxygen System:

    Pilot Handbook of Aeronautical Knowledge, Continuous Flow
    Figure 3: Pilot Handbook of Aeronautical Knowledge, Continuous Flow
  • Portable electrical pulse-demand oxygen systems deliver oxygen by detecting an individual's inhalation effort and provide oxygen flow during the initial portion of inhalation
  • Pulse demand systems do not waste oxygen during the breathing cycle because oxygen is only delivered during inhalation
  • Compared to continuous-flow systems, the pulse-demand method of oxygen delivery can reduce the amount of oxygen needed by 50-85%
  • Most pulse-demand oxygen systems also incorporate an internal barometer that automatically compensates for changes in altitude by increasing the amount of oxygen delivered for each pulse as altitude is increased [Figure 4]

Pulse Oximeters:

  • A pulse oximeter is a device that measures the amount of oxygen in an individual's blood, in addition to heart rate
  • This non-invasive device measures the color changes that red blood cells undergo when they become saturated with oxygen
  • By transmitting a special light beam through a fingertip to evaluate the color of the red cells, a pulse oximeter can calculate the degree of oxygen saturation within one percent of directly measured blood oxygen
  • Because of their portability and speed, pulse oximeters are very useful for pilots operating in non-pressurized aircraft above 12,500' where supplemental oxygen is required
  • A pulse oximeter permits crew-members and passengers of an aircraft to evaluate their actual need for supplemental oxygen [Figure 5]

Servicing of Oxygen Systems:

    Pilot Handbook of Aeronautical Knowledge, Portable Pulse-Demand-System
    Figure 4: Pilot Handbook of Aeronautical Knowledge, Portable Pulse-Demand-System
  • Before servicing any aircraft with oxygen, consult the specific aircraft service manual to determine the type of equipment required and procedures to be used
  • Certain precautions should be observed whenever aircraft oxygen systems are to be serviced
  • Oxygen system servicing should be accomplished only when the aircraft is located outside of the hangars
  • Personal cleanliness and good housekeeping are imperative when working with oxygen
    • Oxygen under pressure and petroleum products create spontaneous results when they are brought in contact with each other
    • Service people should be certain to wash dirt, oil, and grease (including lip salves and hair oil) from their hands and tools before working around oxygen equipment
  • Aircraft with permanently installed oxygen tanks usually require two persons to accomplish servicing of the system
    • One should be stationed at the service equipment control valves, and the other stationed where he or she can observe the aircraft system pressure gauges
  • Oxygen system servicing is not recommended during aircraft fueling operations or while other work is performed that could provide a source of ignition
  • Oxygen system servicing while passengers are on board the aircraft is not recommended
  • Oils and greases, and cannot be used for sealing the valves and fittings of oxygen equipment

Oxygen Requirements:

  • Oxygen requirements are defined in FAR 91.211
  • As Pressure Altitude (PA) increases, the partial pressure of oxygen decreases
  • This decrease in partial pressure prohibits oxygen saturation into the blood
  • Therefore, the oxygen requirement altitudes here are listed as pressure altitudes
    • Sea Level to 12,500' - No oxygen required
    • 12,501' to 14,000' - Required by the required crew if over 30 minutes at this altitude
    • 14,001' to 15,000' - Required to be provided and used by the required flight crew
    • 15,001' to 25,000' - Must be provided for every occupant
    • 25,001' to Unlimited - Required to satisfy the above and an additional 10 minutes for each occupant
    • At FL350 - if one pilot leaves the cockpit then the other must have supplemental oxygen on unless he has a quick donning mask
    • At FL410 - each pilot must be on oxygen at all times

SCUBA Diving Recommendations:

Pilot Handbook of Aeronautical Knowledge, Pulse Oximeter
Figure 5: Pilot Handbook of Aeronautical Knowledge, Pulse Oximeter

O2 Preflight Checklist (PRICE):

  • Pressure
  • Regulator (check quantity)
  • Indicators
  • Connections
  • Emergency

  • Always ensure the system is accessible in flight and operationally checked

Continuous Flow:

  • Usable up to 25,000'
  • Provide a constant delivery of oxygen
  • Cost efficient and simple operation


  • Delivers a constant amount of O2, inefficient at low altitudes
  • Adjustable-flow: Increases O2 duration by allowing user to manually adjust a regulator valve for different altitudes
  • Altitude-compensated: Works like adjustable-flow, but utilizes barometric pressure to automatically adjust O2 for changing altitudes
  • These systems can be either built in or portable
  • Oronasal re-breathers: cover both nose and mouth and use a re-breather bag to mix oxygen and exhaled air
  • Cannula, which insert into the nose
  • As altitude increases, the ratio of O2 in the bag increases, because of decreased ambient air pressure
  • Check valves allow for exhaled air to completely exit the mask at high altitudes
  • General flow rates are 120 liters per hour for crew and 90 liters per hour for passengers


  • Diluter-demand oxygen systems supply oxygen only when the user inhales through the mask
  • An auto-mix lever allows the regulators to automatically mix cabin air and oxygen or supply 100% oxygen, depending on the altitude
  • The demand mask provides a tight seal over the face to prevent dilution with outside air
  • Diluter-demand provides sufficient O2 up to 40,000'
  • A pilot who has a beard or mustache should be sure it is trimmed in a manner that will not interfere with the sealing of the oxygen mask
  • The fit of the mask around the beard or mustache should be checked on the ground for proper sealing


  • Pressure-demand oxygen systems are similar to diluter demand oxygen equipment, except that oxygen is supplied to the mask under pressure at cabin altitudes above 34,000'
  • Pressure-demand regulators create airtight and oxygen-tight seals, but they also provide a positive pressure application of oxygen to the mask face piece that allows the user's lungs to be pressurized with oxygen
  • Pressure-demand is used above 40,000', because even with 100% O2, the atmospheric pressure is too low to allow proper saturation into the blood stream
  • Some systems may have a pressure demand mask with the regulator attached directly to the mask, rather than mounted on the instrument panel or other area within the flight deck
  • The mask-mounted regulator eliminates the problem of a long hose that must be purged of air before 100% oxygen begins flowing into the mask

Chemical Oxygen:

  • Generally used on large aircraft in case of an emergency to provide a 10 minute supply of oxygen
  • Activated by a lanyard on the oxygen mask, as user pulls mask to their face a chemical reaction with sodium chlorate is triggered
  • Once this system is activated, it must burn out and will need to be replaced
  • Chemical oxygen is lightweight and compact
  • Oxygen tanks are normally green in color
  • Oxygen candles used on airliners
  • No more than: 0.005 ml of water per 1 liter of oxygen = 99.5% O2

On-Board Oxygen Generating System (OBOGS):

  • Converts engine compressor bleed air to oxygen-rich breathing air and pressurized air for anti-g system at the correct pressure and temperature
  • Provides continuously available supply of breathing air for crew, while the engine is operating

Case Studies:

  • NTSB Identification: WPR12FA154: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The in-flight loss of control due to the pilot’s impairment as a result of hypoxia. Contributing to the accident was the pilot’s operation of the airplane above 12,500' without the aid of supplemental oxygen
  • NTSB Identification: CEN09LA527: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The in-flight loss of control due to the pilot's impairment as a result of hypoxia. Contributing to the accident was the pilot's decision to operate the unpressurized airplane at an altitude requiring supplemental oxygen without having any oxygen available


  • Note the connection between supplemental oxygen and Hypoxia in our case studies
    • These rules are in place because others have died from it