B737 Oxygen Systems

The Boeing 737 has two independent oxygen systems, one for the flight crew and one for the passengers. The crew system stores gaseous oxygen in a single high-pressure cylinder and delivers it through quick-donning masks, each with a regulator the pilot can set for diluted oxygen, pure oxygen or oxygen under pressure. The passenger system has no central store at all: every passenger service unit carries its own oxygen source, which runs for a fixed time once started. Portable cylinders and smoke hoods complete the equipment, for first aid and for crew members who must move about or fight a fire.

The crew masks and their selectors are a favorite subject of type rating checks, and the passenger system's 14,000 ft threshold reappears in the rapid depressurization recall items. This article follows the 737 Next Generation (NG) flight crew operations manual (FCOM); where a 737 MAX operator's documentation differs, it takes precedence. The general principles are in crew oxygen systems and passenger and portable oxygen.

Oxygen on the 737

Flight crew oxygen system

Crew oxygen comes from a single cylinder, and the system pressure may be as high as 1,850 psi. It feeds a quick-donning mask stowed in a box at each crew station. The oxygen mask regulator is part of the mask, so the pilot selects dilution and pressure on the mask itself.

On the typical installation the mask comes out with one movement of the inflation lever. Squeezing and pulling it up:

• releases the mask from its stowage box;
• releases the OXY ON flag, as the box doors open;
• starts the oxygen and the mask microphone;
• inflates the harness while the lever is squeezed, so that it slips easily over the head.

An oxygen flow indicator shows a yellow cross whenever oxygen is flowing through the mask. It shows that oxygen flows, not how much.

The audio system follows the mask. On the most common variants, opening the left door of the oxygen mask panel enables the mask microphone and disables the boom microphone; to go back to the boom microphone the door must be closed and the RESET/TEST switch pushed. The control wheel push-to-talk switch keys whichever microphone is active, and the flight deck speakers, muted when a pilot transmits through the push-to-talk microphone, are not muted for oxygen mask microphone operation. Other installations select the mask microphone differently. On Southwest Airlines Flight 1380, a 737-700 depressurized by an engine failure in 2018, each pilot's audio control panel had a switch with a MASK position for it. The cockpit voice recorder first captured unintelligible communications, and the pilots later reported some initial confusion about the position of that switch before they were talking through their masks.

Before flight, the oxygen pressure drop test is part of the preliminary preflight. It needs to be done at only one crew member or observer station, and the crew oxygen pressure must not fall by more than 100 psig. A cylinder valve that is not fully open betrays itself in one of three ways: the pressure falls rapidly, falls by more than 100 psig, or recovers only slowly to normal (see B737 preflight and before start procedures).

Crew masks: NORMAL, 100% and EMERGENCY

The regulator delivers oxygen on demand, as the wearer breathes in, and three controls on the mask decide what it delivers:

Positive pressure does two things. At very high cabin altitude it forces enough oxygen into the lungs, and in smoke it keeps any leak around the seal blowing outward, so contaminated air is not drawn in. On airplanes with quick-donning, automatic pressure-breathing masks, the FCOM states that pressure breathing occurs above 27,000 ft. EMERGENCY gives positive pressure all the time, which is why it protects against smoke and harmful vapors, but it also uses the oxygen faster than 100 % or NORMAL, and the FCOM limits it to when conditions require it. One case is written into the FCOM: if a Halon or equivalent extinguisher is to be discharged on the flight deck, everyone there must first be on oxygen at 100 % with EMERGENCY selected.

The cabin altitude warning shows how the masks are used. Whenever the intermittent horn sounds in flight above 10,000 ft MSL, the first recall items are oxygen masks on with the regulators at 100 %, then crew communications established, then the Cabin Altitude Warning or Rapid Depressurization checklist (see B737 memory items). The masks come off only when both pilots have verified on the cabin altitude panel that the cabin altitude is stabilized at or below 10,000 ft. The A320 procedure differs here: once the masks are on, it returns the diluter to N to make the oxygen last through the descent.

View full schematic

Passenger oxygen system and deployment

The passenger oxygen system has no central supply. An individual oxygen source in each passenger service unit (PSU) feeds four continuous-flow masks, and a source with two masks sits above each attendant station and in each lavatory. As on most airliners, these sources are chemical oxygen generators. The NTSB's report on Southwest Flight 1380 describes each passenger service unit of that 737-700 as an oxygen generator, a lanyard-based activation system, a distribution manifold and four masks, and after the depressurization investigators found the generators of all 48 seat sets activated.

Deployment and the PASS OXYGEN switch

A pressure switch activates the system automatically at a cabin altitude of 14,000 ft: the PSU compartments open and the masks drop. The guarded PASSENGER OXYGEN switch on the aft overhead panel has two positions:

• NORMAL (guarded): the masks drop and the system starts automatically if the cabin altitude climbs to 14,000 ft.
• ON: activates the system and drops the masks, for use if the automatic function fails.

When the system operates, the amber PASS OXY ON light comes on, with the OVERHEAD annunciator and MASTER CAUTION. In the preliminary preflight the crew check that the switch guard is closed and the PASS OXY ON light is out; the FCOM cautions that operating the switch deploys the masks.

In flight, the Cabin Altitude Warning or Rapid Depressurization recall items call for the PASSENGER OXYGEN switch to ON if the cabin altitude exceeds, or is expected to exceed, 14,000 ft. The Emergency Descent checklist follows if the airplane is above 14,000 ft MSL and the cabin altitude cannot be controlled, or if cabin pressure is lost (see decompression).

Using the masks

A dropped mask delivers nothing until it is pulled. Pulling one mask down brings down every mask of that unit and starts 100 % oxygen flowing to all of them, and a green in-line flow indicator appears in the transparent hose of any mask receiving oxygen. On most variants the oxygen flows for about 12 minutes and cannot be shut off. If the system has been activated but a PSU compartment stays closed, the passengers or the cabin crew can drop its masks by hand.

The passenger masks give no protection against smoke. Each breath is a mixture of the oxygen and cabin air, so a passenger still inhales whatever is in the cabin. The FCOM therefore forbids the use of passenger oxygen when smoke or an abnormal heat source is present and the cabin altitude is below 14,000 ft.

Chemical oxygen generators

A chemical oxygen generator is a sealed canister with a core of sodium chlorate and iron. Pulling a mask pulls a lanyard that releases a firing pin, and the heat of ignition starts a reaction that releases oxygen until the core is used up. The reaction cannot be stopped and the canister surface can reach about 232 °C (450 °F), so a smell of burning near the units can come from generators working normally. Every fired generator must be replaced. Carried as cargo, generators must be declared and packed as dangerous goods (see dangerous goods).

When the masks drop and nobody acts

A mask deployment is also a warning to the flight deck. On Helios Airways Flight 522 in 2005, a 737-300 whose pressurization mode selector had been left in MAN, the passenger masks deployed as the cabin passed 14,000 ft, with the airplane climbing through about 18,200 ft. The PASS OXY ON light came on, but the master caution, triggered either by the equipment cooling lights or by the deployment, was not canceled, so the second event brought no new alert. The investigators listed the failure to identify the warnings, the deployment indication among them, as a direct cause. The passenger oxygen, designed to last about 12 minutes, kept at least some occupants conscious during the climb. Cabin crew who see the masks down while the airplane keeps climbing should tell the flight deck at once.

Portable oxygen bottles

Portable oxygen cylinders can be located throughout the airplane, but they are normally found in the forward and aft areas of the passenger cabin. They let cabin crew move about on oxygen after a depressurization, when a fixed mask would hold them in one place, and they provide first-aid oxygen for passengers who need it (see in-flight medical emergencies).

• Pressure: 1,800 psi at 70 °F (21 °C). Do not confuse it with the 1,850 psi of the crew system.
• Fittings: a pressure gauge, a pressure regulator and an on-off valve.
• Outlets: two continuous-flow outlets, one regulated to 2 liters per minute for walking about, the other to 4 liters per minute for first aid.

Duration is capacity divided by flow. A smaller cylinder of 4.25 cu ft (120 liters) therefore lasts about 30 minutes at the 4 liter first-aid rate, and about 60 minutes at the 2 liter walk-around rate. How much first-aid oxygen an operator must carry is set by the operating rules (see supplemental oxygen requirements).

Protective breathing equipment

Protective breathing equipment (PBE), also called a smoke hood, is for crew members combating fires or entering areas where smoke or fumes have accumulated. The device is placed over the head and, once activated, provides about 15 to over 20 minutes of oxygen, depending on the type fitted. A hood protects the eyes as well as the lungs and leaves the wearer free to move, which a fixed mask cannot do. PBE may be stowed anywhere in the airplane, but it is normally found in the forward and aft sections of the passenger cabin.

A hood that makes its oxygen chemically cannot be stopped once started, and types differ in how they are activated and checked before flight, so crews train on the model their operator carries. On the flight deck, the pilots' protection against smoke and harmful vapors is the crew mask with EMERGENCY selected, and the smoke vent valve open to pass oxygen into the goggles (see in-flight fire, smoke and fumes). Passengers have no hoods: their protection is getting out of the smoke.

737 and A320 compared

Type rating candidates who know one type are often asked about the other. The main figures, the A320's from the A320 oxygen system:

The wider comparison of the two types is in A320 and B737 systems compared.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.