Emergency oxygen system

Oxygen masks dropping

Aircraft emergency oxygen systems are emergency equipment fitted to commercial aircraft, intended for use when the cabin pressurisation system has failed and the level of oxygen in the cabin atmosphere drops below a safe level. It consists of a number of individual oxygen masks stored in compartments above passenger seats, and some form of central oxygen generator.

Use

Most commercial aircraft are pressurized at a maximum cabin altitude of 8,000 feet, where it is possible to breathe normally without an oxygen mask. If the cabin pressurization level reaches the equivalent of 14,000 feet or higher on the exterior, or a decompression occurs above that flying level and hypoxia is possible, compartments containing the oxygen masks will open automatically, either above or in front of the passenger and crew seats, and the oxygen masks will drop down in front of the passenger. Oxygen masks may also drop on extremely rough landings or during severe turbulence if the oxygen mask panel becomes loose. Rows of seats typically have an extra mask (i.e. 3 seats, 4 masks), in case someone has an infant in their lap, or someone in the aisle needs to grab one.

An oxygen mask consists of a yellow, soft, silicone facial cup with white elastic bands for securing the mask to the passenger's face. This band is adjustable by pulling two ends looped through the facial cup. The mask may also have a concentrator or re-breather bag that may or may not inflate depending on the cabin altitude, which has (in some instances) made passengers nervous the mask was not providing adequate oxygen, causing some to remove them, who thereby suffered hypoxia. All airlines now make a point in the safety video or demonstration to point out that the bag may not inflate. The bag is attached to a tube, connected to the oxygen source in the compartment, allowing for it to drop down and hang in front of the passengers. To operate on all aircraft except the L-1011, they must be pulled sharply toward the passenger who needs it to un-clip the flow pin and start the process of transporting the oxygen to the passenger. Passenger oxygen masks cannot deliver enough oxygen for sustained periods at high altitudes. This is why the flight crew needs to place the aircraft in a controlled emergency descent to a lower altitude where it is possible to breathe without emergency oxygen. While the masks are being used, passengers are not allowed to leave their seat for any reason until it is safe to breathe without the emergency oxygen. If there is a fire on board the aircraft, masks are not deployed, as the production of oxygen may further fuel the fire.

Aircraft safety cards and in-flight safety demonstrations shown at the beginning of each flight explain the location and use of oxygen masks.

Some aircraft, such as the SAAB Series Aircraft and the 1900D, have a mask system where either a mask is stored under the seat or is distributed by the cabin attendant. These masks are removed from packaging and plugged into the socket for oxygen supply.

Mechanism

There are two systems that are typically found on aircraft:

• A gaseous manifold system, which connects all oxygen masks to a central oxygen supply, usually in the cargo hold area. Pulling down on one oxygen mask starts the oxygen supply for that mask only. The entire system can usually be reset in the cockpit or in some other location in the aircraft.
• A chemical oxygen generator system connected to all masks in the compartment. Pulling down on one oxygen mask removes the firing pin of the generator igniting a mixture of sodium chlorate and iron powder, opening the oxygen supply for all the masks in the compartment. Oxygen production cannot be shut off once a mask is pulled, and oxygen production typically lasts at least 15 minutes. During the production of oxygen, the generator becomes extremely hot and should not be touched. A burning smell may be noted and cause alarm among passengers, but this smell is a normal part of the chemical reaction. This system can be found on the MD-80 aircraft, whose system is also unique in the fact that the face masks are clipped to the inside of the compartment door and do not drop out and hang, by the oxygen tube, in front of the passengers.

Usage history

Remarkably, for a widely-deployed piece of safety equipment, some research has suggested that no lives are known to have been saved by use of an emergency oxygen mask - nor any lives lost through the absence of one - whilst carrying oxygen generating apparatus, albeit as cargo, has caused at least one fatal accident (ValuJet Flight 592), in 1996, where expired chemical oxygen generators were loaded as cargo on board the aircraft without being safely deactivated; in transit, it is believed these generators activated; the heat generated from the activated generators caused the boxes they were improperly stored in to catch fire.

In the three cases of in-flight explosive decompression studied, one took place at a sufficiently low altitude for atmospheric oxygen to be sufficient, whilst in the other two cases the systems failed in the accident and did not provide oxygen to the passengers. However, in several other cases, oxygen masks have kept passengers conscious and alert during a decompression and have protected passengers from injury.

The cockpits of aircraft generally contain a separate oxygen system for the flight crew, and effective use of these has no doubt saved many aircraft. Hypoxia, which can cause severe disorientation and unconsciousness, sets in quickly; if a flight crew does not realise the cabin has decompressed, or is too slow to respond, they can quickly lose control of the aircraft. For example, on Helios Airways Flight 522 in 2005, the cabin depressurized slowly during the ascent to cruising altitude, and whilst the passenger oxygen masks were released at 14,000 feet, the crew were disoriented and failed to realise the significance of this; they lost control within a few minutes, having not put on their own oxygen masks. In the 1999 South Dakota Learjet crash, the NTSB report concluded that only a few seconds delay in using their masks following decompression would be enough to incapacitate a flight crew.

In one case, in 2000, a Boeing 737-800 suffered a slow depressurization, coupled with the failure of the cabin altitude warning system. The depressurization was only discovered by the crew due to the automatic deployment of the passenger oxygen masks; this gave them time to respond appropriately.

In 2011, the FAA announced that it had ordered the removal of all oxygen masks from airplane lavatories in the United States for security reasons.