Re: Oxygen masks in aiplanes, what's the process/reaction?

Greetings:

Your question regards Supplemental Oxygen Systems which are specified in Section 121 of the Federal Aviation Regulations (FAR). The specific sections of the FAR include:

Sec. 121.327 Supplemental oxygen: Reciprocating engine powered airplanes.
Sec. 121.329 Supplemental oxygen for sustenance: Turbine engine powered 
airplanes.
Sec. 121.331 Supplemental oxygen requirements for pressurized cabin 
airplanes.
Sec. 121.333 Supplemental oxygen for emergency descent and for first aid;
Sec. 121.335 Equipment standards.
Sec. 121.337 Protective breathing equipment.

http://www.gleim.com/Aviation/part121/index.html

The deployment of supplemental oxygen is a function of the type of aircraft, the number of passengers and the altitude of flight. Based on these factors the regulations specify who should receive supplemental oxygen and for how long (e.g. flight crew, cabin attendants, passengers).

In the passenger compartments the supplemental oxygen system can be automatically activated by a rapid drop in cabin pressure or by the flight crew when required. Some forms of cabin decompression are not severe enough to trigger the automatic deployment. A recording for the passengers to use face masks is automatically activated or it can be triggered by the flight crew.

Accident investigations have shown that the automatic deployment of the oxygen masks is not 100% foolproof, and the cabin attendants and taller passengers sometimes have to aid passengers to deploy their face masks (See accident report below).

In some aircraft supplemental oxygen is contained in high pressure bottles and in other types of aircraft it can be generated by a low pressure chemical process (e.g. DC-10, L-1011). When a typical emergency condition occurs, the flight crew rapidly decreases the altitude of the aircraft to less than 3048m (10,000 ft) so that supplemental oxygen is not required.

When necessary, the oxygen masks are deployed and the passengers put on there face masks and begin breathing supplemental oxygen. Fixed flow oxygen systems do not shut off when people are not breathing in. People only breathe in 27% of the time; the remaining 73% of the cycle is either pause or breathing out. To help regulate the flow of oxygen each face mask contains an inflatable holding chamber that can collect oxygen when the passengers are not breathing in, and release the collected oxygen upon inspiration. However, if the aircraft’s oxygen generating system and flow controls are operating correctly these holding bags will usually not inflate.

If you are interested in the problems encountered with a supplemental oxygen systems during an in flight emergency, the following URL has an accident report that describes some of the problems encountered by passengers during an incident that fortunately had no fatalities.

http://home.earthlink.net/~tristar500/incidents/g-bbah/feb96.html

Best regards, your Mad Scientist
Adrian Popa