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Aircrew Oxygen Mask Utilisation

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Article Information
Category: Flight Technical Flight Technical
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Description

If required by regulation for the type and environment of operations, oxygen masks must be available for the aircrew to provide them with a source of breathable air. The masks are used in the event of a depressurisation event that raises the cabin altitude above an acceptable level, as determined by national regulations, or for a smoke or fumes event. The use of the mask will be determined by organisational policy and national regulations.

Regulations

National regulations governing the required use of oxygen masks by aircrew are based on the information provided in Annex 6 of International Civil Aviation Organisation (ICAO) Standards and Recommended Practices (SARPS). These provide guidance on the requirements for oxygen mask use based on the cabin altitude of the aircraft. The article Aircraft Oxygen Systems provides more information on system requirements and architecture.

Discussion

In the event of a situation requiring the aircrew’s use of supplemental oxygen (e.g. a depressurisation event causing an increase in the cabin altitude to a level requiring supplemental oxygen), the aircrew will be required to don their oxygen masks. The masks are normally of the “quick donning” type that enable the aircrew to don them with one hand and in less than five seconds. Oxygen quantity, mask and microphone functionality are normally checked by the flight crew during the aircraft preflight checks. The masks and regulators should also be inspected and tested by maintenance personnel at predetermined intervals. Aircrew oxygen masks may be full face masks covering the entire face or a mask that covers just the mouth and nose. The flow of oxygen provided through the mask may be diluter demand (where the oxygen is mixed with ambient air), 100% oxygen, or delivered under positive pressure. The type of flow may be selectable by the crew or automatically selected by the aircraft’s oxygen system. Positive pressure is normally used for a smoke or fume event.

Communications considerations: Even though aircrew oxygen masks are equipped with microphones, the quality of communications may be degraded, particularly if positive pressure oxygen is provided. This may affect communications both internal with other crewmembers and cabin personnel within the aircraft, and externally with other aircraft and air traffic control. Under these circumstances, it is recommended that crew disable the "hot microphone" feature.

Accidents and Incidents

  • B733, en-route, northwest of Athens Greece, 2005 On 14 August 2005, a Boeing 737-300 was released to service with the cabin pressurisation set to manual. This abnormal setting was not detected by the flight crew involved during standard checks. They took no corrective action after take-off when a cabin high altitude warning occurred. The crew lost consciousness as the aircraft climbed on autopilot and after eventual fuel exhaustion, the aircraft departed controlled flight and impacted terrain. The Investigation found that inadequate crew performance had occurred within a context of systemic organisational safety deficiencies at the Operator compounded by inadequate regulatory oversight.
  • LJ35, Aberdeen SD USA, 1999 On 25 October 1999, a Learjet 35, being operated on a passenger charter flight by Sunjet Aviation, crashed in South Dakota following loss of control attributed to crew incapacitation.

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