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A pressurised aeroplane is flying at FL 370 and experiences a rapid decompression. Which of the following statements about the oxygen system is correct?
  • A
    In general manual override of the automatic presentation of passenger oxygen masks is not possible.
  • B
    If the automatic mask presentation has been activated, the oxygen flow will start immediately.
  • C
    After an automatic mask presentation for cabin crew and passengers, oxygen starts to flow only after the mask has been firmly pulled.
  • D
    The oxygen masks are automatically presented to flight crew members.

Refer to figure.

The cabin’s fixed oxygen system supplies oxygen to the occupants, in case of cabin depressurization. Most air transport aircraft use Chemical generators to provide passengers with oxygen (Boeings use gaseous). Generators and masks are in containers above the passenger seats, in the lavatories, in each galley, and at each cabin crew station.


For Chemical Oxygen Systems, each container has an electrical latching mechanism that opens automatically to allow the masks to drop, if the cabin pressure altitude exceeds 15 000 ft. The flight crew can override the automatic control. A manual release tool allows crewmembers to manually open the doors in case of electrical failure.

The generation of oxygen begins when the passenger pulls the mask towards the passenger seat. Once the generator is activated, the flow of oxygen is constant (whether or not the mask is being worn) for approximately 15 minutes, until the generator is exhausted. In line with the oxygen supply to the passenger mask, there is a small bag. This should partially inflate during use, indicating that there is a supply of oxygen. A mixture of oxygen together with cabin air will be supplied. Thus the masks will not protect the passenger from smoke and fumes.

The chemical reaction used for oxygen generation creates heat (the generator itself incorporates a heat shield). Therefore, the smell of burning or smoke, and cabin temperature increase, may be associated with the normal operation of the oxygen generators.

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