ATPL Oxygen Questions

An unpressurized aircraft operating above FL100 must generally have supplemental oxygen available for:

• Both pilots essentially immediately, plus cabin crew and a portion of the passengers once more than 30 minutes has been spent between FL100 and FL130
• Both pilots and all passengers, regardless of time spent at altitude
• Both pilots only, with no requirement extending to cabin crew or passengers
• Both pilots immediately, plus cabin crew and all passengers after 30 minutes between FL100 and FL130

Explanation: Regulations generally require flight crew to use supplemental oxygen essentially immediately above FL100 in an unpressurized aircraft, while a more graduated requirement applies to cabin crew and passengers - typically covering cabin crew and only a portion of passengers once more than 30 minutes has been spent in the FL100-FL130 band, reflecting the increasing physiological risk with both altitude and time of exposure.

Oxygen - Unpressurized Flight Requirements

Oxygen supplied to the flight deck crew oxygen system on a modern transport aircraft is typically:

• Gaseous oxygen, diluted with cabin air if required
• Chemically generated oxygen, diluted with ambient air if required
• Gaseous oxygen, diluted with ambient (outside) air if required
• Chemically generated oxygen, diluted with cabin air if required

Explanation: Flight crew oxygen is normally supplied from a gaseous oxygen cylinder system, rather than a chemical generator (typically used instead for the one-shot passenger system), with a diluter-demand regulator mixing in cabin air as needed depending on cabin altitude, rather than drawing in unconditioned outside air directly.

Oxygen - Flight Crew Oxygen Supply

An aircraft certified to operate above 25,000 ft, with a maximum certified passenger capacity of 240, is actually configured for 200 passenger seats. The minimum number of automatically-deployable passenger oxygen masks required to be installed is generally based on:

• About 110% of the number of installed passenger seats (roughly 220 in this case), not the maximum certified capacity or the number of passengers actually carried
• The maximum certified passenger capacity of 240
• The number of installed seats exactly, with no margin (200)
• The number of passengers actually on board a given flight

Explanation: The regulatory mask-count requirement is generally tied to the number of passenger seats actually installed, with roughly a 10% margin built in to allow for things such as a lap-held infant sharing an adult's mask coverage. It is based on neither the aircraft's maximum certified capacity nor how many passengers happen to be aboard on a particular flight.

Oxygen - Passenger Oxygen Mask Provisioning

Emergency oxygen on a transport aircraft is generally provided by:

• Two independent systems - one dedicated to the flight deck, one to the cabin
• A single combined system serving both the flight deck and cabin
• Two systems, each independently capable of supplying both the flight deck and cabin
• Three separate systems, one each for flight deck, passengers, and cabin crew

Explanation: Flight deck and cabin oxygen are generally kept as two independent systems, so a fault, depletion, or contamination affecting one does not also compromise the other - the flight deck's gaseous cylinder-based system is entirely separate from the cabin's chemical-generator-based drop-down system.

Oxygen - System Redundancy

Above which altitude is it generally mandatory for at least one flight deck crew member to be wearing an oxygen mask continuously, regardless of whether both pilots are at their stations?

• 41,000 ft
• 25,000 ft
• 32,000 ft
• 37,000 ft

Explanation: Beyond the well-known rule requiring the remaining pilot to don a mask if the other leaves the flight deck above a lower threshold, some operating rules additionally require at least one pilot to wear an oxygen mask continuously above a very high altitude such as 41,000 ft, reflecting the very short time of useful consciousness following a decompression at that altitude. The exact regulatory altitude and conditions can vary by operating rule set, so this figure is presented with that caveat.

Oxygen - Mandatory Mask-Wear Altitude

The passenger oxygen mask stowage compartment doors are typically released:

• Electrically for a chemical oxygen generator system, or pneumatically for a gaseous oxygen system
• Purely mechanically, via a spring latch only
• Manually, by cabin crew reaching up to open each compartment
• By a lanyard connected to a barometric pressure capsule

Explanation: The stowage door release method depends on which passenger oxygen system is fitted: aircraft using chemical oxygen generators typically release the doors electrically, as part of triggering generator activation, while aircraft using a centralized gaseous supply typically release the doors pneumatically, using the oxygen system's own pressure - both normally triggered automatically by a barometric switch, with a manual backup available to the crew.

Oxygen - Passenger Mask Door Release

The purpose of a diluter-demand valve in the flight crew oxygen system is to:

• Mix supplied oxygen with cabin air, delivering a lower concentration when full undiluted oxygen isn't needed
• Supply oxygen continuously regardless of breathing, rather than only on demand
• Dilute oxygen with air specifically in the passenger drop-down system
• Supply pure oxygen only when the wearer inhales, with no dilution ever

Explanation: A diluter-demand regulator mixes cabin air with the pure oxygen supply, delivering only as much oxygen as needed for the current cabin altitude rather than always delivering 100% oxygen, and can typically be switched to deliver 100% oxygen directly when required, such as in a smoke or fume event.

Oxygen - Diluter-Demand Regulator

Aircraft gaseous oxygen cylinders are typically charged to a pressure in the region of:

• 1800 psi
• 2000 psi
• 1200 psi
• 1000 psi

Explanation: Aircraft oxygen cylinders are commonly charged to roughly 1800 psi when full, though the exact figure and tolerance varies somewhat by installation and cylinder design, giving crews a straightforward pressure-gauge reference to judge remaining oxygen quantity.

Oxygen - Cylinder Charging Pressure