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3 to 1 space and a 5 to 1 weight savings over gaseous ABO. The major disadvantages
are that LOX is stored at its critical temperature of minus 197º F and its volatile nature
when it come in contact with petroleum products. If LOX comes in contact with
exposed skin, severe frostbite may occur.
Sodium chlorate candles (solid-state
oxygen). Sodium chlorate is a chemical
that, when heated to 350º F, will thermally
decompose and release oxygen. Sodium
chlorate candles have the advantage of
saving weight and space over ABO because
it provides a 600 to 1 expansion ratio.
The major disadvantage is that once the chemical reaction (the candle is activated) has
started, it can’t be easily stopped. Additionally, the candle produces a great deal of heat,
and precautions must be taken to avoid a fire hazard.
Molecular sieve oxygen generators (MSOG). The air that we breathe contains basically
21% oxygen, and the remainder is inert gases that play no major physiological role in
the body. MSOGs take ambient air and separates oxygen from inert gases, using that to
supply oxygen to the aircraft. The military has used this system for many years, as well
as medical patients who need a portable oxygen system. Civil aviation hasn’t embraced
MSOG, but it may be more in vogue in future aircraft.
Oxygen Delivery Systems
Continuous flow. This system delivers a continuous flow of oxygen from the storage
container. It is a very economical system in that it doesn’t need complicated masks
or regulators to function. But it is also very wasteful—the flow of oxygen is constant
whether you’re inhaling, exhaling, or pausing in between breaths. This system is typically
used at 28,000 feet and lower.
Diluter demand. The diluter demand system is designed
to compensate for the short-comings of the continuousflow
system. It gives the user oxygen on-demand (during
inhalation) and stops the flow when the demand ceases
(during exhalation). This helps conserve oxygen. Additionally,
the incoming oxygen is diluted with cabin air and provides the
proper percentage of oxygen, depending on the altitude. This system is typically used at
altitudes up to 40,000 feet.
Pressure demand. This system provides oxygen under
positive pressure. Positive pressure is a forceful oxygen
flow that is intended to slightly over-inflate the lungs.
This will, in a sense, pressurize the lungs to a lower
altitude, thus allowing you to fly at altitudes above
40,000 feet, where 100% oxygen without positive
pressure will not suffice.
Oxygen Masks and Cannulas
When considering an oxygen mask, you must ensure that the mask you are using is
compatible with the delivery system you are using.
Nasal cannulas. These are continuous-flow devices and offer
the advantage of personal comfort. They are restricted by
federal aviation regulations to 18,000 feet service altitude
because of the risk of reducing oxygen-blood saturation levels
if one breathes through the mouth or talks too much.
Oral-nasal re-breather. This type is
the most common and least expensive. It is also the simplest in
operation; it has an external plastic bag that inflates every time you
exhale. The purpose of the bag is to store exhaled air, so it may be
mixed with 100% oxygen from the system. These masks supply
adequate oxygen to keep the user physiologically safe up to 25,000
feet.
Quick-don mask. These masks must
demonstrate the ability to be donned
with one hand in 5 seconds or less, while
accommodating prescription glasses.
Quick-don masks are typically suspended
or stored to permit quick and unimpeded
access by cockpit crew. These masks are
typically rated to altitudes up to 40,000 feet.
Airline drop-down units (Dixie cup). The phase-sequential
continuous-flow mask looks similar to a general aviation rebreather.
However, both masks function differently, and the phase
sequential mask allows the user to go to higher altitudes. This mask
uses a series of one-way ports that allow a mixture of 100% oxygen
and cabin air into the mask. Exhalation is vented to the atmosphere;
as a result, the bag does not inflate. This mask can be safely used at
emergency altitudes up to 40,000 feet.
The PRICE Check
Prior to every flight, the pilot should perform the “PRICE” check on the oxygen
equipment. The acronym PRICE is a checklist memory-jogger that helps pilots and
crewmembers inspect oxygen equipment.
• PRESSURE - ensure that there is enough oxygen pressure and quantity to
complete the flight.
• REGULATOR - inspect the oxygen regulator for proper function. If you are
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Pilot Safety Brochures飞行员安全手册(34)
