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Bicester Aviation Services |
BAS Data Sheet No 2Oxygen Systems
As you climb above sea level the total pressure, and hence the partial
pressures reduce until at 10,000 ft the partial pressure of the oxygen in the
lung reaches 80 mbs. This is the minimum that a healthy person can tolerate
and accordingly, if the climb is continued above this height, the first
symptoms of lack of oxygen, or hypoxia, appear. Between 10,000 and 15,000
ft the ability to perform skilled tasks such as aircraft control and navigation
are impaired while between 15,000 and 20,000 ft there is a marked
deterioration of performance, even of simple tasks, together with a loss of
critical judgment and willpower. Thinking is slowed while muscular
incoordination and clumsiness result. Above 20,000 ft the symptoms become
severe, rapidly leading to unconsciousness. The great danger of hypoxia is
that it is insidious and the individual is not aware of the gradual
deterioration in his performance which, together with the sense of general
well-being that is created, means that the onset may not be detected.
Furthermore, as the breathing process is controlled by carbon dioxide levels,
deliberate over-breathing will wash out carbon dioxide from the lungs and
disturb the oxygen transfer mechanism. This is known as hyperventilation
and can accelerate the onset of hypoxia as well as wasting oxygen.
The onset of hypoxia can be delayed by increasing the proportion of
oxygen in the inspired air with the result that the partial pressure of oxygen
in the lung is increased. Assuming that the pilot is breathing 100% oxygen
when the climb is commenced, then the partial pressure of oxygen in his lungs
will not fall below 130 mbs until 34,000 ft is reached. Climbing above this
altitude, even when breathing 100% oxygen, will result in a reducing partial
pressure of oxygen in the lung and breathing 100% oxygen at a height of
40,000 ft is equivalent to breathing air at 10,000 ft. Above 40,000 ft, hypoxia
can only be prevented by employing pressure breathing. In practice, oxygen
systems are never 100% efficient and 35,000 ft is a sensible limit.
There are two types of oxygen systems available to pilots; demand
systems and continuous flow systems. Each will be described separately
together with their advantages and disadvantages.
In a simple demand system, a mask is worn which is connected to a
regulator by a wide bore, corrugated tube. On inhaling, the pressure drop
sensed in the regulator opens an oxygen demand valve allowing oxygen to
flow to the mask. As soon as inhalation ceases, the demand valve closes
shutting off the oxygen supply. Examples of this type of regulator supply
air in sub-aqua breathing equipment and firemans breathing apparatus.
For aircraft use, demand regulators are fitted with air dilution which
ensures economy at the lower levels. These diluter demand regulators can
be recognised by having a lever marked 'Normal'/'100%' which controls the
air inlet valve. The air inlet valve is also controlled by a barometric capsule
which ensures the correct air/oxygen mixture for any given altitude, thus
maintaining adequate oxygen partial pressure in the lung and hence adequate
blood saturation. When 'Normal' is selected at sea level the oxygen demand
valve hardly opens and the user breathes mostly ambient air; by 30,000 ft the
air inlet is fully closed and the user receives 100% oxygen. In gliders the
lever should be wire locked in the 'Normal' position as there is no risk of
cockpit fumes or smoke, and accidental selection of '100%' will rapidly empty
the cylinder. The A-12A regulator is of the diluter demand type and can be
safely used up to 35,000 ft provided the user has a well-fitting mask. The
A-14 regulator is of similar design but provides a positive over pressure at
heights above 35,000 ft. This type of regulator may be used up to 45,000 ft
providing the user has undergone aviation medicine indoctrination.
A recent introduction is the electronic demand regulator marketed by Mountain
High. This is a compact, efficient regulator that is designed to be
used with a cannula. On breathing in, the regulator supplies a pulse
(bolus) of oxygen that depends on height at the start of inspiration.
This bolus of oxygen is then drawn deep into the lungs where it achieves maximum
effect. The system works well and is particularly economical on
oxygen.
Continuous flow oxygen systems rely on a simple regulator that delivers
oxygen at a pre-determined flow rate irrespective of altitude or the needs of
the user. The AAV AIR/24 regulator has two flow rates, 'Normal' (two litres
per minute) and 'High', (four litres per minute), while other types of constant
flow regulators may have a flow control valve marked in thousands of feet.
In order for the system to be efficient, it is necessary to devise some means
of storing oxygen delivered by the regulator during the period that the user
is exhaling. This is usually accomplished by providing a rubber re-breathing
bag to which the oxygen flow is piped. A non-return valve at the mouth of the
bag closes when the user exhales. On inhaling, the valve opens allowing the
oxygen stored in the bag during the time that the user was exhaling, to be
inhaled. A further refinement is a simple flow meter which fits in the oxygen
supply pipe and shows that oxygen is flowing to the mask. A constant flow
system can be safely used up to 20,000 ft on 'Normal' flow and up to 30,000 ft
on 'High' flow. Their use above this height is not recommended.
Whichever oxygen system is in use, it is essential that a well fitting and
compatible mask is used. Failure of a demand valve to open is usually due to
leaks around the mask. Masks are not interchangeable without modification,
a demand mask must be used with a demand system and vice versa. Aviation
masks are designed to operate in the freezing conditions that prevail at
altitudes requiring the use of oxygen. Medical masks, which can normally be
recognised by their clear plastic material, will fail at altitude due to ice
formation in the throat of the mask. Microphones may be fitted to some
aviation masks to facilitate intercommunication. When not in use, masks and
associated tubes should be kept in a cool, dark place; ultimately rubber
components will age and crack, and should then be replaced.
An alternative to a constant flow mask is the cannula, of which there are
two types available. The simple cannula pipes a constant flow of oxygen into
the nostrils. This is extremely wasteful as it not possible to save the oxygen
delivered while exhaling. The preferred cannula is the AEROX oxy-saver
cannula. This has small oxygen reservoirs which store a small amount of
oxygen on exhaling and which is subsequently delivered when inhaling. Cannulas have the big advantage that normal activities such as using a
microphone or eating and drinking can be carried out while the cannula is in
use. Cannulas should not be used above 18,000ft as the amount of oxygen that
can be inhaled is insufficient to ensure adequate oxygenation above that
height. None the less, they are a useful alternative to the mask when
operating in the 12,000 to 18,000ft height band. Cannulas should not be used
with diluter demand regulators other than the Mountain High electronic regulator.
There is no doubt that a diluter demand system is to be preferred for
installation in an aircraft. It makes more effective use of the limited oxygen
quantity and a 630 litre cylinder should last at least 5 hours, the actual
duration depending upon the height. When fitted with a mechanical or
electrical flow indicator, positive indication is given of the flow of oxygen to
the pilot. In conjunction with a well-fitting demand mask it can be used
safely up to 35,000 ft. However, the equipment is expensive to buy initially
and overhaul costs, if needed, are high.
The continuous flow system is a reliable, effective and cheaper
alternative provided it's limitations are well understood. The economizer bag
and flow indicator, if fitted, require careful monitoring to ensure that oxygen
is flowing during flight. For a given cylinder capacity, the duration is less
than when using a diluter demand system; on 'Normal' flow, a 630 litre
cylinder will last some 5 hours which reduces to approximately 2 1/2 hours on
'High' flow. The maximum safe altitude is 30,000 ft and it is not recommended
that this is exceeded.
To obtain safe service from your oxygen system:
High altitude flight and rapid descents have other hazards to the body,
above 30,000 ft nitrogen bubbles released into the blood stream can cause
decompression sickness, and gas can become trapped in the gut, sinuses or
teeth. If you suffer any discomfort at altitude then you should return, land
and seek advice.
Oxygen equipment is life supporting; if it should fail above 25,000 ft then
even a trained pilot will fail to notice before he loses consciousness. Thus it
is prudent to ensure that you:
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Contact: Dickie Feakes at 01869 245948 or 07710
221131. Email: - dickie@bas.uk.net |