RTO MP-062 / HFM-050 - FTP Directory Listing - Nato

KN1-6
Composition of the Inspired Gas
Hypoxia is prevented when the cabin is pressurised in fighter type aircraft by increasing the concentration of oxygen in
the inspired gas as the aircrew are breathing gas from the breathing gas delivery system throughout flight. Ideally the
concentration of oxygen in the gas breathed at altitude should always be such as that the partial pressure of oxygen
(Po2) in the alveolar gas is equal to or greater than the normal value associated with breathing air at ground level i.e.
103 mm Hg (figure 3). This is the standard employed in virtually all military oxygen delivery systems. Should
economy in the use of oxygen demand it then it may be acceptable for the inspired oxygen concentration to be reduced
to the level required to maintain a minimum alveolar PO2 of 75 mm Hg – equivalent to breathing air at an altitude of
5,000 feet (Ernsting, 1966).
Whilst the toxic effects of breathing high concentrations of oxygen do not arise in the low pressure differential cabins
of fighter aircraft, breathing high concentrations of oxygen has two important disadvantages due to the concomitant
reduction of the concentration of nitrogen in the inspired gas. It produces acceleration-induced atelectasis and delayed
otitic barotrauma (Ernsting, 1995b). Thus exposure to +Gz accelerations greater than 3-4 G, whilst breathing 100%
oxygen rapidly induces marked absorption collapse of the basal parts of the lungs which is associated with a large right
to left shunt due to venous blood flowing through collapsed lung without undergoing oxygenation. The conditions also
gives rise to bouts of uncontrollable coughing. The high rate of absorption of gases from non-ventilated cavities when
100% oxygen is breathed also gives rise to delayed otitic barotrauma with ear discomfort and deafness. These effects of
breathing high concentrations of oxygen on the lungs and middle ear cavity are prevented by maintaining the
concentration of nitrogen in the inspired gas at or above 40% (Green, 1963; Haswell et al, 1986). Inflight experience
and limited laboratory studies (Ernsting, 1965) support the theoretical conclusion that at altitude this effect of nitrogen
is a function of the concentration of this gas. Thus in order to avoid acceleration induced lung collapse and delayed
otitic barotrauma the concentration of oxygen at altitude should not exceed 60% (Ernsting, 1995b). This requirement
for the limit of the concentration of oxygen in the inspired gas at altitude eventually conflicts at the higher cabin
altitudes with the concentration of oxygen required to prevent hypoxia (figure 3). The degree of this conflict depends
essentially upon the sustained G capability of the aircraft at high altitude. Thus most current fighter aircraft are
incapable of sustaining +Gz accelerations greater than 2-3G at aircraft altitudes above 35,000-40,000 feet. The +Gz
acceleration performance of the new generation of agile combat aircraft is, however, well maintained at aircraft
altitudes considerably above 50,000 feet. The intersection of the two oxygen concentrations altitude curves at 60%
oxygen at a cabin altitude of 24,500 feet requires close control of the concentration of oxygen delivered by the oxygen
system at cabin altitudes above 18,000 – 20,000 feet if both requirements are to be fully met.
The hypoxia which can follow decompression of the pressure cabin to altitudes above 30,000 feet in spite of 100%
oxygen being delivered to the mask cavity immediately after the decompression is a further factor which determines the
concentration of oxygen in the gas breathed prior to the decompression (Ernsting, 1963; and 1995b). The alveolar PO2
prior to the decompression must be high enough to ensure that the fall of alveolar PO2 induced by the reduction of the
pressure in the respiratory tract does not reduce the alveolar PO2 to below 30 mm Hg. The concentration of oxygen
required in the gas breathed prior to the decompression to prevent the alveolar PO2 falling below 30 mm Hg is a
function of the pressurisation schedule of the pressure cabin and the final cabin altitude (Ernsting, 1995b). This
relationship for a typical fighter aircraft with a maximum aircraft altitude of 50,000 feet is presented in figure 3.
Increasing the differential pressure of the cabin will increase the concentration of oxygen which must be breathed at a
given cabin altitude to prevent hypoxia on rapid decompression (figure 4). Similarly the lower the intrapulmonary
pressure after the decompression, which is determined at high altitude by the pressure breathing-altitude schedule
employed in the partial pressure assembly system, the higher the concentration of oxygen which must be breathed to
prevent hypoxia on rapid decompression (figure 4). Thus the conflict between the minimum concentration of oxygen
required to prevent hypoxia after a rapid decompression and the maximum permitted concentration to avoid
acceleration atelectasis will be increased by an increase in the cabin differential pressure.
Compromises in Future Aircraft
It has been seen that the compromise between pressurisation of the pressure cabin and the concentration of oxygen in
the breathing gas delivered to the aircrew employed over the last forty years has been to employ a maximum cabin
differential pressure of 5.0 (US) to 5.25 (UK) Lb in -2 . The manner in which this cabin altitude has varied with aircraft
altitude has, however, differed – the US military specification has employed an isobaric schedule, whilst in the UK and
FR the maximum pressure differential has only been operative at aircraft altitudes above 35,000-40,000 feet (figure 1).
The latter pressurisation schedule has the advantage in avoiding very high rates of increase of the absolute pressure in
the cabin during descent of the aircraft from high altitude (Ernsting, 1995a).