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became negligible [the incidence remained high however in lower cabin differential pressure reconnaissance aircraft in
which the crew were exposed for several hours to cabin altitudes as high as 26,000 to 28,000 feet].
The extensive studies conducted by the Armstrong Laboratory using Doppler ultrasound to detect and semi-quantify
the occurrence of venous gas emboli in the right side of the heart have demonstrated that significant quantities of
venous gas emboli occur in subjects exposed for several hours to altitudes as low as 15,000 feet when the inspired gas
contains nitrogen (Webb and Pilmanis, 1993). This group of investigators has advocated that the cabin altitudes of
combat aircraft, which may fly at very high altitudes for several hours, should not exceed 16,000 feet (Webb et al,
1993). Although serious symptoms of decompression sickness are extremely rare at cabin altitudes below 20,000 feet,
serious decompression sickness could well develop rapidly after the rapid decompression to high altitude of a pilot with
severe venous gas embolism. Further experimental evidence is urgently required with respect to the possibility and the
incidence of symptoms of decompression sickness at altitudes between 16,000 and 20,000 feet under the conditions to
be expected in modern and future combat aircraft, viz. the crew performing light work and breathing gas containing 30-
40% nitrogen. At present it is considered that decompression sickness should be avoided by not allowing the cabin
altitude to exceed 18,000 feet. Short duration exposures to cabin altitudes as high as 20,000 feet are very unlikely to
produce a significant incidence of decompression sickness.
Ventilation of the Middle Ears and Sinuses
Failure to ventilate the middle ear during descent gives rise to increasing discomfort in the ear and deafness, and
eventually to otitic barotrauma. The discomfort produced in the ear by descent and the need to occlude the nostrils to
perform the Frenzel manoeuvre can distract the aircrew member from his primary task. It is vital to avoid otitic and
sinus barotrauma, which often prevent the individual flying for several days. It is highly desirable that in war aircrew
with an upper respiratory tract infection can continue to fly operationally. All these considerations argue for the
minimum rate of increase and decrease of cabin altitude during flight. Other factors which have been discussed in the
introduction to this paper do not allow this solution in high-performance combat aircraft, although the isobaric cabin
pressurisation schedule of the United States Military Specification for low pressure differential cabins provides the
ideal of no change of cabin altitude during aircraft manoeuvres between altitudes of 8,000 and 23,000 feet (Figure 1).
The rate of increase of pressure within the cabin on descent of a combat aircraft should be as low as possible to avoid
ear discomfort and the minimise the frequency with which the nose must be occluded to perform the Frenzel
manoeuvre and the incidence of otitic barotrauma. Aeromedical advice in the United Kingdom has recommended
(Roxburgh, 1964) that on descent, the rate of increase of absolute pressure in the cabin should not exceed 2.0 Lb in -
2 -2
/min over a change of pressure of 1.0 Lb in . This advice was based on the incidence of otitic barotrauma in flight and
hypobaric chamber studies.
Integrated Physiological Requirements
It is considered that in respect of the avoidance of hypoxia due to failure of the oxygen supply and decompression
sickness, the cabin altitude of a combat aircraft should not exceed 20,000 feet when the aircraft is at its service ceiling.
Furthermore, it is highly desirable that the cabin altitude does not exceed 18,000 feet on the vast majority of operational
sorties. Avoidance of ear discomfort and the minimisation of the frequency with which the nostrils have to be occluded
to introduce gas into the middle ear, and of the risk of otitic barotrauma, requires that the rate of increase of cabin
pressure during descent of the aircraft should be as low as possible, and ideally should not exceed 2.0 Lb -2 /min for an
increase of pressure of 1.0 Lb in -2 .
Physiological Aspects of Rapid Decompression of the Cabin
The pressurisation of the cabin of a combat aircraft may fail due to cessation of the flow of air into the cabin (due to
engine failure of malfunction of the environmental control system), opening of the cabin outlet valve (due to a failure
of the control system or selection by the pilot) or a defect in the pressure cabin structure (due to enemy action, human
error, mechanical failure or as a prelude to ejection). The cabin altitude-time profile on decompression is determined by
the ratio of the effective area of the defect in the structure to the volume of the cabin, the magnitude of the air flow into
the cabin, the magnitude of the aerodynamic suction and the altitude-time profile of the aircraft, including the initial
aircraft altitude, the time taken to initiate descent and the rate of descent of the aircraft.
The hazards of a rapid decompression of the cabin include physical injury due to the structural failure and very high air
flow velocities in the cockpit, lung barotrauma, hypoxia, decompression sickness, cold injury and hypothermia. Of
particular concern in relation to the cabin differential pressure at the instant at which the decompression occurs are
pulmonary barotrauma and hypoxia.