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Cabin Pressurisation – The Mechanism

It is not practical to maintain sea level conditions in flight. For example, a cabin differential pressure of 1 Kg/sq cm (nearly 1 ATA) generates more than 1,000 gm/cm2 pressure on the cabin wall and the transparency. If the cabin altitude of 2,500 m (8,000 ft) is accepted,the pressure differential would now be reduced by as much as a quarter to 750 gm/sq cm. This has been gainfully utilised in pressurisation schedule of commercial aircraft. In addition, combat aircraft may have an added risk of sudden loss of cabin pressure due to enemy action, where the greater cabin differential is hazardous. Hence, aircraft are designed with pressure cabins which represent a compromise between the physiological ideal and the aircraft role and performance.

Such an acceptable physiological compromise has defined the cabin pressurisation schedule in aviation, where the high pressure differential cabins usually maintain cabin altitude of 2,500 m (8,000 ft) or below almost throughout the flight envelopes. Low pressure differential cabins, on the other hand, never allow cabin altitude to rise above 6 Km (20,000 ft), even if the aircraft cruises at its service ceiling.

Passenger carrying aircraft require high differential cabins otherwise passenger would require personal oxygen equipment to prevent hypoxia. Such a situation is unacceptable to passengers. The only exceptions being a medical evacuation or when a physically debilitated person must travel where either on-board Oxygen is used or a personal Oxygen system must be carried by the patient.

Combat aircraft on the other hand, supplement the low pressure differential with the help of mandatory on-board Oxygen system. This ensures that the aircrew remain in their optimal physiological state to withstand the aviation and operational stresses.

Pressurisation of the cabin is achieved by tapping air from the compressor of the engine. This air is passed through heat exchangers before being fed into the cabin. A relief valve controls the amount of pressurisation.

Generally, aircraft pressurisation commence, say, from an altitude of about 5000 feet. While the aircraft continues ascending, the cabin pressurisation can be maintained at around the same level, till the maximum pressure differential is attained. If the aircraft continues its ascent, the cabin altitude rises too, hereafter maintaining the same pressure differential.

 In commercial aircraft the pressurisation commences at lower altitudes and cabin altitude is maintained till the ceiling of the aircraft. However, the cabin pressurisation in combat aircraft commences at higher altitudes and is maintained at the defined level till the maximum pressure differential is reached. From this point upwards, an increase in altitude is associated with increase in cabin altitude at the defined pressure differential.

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Reference

1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.

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Acknowledgement Image courtesy Wikimedia Commons

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