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Cabin Pressurisation – An Introduction

On 26 January 2011, a Qantas Boeing 737-400 made an emergency descent of about 8000m [*], when the aircraft lost cabin pressure after about 30 minutes of flight. This flight, with 99 passengers, from Adelaide to Melbourne had oxygen masks dropping in the cabin, causing a scare amongst its 99 passengers. 



Climbing to altitude in an unpressurised aircraft, results in threats of hypoxia, decompression sickness, besides exposing the occupants to hypothermia. These physiological disturbances occurring due to reducing environmental pressures can be overcome by artificially raising the pressure in the occupant compartment, i.e. cabin pressurisation.



Although it may be ideal to maintain sea level conditions throughout the flight envelope of the aircraft, this could lead to serious issues in terms of the design. There shall be a requirement of a powerful pressurisation system, which in turn adds to the weight penalty. So also, such a system increases the fuel requirement while reducing the aircraft performance. Most importantly, such a design is a structural safety hazard with a very large pressure differential across the walls of the pressurised cabin and the ambient. Therefore, compromises are made between the ideal requirement of aircraft cabin at 1 ATA and the likelihood of failure of cabin pressurisation. Hence, the compromises are defined either as comfort oriented, as in commercial aviation, or strategic role dependent, as in combat aircraft.



In passenger aircraft, the cabin pressurisation system is meant for keeping the passengers comfortable. The cabin altitude is maintained at such levels where there is no necessity to provide Oxygen for either the passengers or the crew. In combat aircraft, where the risk of enemy action is always there and loss of cabin pressure is to be anticipated more frequently, pressurisation system provides lower levels of pressurisation.



It is convenient to express the amount of pressurisation being afforded in terms of ‘Pressure Differential’. The term ‘pressure differential’ implies the difference in the cabin/cockpit pressure and the ambient pressure. For example, an aircraft flying at 40,000 ft (12 Km approx.) has the following parameters:

  • Ambient Pressure at 40,000 ft = 2.72 psi ( = 141 mm Hg/ 187.9 mb/ 20 K pa/ 0.21 kg.sq cm)

    Say, the Cabin Altitude is to be maintained at 10,000 feet. This shall correspond to –

  • Cabin Pressure equivalent to 10,000 ft altitude = 10.1 psi ( = 523 mm Hg/ 697.1 mb/ 65 K pa/ 0.6 kg.sq cm)

    Therefore, since Pressure Differential = Pressure at Cabin Altitude – Ambient Pressure

  • Pressure Differential = 10.1 psi – 2.72 psi = 7.38 psi ( = 382 mm Hg/ 509.2 mb / 45 K pa / 0.4 kg.sq cm).

Aircraft have either high pressure differential or low pressure differential. High pressure differential (8 to 10 psi) is available in commercial aircraft, while low pressure differential (3.5 to 5 psi) is meant for combat aircraft.

The advantage of a high pressure differential cabin is that it protects the occupants of the cabin from any serious effects of hypoxia without the need for personal breathing equipment. Although at a conventional cabin altitude of 2,500 m (8,000 ft) there is a small decrement in crew performance due to hypoxia, including some loss of night visual acuity, this is neither significant nor affects the safety of the flight. Additionally, there is no risk of decompression sickness at this altitude. As the cruising altitude of the passenger carrying aircraft increases, the cabin pressure differential continues to increase concurrently .

Large bomber and military transport aircraft, with ability to operate at high altitude, also have high differential cabins. However, these aircraft usually have facility select a low pressure differential when in combat zone.

Conventional low pressure differential cabin is found in the fighter aircraft, where the aircrew uses personal oxygen equipment routinely. Such aircraft usually have a small pressure cabin, which results in a rapid loss of cabin pressure if the canopy is damaged.

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Reference

  • Qantas Jet’s 8000m Horror Dive

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

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

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

In case you wish to share your experiences, offer comments/feedback on this Blog, you may use the form below or send an e-mail to avmed@avmed.in

Acknowledgement Image courtesy Wikimedia Commons

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