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AP4ATCO - Cabin Pressurisation
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- The following SKYbrary Articles:
a) article description Cabin pressurization principles. Cabin decompression.
b) source (IANS) 6.2.2
c) additional sources
d) SKYbrary source: * FAA Pilot’s Handbook of Aeronautical Knowledge – chapter 6 Airbus - Cabin Operations - Cabin Decompression Awareness Pressurisation Problems: Guidance for Controllers
Modern aircraft operate at high altitudes and can achieve high rates of climb. In order to take advantage of these properties the interior of an aircraft flying at high altitude is pressurized to allow passengers and crew to function normally without any need for additional oxygen. Cabin pressurization systems are designed to produce conditions equivalent to those at approximately 8000 feet.
In a typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit capable of containing air under a pressure higher than outside atmospheric pressure. On aircraft powered by turbine engines, bleed air from the engine compressor section is used to pressurize the cabin. Superchargers may be used on older model turbine-powered aircraft to pump air into the sealed fuselage. Piston-powered aircraft may use air supplied from each engine turbocharger through a sonic venturi (flow limiter). Air is released from the fuselage by a device called an outflow valve. By regulating the air exit, the outflow valve allows for a constant inflow of air to the pressurized area.
For more information about system used check SKYbrary’s article: Aircraft's pressurisation system
When the aircraft is climbing, the change of cabin pressure is proportional to the change of the ambient pressure, in order to control the structural stress on the fuselage from the inside. This is performed automatically by sophisticated control system. However, if the cabin pressure is manually controlled or in case of system degradation, care should be taken to ensure that the climb rates are safe and ensure that the structural stress is not exceeding the maximum limit. The maximum rate of climb and ceiling are affected. When exceeded the aircraft structure is overstressed from inside and structural failure (explosion) is possible.
The parameter here is called differential pressure. It is the difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air-conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure and is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors.
Pressurization of the aircraft cabin is an accepted method of protecting occupants against the effects of hypoxia. Passengers comfort is also a factor. Usually the best comfort is achieved at rates of climb of 1500 feet per minute.
Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gauge indicates the difference between inside and outside pressure. This gauge should be monitored to assure that the cabin does not exceed the maximum allowable differential pressure. A cabin altimeter is also provided as a check on the performance of the system. In some cases, these two instruments are combined into one. A third instrument indicates the cabin rate of climb or descent. A cabin rate-of-climb instrument and a cabin altimeter are illustrated below:
Decompression is defined as the inability of the aircraft’s pressurization system to maintain its designed pressure differential. This can be caused by a malfunction in the pressurization system or structural damage to the aircraft.
Physiologically, decompressions fall into two categories:
1. Explosive decompression—a change in cabin pressure faster than the lungs can decompress, possibly causing lung damage. Normally, the time required to release air from the lungs without restrictions, such as masks, is 0.2 seconds. Most authorities consider any decompression that occurs in less than 0.5 seconds to be explosive and potentially dangerous.
During an explosive decompression, there may be noise, and one may feel dazed for a moment. The cabin air fills with fog, dust, or flying debris. Fog occurs due to the rapid drop in temperature and the change of relative humidity. Normally, the ears clear automatically. Air rushes from the mouth and nose due to the escape of air from the lungs, and may be noticed by some individuals.
Explosive depressurisation/decompression is more likely to occur in small volume pressurized aircraft, such as military jets or VLJs than in large pressurized aircraft. That is because of much smaller cabin volume ratios in such planes compared to airliners. A typical small pressurized aircraft can be expected to decompress on the order of 10 to 200 times faster than large aircraft (cabin volume of Learjet is over 200 times smaller that volume of B747’s cabin).
2. Rapid decompression—a change in cabin pressure in which the lungs decompress faster than the cabin, resulting in no likelihood of lung damage.
Rapid decompression decreases the period of useful consciousness because oxygen in the lungs is exhaled rapidly, reducing pressure on the body. This decreases the partial pressure of oxygen in the blood and reduces the pilot’s effective performance time by one-third to one-fourth its normal time. For this reason, an oxygen mask should be worn when flying at very high altitudes (35,000 feet or higher). It is recommended that the crewmembers select the 100 percent oxygen setting on the oxygen regulator at high altitude if the aircraft is equipped with a demand or pressure demand oxygen system.
3. Gradual or slow decompression - usually dangerous only when it has not been detected at an early stage. Automatic visual and aural warning systems do not always provide an indication of a slow decompression until its effects have become significant.
Decompression possible effects
The primary danger of decompression is hypoxia. Quick, proper utilization of oxygen equipment is necessary to avoid unconsciousness. Another potential danger that pilots, crew, and passengers face during high altitude decompressions is evolved gas decompression sickness. This occurs when the pressure on the body drops sufficiently, nitrogen comes out of solution, and forms bubbles that can have adverse effects on some body tissues.
Decompression caused by structural damage to the aircraft presents another type of danger to pilots, crew, and passengers––being tossed or blown out of the aircraft if they are located near openings. Individuals near openings should wear safety harnesses or seatbelts at all times when the aircraft is pressurized and they are seated. Structural damage also has the potential to expose them to wind blasts and extremely cold temperatures.
In case of decompression EPT (Effective Peformance Time) or TUC (Time of Useful Consciousness) is the amount of time in which a person is able to effectively or adequately perform flight duties with an insufficient supply of oxygen. Refer to SKYbrary’s article Time of Useful Consciousness to find out EPT/TUC at various altitudes.
Rapid descent from altitude (usually to 8000 – 10000 feet) is necessary if these problems are to be minimized. Automatic visual and aural warning systems are included in the equipment of all pressurized aircraft.
Guidance for controllers
A wide range of practical problems could arise during decompression and the following emergency descent. To find them out as well as suggested controller’s actions please refer to SKYbrary article: “Pressurisation Problems: Guidance for Controllers”
1. [Question type: true or false]
Q: Change of cabin pressure is proportional to the change of the ambient pressure, in order to control the structural stress on the fuselage. A1:True A2: False
Correct answers: A1.
2. [Question type: multiple response]
Q: Choose decompression possible effects for ATCO: A1:Hypoxia A2: RTF communication reduced quality A3:Aircraft structural damage A4: Rapid descent A5: Possible diversion
Correct answers: A2, A3, A4, A5
26. Factors affecting aircraft performance during cruise