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Pressureization

Introduction:

  • Pressurization systems are part of the life support systems required to keep us alive for high altitude operations
Pilot Handbook of Aeronautical Knowledge, Airplane Pressurization System
Figure 1: Pilot Handbook of Aeronautical Knowledge,
Airplane Pressurization System

Pressurization:

  • The pressurization of an aircraft to allow high altitude operations due to loss in pressure and protecting occupants against the effects of hypoxia
    • 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
  • High altitude operations allow for lower fuel consumption for a given airspeed (efficiency) and avoidance of weather and turbulence above storms
  • Oxygen masks prevent hypoxia but they do not help with sinus and ear blocks or decompression sickness, also oxygen masks can be uncomfortable
  • Pressurized air is generally obtained from an aircrafts turbocharger or compressor section of turbine aircraft
    • Piston-powered aircraft may use air supplied from each engine turbocharger through a sonic venturi (flow limiter)
    • Gas-turbine-powered aircraft use air supplied from the compressor stage of the engine which is conditioned for the cabin
  • A cabin pressurization system typically maintains a cabin pressure altitude of approximately 8,000' at the maximum designed cruising altitude of an aircraft [Figure 1:]
    • This prevents rapid changes of cabin altitude that may be uncomfortable or cause injury to passengers and crew
  • The pressurization system permits a reasonably fast exchange of air from the inside to the outside of the cabin to eliminate odors and to remove stale air
  • Definitions:
    • Aircraft altitude: the actual height above sea level at which the aircraft is flying
    • Ambient temperature: the temperature in the area immediately surrounding the aircraft
    • Ambient pressure: the pressure in the area immediately surrounding the aircraft
    • Cabin altitude: cabin pressure in terms of equivalent altitude above sea level
    • Differential pressure: 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
    Pilot Handbook of Aeronautical Knowledge, Standard Pressure Chart
    Figure 2: Pilot Handbook of Aeronautical Knowledge,
    Standard Pressure Chart
  • The cabin pressure control system provides cabin pressure regulation, pressure relief, vacuum relief, and the means for selecting the desired cabin altitude in the isobaric and differential range
  • In addition, dumping of the cabin pressure is a function of the pressure control system
  • A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish these functions
  • The cabin pressure regulator controls cabin pressure to a selected value in the isobaric range and limits cabin pressure to a preset differential value in the differential range [Figure 2]
  • When an aircraft reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed, a further increase in aircraft altitude will result in a corresponding increase in cabin altitude
  • Differential control is used to prevent the maximum differential pressure, for which the fuselage was designed, from being exceeded
  • This differential pressure 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
  • The cabin air pressure safety valve is a combination pressure relief, vacuum relief, and dump valve
  • The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure
  • The vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure
  • The flight deck control switch actuates the dump valve
  • When this switch is positioned to ram, a solenoid valve opens, causing the valve to dump cabin air to atmosphere
  • The degree of pressurization and the operating altitude of the aircraft are limited by several critical design factors
  • Primarily, the fuselage is designed to withstand a particular maximum cabin differential pressure
  • 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 in [Figure 3]

Pilot Handbook of Aeronautical Knowledge, Pressurization Instruments
Figure 3: Pilot Handbook of Aeronautical Knowledge,
Pressurization Instruments

Cabin Pressure Altitude:

  • The equivalent altitude inside of the cabin

Cabin Differential Pressure:

  • The difference in pressure between the cabin and the outside air

Sonic Venturi:

  • Limits the amount of air taken from turbo by accelerating air to sonic speeds creating a shock wave which acts as a barrier
  • This air is very hot and must be run through a heat exchanger to cool it
  • After being cooled, air is sent to the cabin via heating and ventilation outlets

Regulation:

  • Outflow valve: Allows for air to exit the cabin at a controlled rate which results in the cabin becoming pressurized
  • Safety/Dump Valve: If the outflow valve fails, the dump valve will release excess pressure (can be manually activated) by a squat switch to prevent pressurization on the ground
  • Vacuum Relief Valve: Allows ambient air into the cabin

Instrumentation:

  • Cabin/differential Pressure Indicator: Works like an altimeter but has two references, outside air pressure and cabin pressure
  • Cabin Rate of Climb Indicator: Indicates the rate of change in cabin pressure

Pressurization Controls:

  • Basic Preset: When cabin pressure reaches a preset value (about 8,000')
  • The Outflow Valve begins closing until max cabin differential pressure is reached and then cabin altitude begins to climb
  • Cabin rate of climb will be slightly less than airplane rate of climb due to higher air density in the cabin
  • Cabin Pressure Control: Pilot selects altitude pressurization begins and can preset the rate at which the cabin pressurizes
  • Differential Range System: Works to prevent exceeding differential pressure limits
  • Isobaric Range: Works to maintain a preset cabin pressure

Cabin Decompression:

  • Decompression: the inability of the aircraft's pressurization system to maintain its designed pressure differential
  • Problems can be caused by a malfunction in the pressurization system or structural damage to the aircraft
  • 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 seat-belts 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
  • Rapid descent from altitude is necessary if these problems are to be minimized
  • Automatic visual and aural warning systems are included in the equipment of all pressurized aircraft
  • Gradual Decompression:
    • Slow decompression is dangerous because it may be hard to detect until after you are already experiencing the effects of hypoxia. Annunciation lights are installed to aid in detection
  • Rapid Decompression:
    • A change in cabin pressure in which the lungs decompress faster than the cabin, resulting in no likelihood of lung damage
    • Decompression in 1-10 seconds usually associated with major structural damage
    • Cabin will fill with fog because of immediate condensation of water vapor
    • The cabin will become extremely cold because of immediate loss of heated air
    • Also at high altitudes you will only have up to 12 seconds of useful consciousness
    • 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' or higher)
    • It is recommended that the crew-members select the 100% oxygen setting on the oxygen regulator at high altitude if the aircraft is equipped with a demand or pressure demand oxygen system
  • Explosive Decompression:
    • Refers to a sudden marked drop in the pressure of a system that occurs faster than the lungs can decompress
    • Generally it results from some sort of material fatigue or engineering failure, causing a contained system to suddenly vent into the external atmosphere
    • Lungs take about 0.2 seconds to decompress without restriction (masks)
    • Anything less than 0.5 seconds is considered explosive decompression
    • Associated with explosive violence and is potentially dangerous
    • During an explosive decompression, there may be noise, and one may feel dazed for a moment
    • The rapid loss in pressure may cause a cloud to form due to the rapid drop in temperature and change in relative humidity
    • Dust or loose objects may become airborne and move around the cabin

Decompression Sickness:

  • When an occupant of any aircraft is observed or suspected to be suffering from the effects of DCS, 100% oxygen or available aircraft oxygen will be started and the pilot shall immediately descend to the lowest possible altitude and land at the nearest civilian or military installation suitable for safe landing and obtain qualified medical assistance
  • Consideration shall be given to whether the installation is in proximity to a medical re-compression chamber
  • It is extremely important to be able to recognize symptoms and convey this and the altitude profile to medical support

Pressurization System Errors:

  • Cabin does not decompress:
    • Outflow valve is blocked, a safety valve should decompress the aircraft, triggered with WOW (Weight on Wheels)
  • Cabin does not pressurize:
    • Outflow valve is stuck open

References: