AS Chpt 10

• A one square inch column of air stretching from sea level into space weighs 14.7 pounds. Therefore, it can be stated that the pressure of the atmosphere, or atmospheric pressure, at sea level is 14.7 psi
• Atmospheric pressure:

  • Aka barometric pressure
  • Measured with a barometer, units in inch of mercury or mm of mercury
• Atmospheric pressure - Measurements:

  • Observing the height of mercury in a column when air pressure is exerted on a reservoir of mercury into which the column is set
  • The column must be evacuated so air inside does not act against the mercury rising
  • A column of mercury 29.92 inches high weighs the same as a column of air that extends from sea level to the top of the atmosphere & has the same cross-section as the column of mercury
• Aviators often interchange references to atmospheric pressure between linear displacement (e.g. inches of mercury) & units of force (e.g. psi)
• Standard atmospheric pressure at sea level is also known as 1 atmosphere or 1 atm.
• The atmospheric pressure decreases with increasing altitude because the column of air that is weighed is shorter. The decrease in pressure is a rapid one & at 50,000 feet, the atmospheric pressure has dropped to almost one-tenth of the sea level value
• Temperature & altitude:

  • Temperature variations in the atmosphere are of concern to aviators
  • Weather systems produce changes in temperature near the earth’s surface
  • Temperature also changes as altitude is increased
• Temperature & altitude:

  • Most civilian aviation takes place in the troposphere (On average, it ranges from the earth’s surface to about 38,000 feet above it.) in which temperature decreases as altitude increases
  • The rate of change is somewhat constant at about –2 °C or – 3.5 °F for every 1,000 feet of increase in altitude
  • The upper boundary of the troposphere is the tropopause. It is characterized as a zone of relatively constant temperature of –57 °C or –69 °F
• Pressurisation term - Cabin altitude:

  • Given the air pressure inside the cabin, the altitude on a standard day that has the same pressure as that in the cabin
  • Rather than saying the pressure inside the cabin is 10.92 psi, it can be said that the cabin altitude is 8,000 feet (MSL)
• Pressurisation term - Cabin differential pressure:

  • The difference between the air pressure inside the cabin & the air pressure outside the cabin.
  • Cabin pressure (psi) – ambient pressure (psi) = cabin differential pressure (psid or Δ psi)
• Pressurisation term - Cabin rate of climb:

  • The rate of change of air pressure inside the cabin, expressed in feet per minute (fpm) of cabin altitude change
• Rate of change of pressure:

  • Common for aircraft to climb at a rate of 1000 ft/min or more
  • Engines are designed to maintain their performance with the changing atmospheric conditions during a climb
  • However, the effect of rapid altitude changes on the human body causes physical pain & discomfort
  • If the cabin pressure decreases violently, the nitrogen & other gases in solution in the blood steam expand rapidly in the form of bubbles
  • This causes acute pain & injury, but would not normally occur except in the event of explosive decompression
• If however, the rates of change are large but not violent, the most common effects are: 

  • Sickness 
  • Expansion of gases in the abdomen (uncomfortable) 
  • Expansion of gases in the ear
• Pressurizing an aircraft cabin assists in making flight possible in the hostile environment of the upper atmosphere. The degree of pressurization and the operating altitude of any aircraft are limited by critical design factors
• A cabin pressurization system must accomplish several functions:

  • To ensure adequate passenger comfort & safety
  • It must be capable of maintaining a cabin pressure altitude of approx 8,000 ft or lower regardless of the cruising altitude of the aircraft. This is to ensure that passengers & crew have enough oxygen present at sufficient pressure to facilitate full blood saturation
• A cabin pressurization system must accomplish several functions:

  • A pressurization system must also be designed to prevent rapid changes in cabin pressure, which can be uncomfortable or harmful to passengers & crew
• A cabin pressurization system must accomplish several functions:

  • A pressurization system should circulate air from inside the cabin to the outside at a rate that quickly eliminates odors & removes stale air
• A cabin pressurization system must accomplish several functions:

  • Cabin air must also be heated or cooled on pressurized aircraft. Typically, these functions are incorporated into the pressurization source
• Structure consideration - Positive differential:

  • To pressurize, a portion of the aircraft designed to contain air at a pressure higher than outside atmospheric pressure must be sealed. Compressible seals around doors combine with various other seals, grommets, and sealants to essentially establish an air-tight pressure vessel. This usually includes the cabin, flight compartment, & the baggage compartments
• Structure consideration - Positive differential:

  • Air is then pumped into this area at a constant rate sufficient to raise the pressure slightly above that which is needed. Control is maintained by adjusting the rate at which the air is allowed to flow out of the aircraft
• Structure consideration - Positive differential:

  • Controlling the cabin to ground level values provides maximum comfort; however, if the cabin altitude was held at sea level whilst the aircraft was climbing, the resultant differential pressure would be much higher than is necessary
• Structure consideration - Positive differential:

  • Thus an important point arising when deciding on cabin altitude and rate of change values is the loss of payload & increased fuel consumption due to increased structural weight required to give the necessary strength to allow very high differential pressure to be used if low cabin altitudes are to be established at very great heights
• Structure consideration - Negative differential:

  • To save weight, the aircraft designer only considers the inside pressure to be greater than the outside. In the design, the fuselage does not cater for a reverse or negative differential pressure and yet, such an instance can occur
• Structure consideration - Negative differential:

  • In the case of perfectly sealed fuselage being flown with a greater pressure inside than outside & then suddenly being dived to a low altitude where the outside atmospheric pressure would be greater than inside
• Structure consideration - Negative differential:

  • To cater for such an emergency would complicate the structural design so an inward relief valve is fitted, set to open generally at a negative differential pressure of 0.5 psi
• Pressure relief setting for different aircraft:

  • It will be seen from the table that if the normal equipment fails to control normal positive differential, there is a secondary safeguard to ensure that the differential pressure does not reach the maximum proof test figure. This secondary safeguard is known as safety valve and is set so that normally it does not come into operation:

  
  

  • Boeing 707:

  Normal working pressure: 8.6 psi

  Safety valve pressure: 9.42 psi

  Inward relief valve: 0.5 psi
• Structure consideration:

  • A key factor in pressurization is the ability of the fuselage to withstand the forces associated with the increase in pressure inside the structure versus the ambient pressure outside. This differential pressure can range from 3.5 psi for a single-engine reciprocating aircraft, to approximately 9 psi on high-performance jet aircraft
• Differential pressure (psid) is calculated by subtracting the ambient air pressure from the cabin air pressure
• Sources of pressurised air:

  • The source of air to pressurize an aircraft varies mainly with engine type
  • Reciprocating aircraft have pressurization sources different from those of turbine-powered aircraft
  • Note that the compression of air raises its temperature
  • A means for keeping pressurization air cool enough is built into most pressurization systems
  •  It may be in the form of a heat exchanger, using cold ambient air to modify the temperature of the air from the pressurization source
• Reciprocating Engine Aircraft - 3 typical sources of air used to pressurize reciprocating aircraft:

  • Supercharger
  • Turbocharger
  • Engine driven compressor
• Supercharger:

  • Mechanically driven by the engine
  • Despite engine performance increases due to higher induction system pressure, some of the engine output is utilized by the supercharger
  • Have limited capability to increase engine performance
• Supercharger:

  • If supplying both the intake & the cabin with air, the engine performance ceiling is lower than if the aircraft were not pressurized
  • Must be located upstream of the fuel delivery to be used for pressurization
• Turbocharger:

  • Driven by engine exhaust gases
  • The turbocharger impeller shaft extends through the bearing housing to support a compression impeller in a separate housing
• Turbocharger:

  • By using some of the turbocharger compressed air for cabin pressurization, less is available for the intake charge, resulting in lower overall engine performance
• Turbocharger:

  • The otherwise wasted exhaust gases are put to work in the turbocharger compressor, enabling high altitude flight with the benefits of low drag and weather avoidance in relative comfort and without the use of supplemental oxygen
• Turbine engine aircraft:

  • The main principle of operation of a turbine engine involves the compression of large amounts of air to be mixed with fuel & burned
  •  Bleed air from the compressor section of the engine is relatively free of contaminants
• Turbine engine aircraft:

  • It is a great source of air for cabin pressurization
  • However, the volume of air for engine power production is reduced
  • The amount of air bled off for pressurization compared to the overall amount of air compressed for combustion is relatively small but should be minimized
• Turbine engine aircraft:

  • The pressurizing an aircraft using turbine engine compressor bleed air is to have the bleed air drive a separate compressor that has an ambient air intake
• Turbine engine aircraft:

  • A turbine turned by bleed air rotates a compressor impeller mounted on the same shaft.
  • Outside air is drawn in and compressed
• Turbine engine aircraft:

  • It is mixed with the bleed air outflow from the turbine and is sent to the pressure vessel
  • Turboprop aircraft often use this device, known as a turbocompressor