Aircraft Cabin Pressurization Basics

• Aircraft pressurization makes flight possible in the hostile upper atmosphere.
• Critical design factors limit the degree of pressurization and the operating altitude.

Pressure of the Atmosphere

• Gases of the atmosphere (air) have weight, equivalent to 14.7 pounds for a one square inch column from sea level into space.
• Atmospheric pressure at sea level is 14.7 psi.
• Atmospheric pressure is also known as barometric pressure and is measured with a barometer, expressed in inches or millimeters of mercury.
• Measurements are taken by observing the height of mercury in a column exerted on a mercury reservoir.
• The barometer column is evacuated to prevent air from acting against the mercury rising.
• Aviators use interchangeable references to atmospheric pressure between linear displacement and units of force.
• Standard atmospheric pressure at sea level is 1 atmosphere (1 atm).
• Standard atmospheric pressure measurements are all equal to each other.
• Atmospheric pressure decreases with increasing altitude due to the shorter column of air being weighed.
• At 50,000 feet, the atmospheric pressure is about one-tenth of the sea level value.

Temperature and Altitude

• Temperature changes in the atmosphere are relevant to aviators.
• Most civilian aviation occurs in the troposphere (up to about 38,000 feet), where temperature decreases as altitude increases.
• The rate of temperature change is about -2°C or -3.5°F for every 1,000 feet of altitude increase.
• The upper boundary of the troposphere is the tropopause, characterized by a relatively constant temperature of -57°C or -69°F.

Pressurization Terms

• Cabin altitude is the equivalent altitude on a standard day with the same air pressure as inside the cabin.
  • For example, cabin altitude is 8,000 feet (MSL) when the pressure inside the cabin is 10.92 psi.
• Cabin differential pressure is the difference between air pressure inside and outside the cabin, calculated as: Cabin pressure (psi) - ambient pressure (psi).
• Cabin rate of climb is the rate of change of air pressure inside the cabin, measured in feet per minute (fpm) of cabin altitude change.

Rate of Change of Pressure

• Aircraft commonly climb at 1000 feet per minute or faster, and engines maintain performance with changing atmospheric conditions.
• Rapid altitude changes affect the human body, causing physical pain and discomfort.
• When cabin pressure decreases violently, nitrogen and other dissolved gases in the blood expand into bubbles.
• Large, but non-violent, rates of change can cause:
  • Sickness
  • Abdominal expansion
  • Ear expansion

Pressurization Issues

• Pressurizing an aircraft cabin is critical for making flight possible in upper atmosphere.
• Cabin pressurization systems ensure adequate passenger comfort and safety.
• Cabin pressure altitude is at approximately 8,000 feet or lower regardless of aircraft's cruising altitude to ensure sufficient oxygen for passengers and crew.
• Pressurization systems prevent fast cabin pressure changes that are uncomfortable or dangerous to people onboard.
• Circulating cabin air eliminates odors and stale air.
• Cabin air is heated or cooled, which is typically part of pressurization source.

Structural Considerations

• For pressurization, the section of the aircraft intended to hold air at higher pressure than the outside must be sealed.
• Compressible seals surround doors and combine with other grommets and sealants to create an airtight vessel.
• This includes the cabin, the flight and baggage compartments.
• Air gets pumped into the sealed area at a rate to raise pressure slightly above needed levels.
• The rate at which the air flows out of the aircraft controls this pressure.
• Controlling cabin to ground level values maximizes comfort.
• Loss payload and increased fuel consumption derive from the increased structural weight required to give the necessary strength to facilitate very low cabin altitudes.

Negative Differential

• Aircraft designs only consider the inside pressure being greater than the outside for weight-saving purposes.
• Fuselage designs do not account for reverse or negative differential pressure, although these can occur.
• Reverse pressure can occur if a perfectly sealed fuselage is suddenly dived to a low altitude, exceeding the internal pressure.
• Inward relief valves are set to open at a negative 0.5 psi to address this situation.

Pressure Relief Setting for Different Aircraft

• Secondary safeguards, such as safety valves, ensure the differential pressure doesn't surpass the maximum proof test figure if the normal equipment fails.
• Safety valves are only required when an error occurs.
• Inward relief valve will open should the pressure drop too low
• Boeing 707 Example:
  • Normal Working Pressure: 8.6 psi
  • Safety Valve Pressure: 9.42 psi
  • Inward Relief Valve: 0.5 psi

Fuselage

• A key factor in pressurization is the fuselage's ability to endure the forces associated with the pressure difference inside versus outside.
• Differential pressure ranges from 3.5 psi for single-engine planes to 9 psi for high-performance jets.

Sources of Pressurized Air

• Varies mainly with engine type.
• Reciprocating aircraft have different sources than turbine-powered aircraft.
• Compression of air raises its temperature, thus, most pressurization systems include a means of cooling.
• This commonly comes in the form of a heat exchange that uses cold ambient air.

Reciprocating Engine Aircraft

• There are three sources of air typically used for pressurizing:
  • Supercharger
  • Turbocharger
  • Engine driven compressor

Turbine Engine Aircraft

• The main principle involves the compression of large amounts of air to be mixed with fuel and burned so bleed air is relatively free of contaminants.
• Bleed air is a great source of air for cabin pressurization.
• Drawing air reduces engine power, so the amount of air bled for pressurization should be minimized but as much as needed.

Supercharger

• Mechanically engine driven that makes use of output power during the induction phase
• Due to taking engine power, these have a limited capability to increase engine performance
• Must be positioned upstream of the fuel delivery system, should it supply the intake and the cabin with air

Turbocharger

• Driven by engine exhaust gasses from a turbocharger impeller shaft, in a bearing housing
• Less air gets added during intake, resulting in lower engine performance
• Allows for high altitude flights, weather avoidance, and comfort without the need for extra oxygen

Pressurizing Using Turbine Compressor Bleed Air

• A separate compressor draws an ambient air intake
• This turbine gets rotated by another bleed airflow in a turbo compresser
• This bleed system allows for careful control of pressurizing air temp to provide high levels of comfort

Control of Cabin Pressure

• Cabin pressurization is controlled through two modes of operation:
  • Isobaric Mode
  • Constant Pressure Mode
• Isobaric mode maintains cabin altitude at a single pressure despite altitude changes while constant pressure differential maintains uniform pressure between the cabin and outside
• Ex. The flight crew may maintain a cabin altitude of 8,000 Ft.

Constant Differential Mode

• Pressure is always lower than the maximum differential pressure for which the airframe is designed.
• When in isobaric mode, the system maintains the altitude selected by the crew, but when the aircraft climbs to altitude, differential pressure above design results.
• The system automatically switches from isobaric to constant differential mode.

Cabin Rate or Climb

• The rate of change can be adjusted manually or automatically by the flight crew
• Typical rates of change are 300-500 fpm
• Modes of operation include automatic standby or manual operation.

Cabin Pressure Controller

• Controls the cabin air pressure by adjusting cabin altitude, rate of change, and barometric pressure
• This is controlled from the pressurization panel in the cockpit
• The controller then controls the regulators that adjust the position of the outflow valve at the rear of the vessel for cabin adjustments.

Modern Pressure Control Implementation

• Modern aircraft will often combine various strategies, be it pneumatic, electric, or electronic
• Selections for the barometric setting are set on this panel in the cockpit
• Electric signals are sent from the selector to the regulator that resides outside the pressurized area
• Modern system will often have the panel set to automatic, and a maintenance mode to override the automatic
• A switch is additionally there to control the position of the outflow valve

Cabin Air Pressure Regulator & Outflow Valve

• Cabin pressure is controlled through regulating airflow in and out of the cabin
• Outflow valves modulates to to control cabin pressure
• Pneumatically controlled regulators will use settings on the cockpit panel settings to adjust cabin air pressure

All Pneumatic Cabin Pressure Regulators

• Electric cabin pressure controllers can be used on air transport to manipulate the outflow valves
• Outflow valves can be positioned with signals from the cockpit panel selectors

Cabin Air Pressure Safety Valve Operation

• If the pressurization system malfunctions, key systems exist to prevent human or structural damage.
• A safety valve offers over-pressurization prevention
• A pressure relief valve set to predetermined pressure differential

Safety Valves

• All aircraft have safety valves set to open between 8 and 10 psid.

Negative Pressure Relief Valve

• Ensures that air pressure outside of the aircraft does not go below air pressure inside.
• A spring loaded relief valve helps to allow air to enter the system when needed.
• Equalization helps to maintain an ambient pressure around 0.3psi.

Pressurization Dump Valves

• Used by certain aircraft that are operated either automatically or manually by a switch
• Allows the removal of smoke, contamination, or control air in emergencies

Pressurization Gauges

• All pressurization systems commonly have a consistent three cockpit indications that commonly use light and alerts to advise the crew
• Cabin altimeter
• Altimeter rate of climb
• Cabin differential pressure indicator
• Cabins are also equipped with an orange PSI pointer on the right
• Should the cabin altitude exceed 10k feet, there will be indication and warning in the pilot's seat

Pressurization Operations

• Normal mode of operation is commonly in automatic operations with a standby controller if there is an error
• Manual mode allows for the crew to position the outflow valve directly, depending on what system (pneumatic or electric)

During Grand Ops

• Weight-on-wheels switches are used to control the safety valve to hold it open during take off

Throttle Switch

• Used to transition smoothly from an unpressurized to pressurized airplane system.
• If that WoW switch detects closed ground and power is given with the throttle, that means pressurization can occur while the ground is happening

During Take off

• Outflow valves often require a full close, but passengers barely notice due to the slight pressurization beginning early

During Flight

• The pressurization controller automates the pressurization components until the aircraft lands again
• Closing the WOW switch will then open the safety valves on the ground

Maintenance

• Maintenance testing typically occurs in testing manual and allows for total control from the technician

This quiz covers cabin pressurization in aircraft, including optimal cabin altitude, pressure vessel design, and factors influencing cabin pressure. It explores air sources for pressurization and the consequences of different cabin altitudes.