Atmospheric Pressure

Pressure of the Atmosphere

• The atmosphere's gases have weight, even though they are invisible.
• A one-square-inch air column stretching from sea level to space weighs 14.7 pounds.
• Atmospheric pressure at sea level is 14.7 psi.
• Atmospheric pressure is also known as barometric pressure.
• Barometric pressure is measured with a barometer, in inches of mercury or millimeters of mercury.
• Atmospheric pressure measurements observe the height of mercury in a column connected to a mercury reservoir.
• The barometer column is initially vacuumed to prevent air from affecting the mercury's rise.
• A 29.92-inch mercury column weighs the same as an equivalent air column from sea level to the top of the atmosphere.
• Aviators use both linear displacement (inches of mercury) and force units (psi) to refer to atmospheric pressure.
• Standard atmospheric pressure at sea level is 1 atmosphere (1 atm).
• 1 atm is equivalent to 14.7 psi, 29.92 inches of mercury, 1013.2 hPa, 1013.2 mb and 760 mm Hg.
• Atmospheric pressure decreases with increasing altitude because the air column being weighed becomes shorter.
• At 50,000 feet, atmospheric pressure drops to about one-tenth of its sea level value.

Temperature and Altitude

• Temperature variations are important to aviators, affected by weather systems and altitude.
• Most civilian aviation occurs in the troposphere, roughly from the earth's surface to 38,000 feet.
• Within the troposphere, temperature decreases with altitude.
• The rate of temperature change is approximately -2°C or -3.5°F per 1,000 feet of altitude increase.
• The tropopause marks the upper boundary of the troposphere.
• The tropopause is characterized by a relatively constant temperature of -57°C or -69°F.

Pressurisation Terms

• Cabin altitude is the equivalent altitude on a standard day with the same pressure as inside the cabin.
• Instead of saying cabin pressure is 10.92 psi, cabin altitude can be stated as 8,000 feet (MSL).
• Cabin differential pressure is the difference between the air pressure inside and outside the cabin, also expressed as, Cabin pressure (psi) - ambient pressure (psi) = cabin differential pressure (psid or Δ psi).
• Cabin rate of climb is 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

• Aircraft commonly climb at 1000 feet per minute or more, with engines designed to maintain performance.
• Rapid altitude changes can cause physical pain and discomfort.
• Rapid decreases in cabin pressure cause nitrogen and other dissolved gases in the bloodstream to form bubbles.
• This condition causes acute pain and injury, and is known as explosive decompression.
• If pressure change rates are large but not violent, common effects include sickness, abdominal expansion, and ear discomfort.

Pressurization Issues

• Pressurizing an aircraft cabin is essential for flight in the upper atmosphere.
• Design factors limit the degree of pressurization and the aircraft's operating altitude.
• Cabin pressurization systems must ensure passenger comfort and safety.
• Systems must maintain a cabin pressure altitude of approximately 8,000 feet or lower.
• This ensures sufficient oxygen for passengers and crew to facilitate full blood saturation.
• Pressurization systems must prevent rapid cabin pressure changes to avoid discomfort or injury.
• Cabin air should circulate from inside to outside to eliminate odors and stale air.
• Cabin air must be heated or cooled, with these functions integrated into the pressurization source.

Altitude vs Oxygen Pressure

• The ability to maintain a cabin pressure altitude of approximately 8,000 feet or lower is essential.
• The altitude is controlled regardless of the aircraft's cruising altitude.
• This ensures sufficient oxygen for crew and passengers to facilitate full blood saturation.

Structure Consideration: Positive Differential

• Pressurizing requires sealing a section of the aircraft to contain air at a higher pressure.
• Compressible seals around doors, with other seals, grommets, and sealants, form an airtight pressure vessel.
• Areas included in the pressure vessel, are the cabin, flight compartment, and baggage compartments.
• Air is pumped in at a controlled rate to raise the pressure slightly above what is needed.
• Control is maintained by adjusting the rate at which air is allowed outflow.
• Maintaining ground level cabin pressure for comfort can lead to higher differential pressure during climb.
• Deciding on cabin altitude and rate of change involves considering payload loss and increased fuel consumption.
• Higher structural weight is necessary, to accommodate high differential pressure, if low cabin altitudes are desired at very high heights.

Structure Consideration: Negative Differential

• Aircraft design usually only accounts for inside pressure being greater than outside pressure to save weight.
• Aircraft fuselages aren't designed for reverse or negative differential pressure but it can occur.
• This occurs with a perfectly sealed fuselage that is inside atmospheric pressure is then dived to a lower outside altitude where atmoshpheric pressure would be greater inside causing a negative differential.
• These negative differential emergencies are solved via an inward relief valve
• Inward relief valve is fitted, set to open generally at a negative differential pressure of 0.5 psi.

Structure Consideration: Pressure Relief

• If normal pressure controls fail a secondary safeguard ensure differential pressure does not exceed proof test figures.
• Safety valves are the secondary safeguard and only operate if this secondary safeguard is not available.
• See the table below on important B707 pressure measure figures.

| | BOEING 707 |

• Pressure Relief Settings for Different Aircraft

Structure Consideration

• Pressurization depends on the fuselage's ability to withstand increased internal pressure versus outside pressure.
• Differential pressure ranges from 3.5 psi in single-engine reciprocating aircraft to about 9 psi in high-performance jets.
• Differential pressure (psid) is figured by subtracting the ambient air pressure from the cabin air pressure.

Sources of Pressurised Air

• The air source for pressurization depends on the engine type.
• Reciprocating and turbine-powered aircraft use different pressurization sources.
• Compressing air raises its temperature, for example, to keep the air cool, most systems use a heat exchanger, utilizing cold ambient air.

Reciprocating Engine Aircraft

• Three typical air sources used to pressurize reciprocating aircraft: supercharger, turbocharger, and engine-driven compressor.

Turbocharged Aircraft

• Driven by engine exhaust gases.
• The turbocharger impeller shaft extends through the bearing housing to support a compression impeller in a separate housing.
• Using turbocharger compressed air for cabin pressurization reduces the available intake charge.
• Nonetheless, it puts wasted exhaust gases to work in the turbocharger compressor.
• Turbocharging enables high-altitude flight for the engine benefits of low drag and weather avoidance in relative comfort.

Supercharged Aircraft

• Superchargers are mechanically driven by the engine.
• Superchargers can increase engine performance by increasing pressure
• Performance increases are limited by power lost from the engine output
• Pressurizing both the intake and the cabin lowers the engine performance ceiling ceiling more than if the aircraft was not pressurized.
• Superchargers must be upstream of the fuel delivery for pressurization.

Turbine Engine Aircraft

• Pressurizing aircraft with turbine engines involves using bleed air from the engine compressor to drive a separate compressor with an ambient ait intake.
• A turbine, turned by bleed air, rotates a compressor impeller on the same shaft.
• Outside air is compressed, mixed with bleed air outflow from the turbine, and sent to the pressure vessel. Turboprop aircraft use this device, known as a turbocompressor, often.
• Turbine powered aircraft often use an air cycle air conditioning and pressurization system.
• Pressurization and temperature are precisely controlled through an elaborate system using bleed air.
• The elaborate system also uses heat exchangers, a compressor, and an expansion turbine.

Control of Cabin Pressure

• Aircraft cabin pressurization is controlled via two modes of operation: isobaric and constant differential.
• The isobaric mode keeps cabin altitude at a consistent pressure despite altitude changes.
• For example, the flight crew may select to maintain a cabin altitude of 8,000 feet (10.92 psi).

Cabin Pressure Control: Constant Differential

• The constant differential mode maintains a constant pressure difference between the internal and ambient air pressure, regardless of altitude changes.
• Pressure differential is lower than the maximum differential pressure for which the airframe is designed, maintaining integrity.
• Automatic mode switches from isobaric to constant differential if altitude changes create excess differential pressure.
• The rate of change of cabin pressure, also known as the cabin rate of climb or descent, is controlled automatically or manually by the flight crew.
• Typical rates of change for cabin pressure are 300 to 500 fpm.
• Modes of pressurization also refer to automatic, standby, and manual operation of the pressurization system.

Cabin Pressure Controller

• The cabin pressure controller regulates cabin air pressure. and is made via direct connection to the panel in the cockpit.
• Input includes, cabin altitude, rate of cabin altitude change, and barometric pressure setting.
• The regulator controls the outflow valve position, typically at the rear of the aircraft pressure vessel and that is valve position determines cabin pressure. Modern aircraft use pneumatic, electric, and electronic control of cabin pressurization.
• Electric signals pass from the selector to the cabin pressure controller, also known as the pressure regulator.
• It is remotely located near the cockpit but inside the pressure vessel of the aircraft.
• Modern pressurization control is fully automatic with variable selections made on the control panel.
• All systems have a manual mode that overrides automatic control and is mostly used on the ground during maintenance.
• A separate switch is used in manual mode, to fully open or close, and control cabin pressure

Cabin Air Pressure Regulator and Outflow Valve

• Cabin pressurization is controlled by regulating the amount of airflow.
• A cabin outflow valve modulates to establish the amount of internal pressure being maintained.
• Some outflow valves regulate pressure, while others control the valve mechanism unit.
• Some systems work to change from the pressure control/panel settings set by the pilot in the cockpit.
• A separate system can be is used to regulate pressure.
• Many transport aircraft have an outflow valve that operates electrically.
• Signals are sent from a remotely located cabin air pressure controller.
• The controller positions the valve(s) based on cockpit pressure and panel settings according to predetermined schedules.

Cabin Air Pressure Safety Valve Operation

• Aircraft pressurization systems prevent human and structural damage if the system fails or becomes inoperable.
• Overpressurisation is prevented to ensure structural integrity.
• A cabin air safety valve acts as a pressure relief and is set to open at a predetermined pressure differential.
• It allows airflow from the cabin to prevent pressures from exceeding design limitations.
• Safety valves in large transport aircraft open automatically when the range is 8 and 10 psid. The aircraft should be inspected for damage.

Cabin Air Pressure Safety Valve Operation

• A negative pressure relief valve, included on pressurized aircraft, prevents external pressure exceeding cabin air pressure.
• A spring-loaded relief valve opens inward, it ensures high outside pressures don't cause difficulty when opening the cabin door.
• Inward pressure is capable of causing structural damage since cabin pressure is designed to be kept greater than ambient.
• It allows air to equalize if the ambient pressure exceeds cabin pressure by 0.3psi.
• Some aircraft have pressurization dump valves.
• Pressurization dump valves are safety valves operated automatically or manually by a switch in the cockpit.
• They are quickly, and automatically, activated when the aircraft has issues with smoke or other contamination.

Pressurisation Gauges

• Three cockpit indications alert the crew with warning lights and advisories regarding pressurization.
• The three pressure gauges, commonly seen together, are the cabin altimeter, the cabin rate of climb, and the differential pressure indicator.
• The cabin gauge is a triple combination gauge.
• The long pointer is similar to a vertical speed indicator on the left of the gauge and indicates rate of change of cabin pressure/altitude
• The orange pointer to right indicates differential pressure.
• The pilot will be informed via a warning indication and warning horn when the cabin pressure altitude exceeds 10,000 feet.

Pressurisation Operations

• The normal mode for most pressurization systems is automatic control, a manual can also be selected.
• Standby Modes also provide automatic control with different inputs and a standby controller.
• A manual mode is available should the automatic and standby modes fail.
• The manual mode allows the crew to directly position the outflow valve through pneumatic or electric control based on the system.
• During ground operations and prior to takeoff, a weight-on-wheels (WOW) switch controls the position of the pressurization safety valve.
• The safety valve is held open until the aircraft takes off. These valves are usually closed after the weight of the aircraft is no longer on the wheels so the cabin can pressurize.
• Throttle position switches are utilized to cause a smooth transition from an unpressurized to a pressurized cabin.
• Partial closing of the outflow valve(s) when the WOW switch is closed begins pressurization during the aircraft rolling out of the terminal to the runway. On takeoff, outflow valve(s) must fully close to meet the rate of climb for aircraft. Passengers do not experience a harsh sensation because the cabin has already be pressurized slightly.
• In flight, a pressurization controller automatically controls the sequence of operation of the pressurization.
• When the WOW switch closes while landing, the safety valve(s) open.
• Some outflow valve(s) also make pressurization impossible on the ground in automatic mode.
• Technicians conduct maintenance testing of the system in manual mode to fully open/close/control certain features.

The gases in the atmosphere have weight, resulting in atmospheric pressure. At sea level, this pressure is 14.7 psi, also known as barometric pressure. Barometers measure this pressure, often in inches or millimeters of mercury, reflecting the height of a mercury column.