Basics of Air Pressurisation System (ATA 21)

Pressure Vessel

Aircraft pressurization is a crucial aspect of modern aviation, ensuring the comfort and safety of passengers and crew at high altitudes. The pressurization system works by isolating a reinforced section of the fuselage known as the pressure vessel. This pressure vessel is a specially designed structure that typically encompasses the cabin, cockpit, and potentially cargo zones of the aircraft. Within this confined space, air is introduced and regulated by an outflow valve positioned at the back of the vessel. The outflow valve plays a key role in maintaining the desired cabin pressure by controlling the rate at which air exits the pressurized area.

This process allows for a controlled and comfortable environment inside the aircraft, despite the changing external atmospheric conditions as the plane ascends or descends. The pressurization system is a critical component that enables commercial aircraft to fly at high altitudes safely, providing a stable and breathable atmosphere for everyone on board.

Outflow valves

1. The valve has a moving gate designed to cover or uncover an aperture in the fuselage skin.

2. An increase in the aperture size will cause cabin pressure to fall (cabin altitude to ascend), whereas a decrease in the aperture size results in an increase in cabin pressure (cabin altitude to descend).

3. The gate is driven by one of two electrically driven motors, either AC/DC motor being determined by flight crew input, whose input signals come from the controller when in the auto or standby modes, or directly from a control panel switch when in the manual mode.

4. 4 Inward and Outward Safety Relief Valves Fuselage frames are designed to accept tensile loads associated with and outward force from within the pressure cell.

5. An inward relief valve will open and equalise the pressure if the inward or negative differential exceeds about 0.5 psid.

6. 2 outward relief valves are fitted to prevent the maximum outward differential pressure from exceeding the structural limit. This will typically be around 8.5psid.

7. Even though the main pressure control is electronic, the safety relief valves are mechanical operated and are completely independent of any automatic control system.

Safety Valves

1. Cabin Air Pressure Safety Valve

   • The pressure relief valve prevents cabin pressure from exceeding the predetermined cabin to ambient pressure differential. A negative pressure relief valve and pressure dump valve may also be incorporated into this valve assembly.

2. Postive Pressure Relief Valves

   • Every pressurized aircraft has a maximum pressure differential limit. Exceeding this limit (pumping too much air pressure into the fuselage) can cause damage – even blow out doors and windows. To protect the aircraft from over pressurizing, positive pressure relief valves are installed. The devices (sometimes called butterfly valves) are spring-loaded to vent excess air pressure when cabin pressure exceeds the maximum limit.

3. Negative Pressure Relief Valve

   • Negative pressure differential means the pressure outside the cabin is greater than the pressure inside the cabin. This situation could occur during a rapid descent. Negative pressure is bad because it pushes inward on doors and windows. These components are not designed for this type of force.

   • Spring-loaded devices are used to protect the fuselage from damage. Air pressure of less than 1.0 psi against the outside of the doors causes them to open inward against the spring load, venting air into the fuselage to equalize the pressure.

4. Dump Valve

   • This valve is normally solenoid actuated by a cockpit switch. When the solenoid is energised the valve opens dumping cabin air to atmosphere. Cabin pressure will decrease rapidly until it is the same as the outside air pressure and cabin altitude will increase until it is the same as the flight altitude.

5. Ditching valve

   • If any of the cabin control valves were situated below the water level and the aircraft ditch in the water, the cabin would quickly flood. To prevent this happening, either a mechanical or electrical ditching selection, can be made by the crew to seal off all pressurisation valves and inlets.

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.

Sealing

Sealing of the pressure vessel is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressure tight area.

All panels and large structural components are assembled with sealing compounds. Access and removable doors and hatches have integral seals. Some have inflatable seals.

Pressurisation system’s function is to raise the pressure inside the vessel and not to move large volume of air.

Larger reciprocating engine powered aircraft

They receive air from engine driven compressors driven through an accessory drive or by an electric or hydraulic motor. Multi engine aircraft have more than one air compressor. These are interconnected through ducting but each have a check valve or isolation valve to prevent pressure loss when one system is out of action.

Turbine powered aircraft

They use engine compressor bleed air. The air supplied from a gas turbine engine compressor is contamination free and can be suitably used for cabin pressurisation. Some aircraft use an independent compressor driven by the engine bleed air. The bleed air drives the coupled compressor which pressurises the air and feeds it into the cabin. Some aircraft use a jet pump to increase the amount of air taken into the cabin.

The jet pump is a venturi nozzle located in the flush air intake ducting. High velocity air from the engine flows through this nozzle. This produces a low pressure area around the venturi which sucks in outside air. This outside air is mixed with the high velocity air and is then passed into the cabin

Small reciprocating engine powered aircraft

They receive their pressurization air from the compressor of a coupled turbocharger which are driven by the engine exhaust gases flowing through a turbine. A centrifugal compressor is coupled to the turbine.

The compressors output is fed to the engine inlet manifold to increase manifold pressure which allows the engine to develop its power at altitude. Part of this compressed air is tapped off after the compressor and is used to pressurise the cabin. The air passes through a flow limiter (or sonic venturi) and then through an intercooler before being fed into the cabin.

Sonic Venturi

A sonic venturi is fitted in line between the engine and the pressurisation system.

When the air flowing across the venturi reaches the speed of sound a shock wave is formed which limits the flow of air to the pressurisation system.

Control and Indication

There are 3 modes of pressurisation:

• Unpressurised mode

• Isobaric mode

• Constant–differential pressure mode.

Unpressurised Mode

In this mode the outflow valve remains open and the cabin pressure is the same as the outside ambient air pressure. This mode is usually from sea level up to 5000 ft but does vary from aircraft to aircraft.

Isobaric Mode

In this mode the cabin pressure is maintained at a specific cabin altitude as flight altitude changes. The cabin pressure controller begins to open or close (modulates) the outflow vlaves to maintain the selected cabin altitude as the flight altitude changes up or down.

The controller will then maintain the selected cabin altitude up to the flight altitude that produces the maximum differential pressure for which the aircraft structure is rated.

At this point the constant differential mode takes control.

Isobaric Control System

The isobaric control system of the pressure regulator incorporates:

• Evacuated capsule

• Rocker arm

• Valve spring

• Ball type metering valve

One end of the rocker arm is connected to the valve head by the evacuated capsule and the other end of the arm holds the metering valve in a closed position. A valve spring located on the metering valve body tries to move the metering valve away from its seat as far as the rocker arm allows. When the cabin air pressure increases enough for the reference chamber air pressure to compress the evacuated capsule the rocker arm pivots around its fulcrum and allows the metering valve to move away from its seat an amount proportional to the compression of the capsule. When the metering valve opens reference pressure air flows form the regulator to atmosphere through the atmospheric chamber. When the regulator is operating in the isobaric range, cabin pressure is held constant by reducing the flow of reference chamber air through the metering valve. This prevents a further decrease in reference pressure. The isobaric control responds to slight changes in reference pressure by modulating to maintain a constant pressure in the chamber throughout the isobaric range of operation. Whenever there is an increase in cabin pressure the isobaric metering valve opens which decreases the reference pressure and causes the outflow valve to open which then decreases the cabin pressure.

Constant–differential pressure mode

Cabin pressurisation puts the aircraft structure under a tensile stress as the cabin pressure expands the pressure vessel.

The cabin differential pressure is the ratio between the internal and external air pressures.

At maximum constant-differential pressure as the aircraft increases in altitude the cabin altitude will increase but the internal/external pressure ratio will be maintained. There will be a maximum cabin altitude allowed and this will determine the ceiling at which the aircraft can operate.

Constant–differential control system

The differential control system of the pressure regulator incorporates:

• Diaphragm

• Rocker arm

• Valve spring

• Ball type metering valve

One end of the rocker arm is attached to the head by the diaphragm which forms a pressure sensitive face between the reference chamber and the atmospheric chamber.

Atmospheric pressure acts on one side of the diaphragm and reference chamber pressure acts on the other. The opposite end of the rocker arm holds the metering valve in a closed position. A valve spring located on the metering valve body tries to move the metering valve away from its seat as far as the rocker arm allows. When reference chamber pressure increases to the system differential pressure limit set above the decreasing atmospheric pressure it collapses the diaphragm which is set at differential pressure and opens the metering valve. Air flows from the reference chamber to atmosphere through the atmospheric chamber, which causes a reduction in the reference pressure. This reduction in reference pressure causes the outflow valve to open to reduce the cabin pressure to maintain the system pressure differential.

Cockpit POV

Most pressurisation systems have 3 basic cockpit indicators

• Cabin altitude

• Cabin rate of climb

• Pressure differential indicator

The cabin altitude gauge measures the actual cabin altitude. The cabin rate of climb indicator tells the pilot the rate that the cabin is either climbing or descending. (I.e. the rate at which the cabin loses or gains pressure)

A typical maximum climb rate is 500ft per minute and the maximum descent rate is 300ft per minute. The control can be automatic or manual depending on aircraft type.

The differential pressure gauge reads the difference between the cabin and the outside air pressures. This differential pressure is normally controlled and maintained to a structural limitation around 7psid.

This depends on the aircraft type and the operating ceiling of the aircraft. The differential pressure gauge may be combined with the cabin altitude

Safety and Warning Devices

On ground, test the pressurisation system with the engines running, at least 3 men are required inside the aircraft for safety reasons.

Both air conditioning and pressurisation systems use safety and warning devices to protect the aircraft from possible catastrophic failures. Some of the protection devices may be inhibited in certain stages of flight; landing or take off where the extra distractions caused by such warnings may be too much for the crews to deal with safely. With the air conditioning system the main concerns are with overheating of the air conditioning packs and extraction and ventilation fans, as well as hot air leaks from ducting which could damage surrounding structure or components.

Overheating

Most packs systems are protected from overheating by a thermal switch downstream of the pack outlet.

If the outlet temperature reaches a pre determined figure the switch will operate causing the pack valves to shut, preventing air from getting to the packs, as well as sending a warning signal to the cockpit central warning panel with associated caution/warning lights and aural chimes and to illuminate a fault light on the pack selector switch.

Once the system has cooled down sufficiently the crew may have an option to reselect the overheated system. The overheat may have been caused by a fault in the automatic temperature control system in which case the pilot may be able to control the system manually via a manual selector switch on the cockpit controller.

Extraction or ventilation fans will be protected in much the same way. An overheat will signal the central warning panel with associated caution/warning lights and aural chimes. The fan may be isolated automatically or manually. Once the fan has cooled down it may be possible to re-select if required.

Fans may also be protected from over or under speeding, which will also have an effect on the system temperatures. Speed sensors on the fan will indicate a fault when over or under speed limits are reached and a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes.

Duct Hot Air Leakage

Any ducting that includes joints is liable to leak under abnormal conditions. A duct protection system will include fire-wire elements around the hot zones such as engine air bleeds, air conditioning packs and auxiliary power units if fitted. The sensing elements will be the thermistor type. As the temperature around the wire increases the resistance decreases until an electrical circuit is made. When the circuit is made a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes. The leaking duct may be isolated automatically or may require the pilot to take action to close off the air valves. The faulty system will then remain out of use.

Excess Cabin Altitude

If the cabin altitude was allowed to increase unchecked the crew and passengers could unknowingly suffer the effects of hypoxia. This dangerous condition is obviously undesirable especially for the aircrew. Most aircraft give a warning on the CWP with associated audio and visual warnings when the cabin altitude reaches 10,000 ft.

Smoke Detection

Smoke detectors may be fitted within the cabin; avionics bay and cargo areas to monitor systems, which if become faulty may generate smoke on overheating, or are may be liable to catch fire. These detectors will send a signal to the CWP with associated lights and audio warnings. They may also automatically switch on extractor fans, which will remove the smoke overboard and away from the cabin and cockpit areas. In this event, the pilot may have a switch or control lever to operate a valve to isolate the cockpit air conditioning ducting from the rest of the aircraft to prevent any smoke from getting to the cockpit.