Pressurization - Pneumatics - Airframes & Aircraft Systems #41

Summary

This lesson delves into the pressurization control systems of modern airliners, ensuring a safe and comfortable cabin environment for passengers and crew at high altitudes. It explains how pressurization systems maintain cabin altitude at or below 8,000 feet, despite flying at much higher altitudes, and the safety devices like relief valves and blow out panels that prevent structural failure. The lesson also covers how differential pressure is managed and the importance of safety warnings when cabin altitude exceeds safe limits.

Takeaways

• 🛫 Modern airliners are equipped with pressurization control systems to optimize efficiency at high altitudes.
• ✈️ Pressurization is necessary for normal passenger and crew function above 10,000 feet, where oxygen levels drop significantly.
• 🌐 Regulatory requirements mandate that cabin pressurization systems simulate conditions no higher than 8,000 feet altitude.
• 💪 Aircraft structures must be robust to handle the cyclical stresses caused by pressurization and depressurization.
• 🔢 The maximum differential pressure between the cabin and outside air is set by the manufacturer to ensure structural integrity.
• 📉 A reduced maximum differential pressure due to defects can lower the aircraft's maximum operating altitude.
• 🚫 Pressurized areas include the cabin, flight deck, and cargo compartments, while landing gear bays and radomes remain unpressurized.
• 🔄 Cabin pressurization is managed by regulating the mass flow of air entering and exiting the cabin through outflow valves.
• 🛡 Safety devices like relief valves and blow out panels are essential to prevent over-pressurization and ensure rapid depressurization if needed.
• ⚠️ Warnings are issued when cabin altitude exceeds 10,000 feet, with both audible and visual alerts for the crew.

Q & A

Why is cabin pressurization necessary in modern airliners?

Cabin pressurization is necessary to allow passengers and crew to function normally at high altitudes without the need for additional oxygen, as the effects of lack of oxygen can become apparent at altitudes above 10,000 feet.

What is the regulatory requirement for cabin pressurization in terms of altitude?

It is a regulatory requirement that cabin pressurization systems are designed to produce conditions equivalent to a maximum of 8,000 feet in the aircraft cabin.

How does the pressurization system affect the aircraft's structural integrity?

The difference in pressure between the pressurized hull and the atmosphere produces cyclical stresses that can lead to fatigue and structural failure over time.

What is the maximum differential pressure that modern transport aircraft can typically withstand?

The normal operating maximum differential pressure for modern transport aircraft is typically between eight and nine pounds per square inch (psi) or between 552 and 621 hectopascals.

How is the aircraft's maximum operating altitude determined?

The aircraft's maximum operating altitude is determined by the maximum differential pressure and the barometric pressure at a given altitude. For example, with a maximum differential pressure of 8.2 psi, the maximum operating altitude would be around 40,000 feet.

What is the purpose of the safety valve in the pressurization system?

The safety valve is a mechanical outward pressure relief valve that prevents the structural limit from being exceeded by relieving positive pressure in the cabin when the normal maximum pressure differential is exceeded.

How does the dump valve function in the pressurization system?

The dump valve is manually operated and enables the crew to reduce the cabin pressure to zero for emergency depressurization or to serve as the air outlet during manual operation of the pressurization system.

What are blow out panels and why are they important?

Blow out panels are fitted in the floor between passenger and cargo compartments to prevent excessive differences in pressure between these areas, such as in the event of a cargo door opening in flight.

What is the purpose of the ditching control in the pressurization system?

The ditching control is used to close all discharge valves in the event of a forced landing on water, reducing the flow of water into the cabin.

What warning system is in place if the cabin altitude exceeds 10,000 feet?

An oral or visual warning system is in place, which may include a horn and a red light in prominent view of the pilot, to alert the crew when the cabin altitude exceeds 10,000 feet.

How does the constant mass flow of air affect cabin pressurization?

Cabin pressurization is achieved by maintaining a constant mass flow of air entering the cabin from the conditioning system and then varying the rate at which it is discharged to the atmosphere through outflow valves.

Outlines

00:00

🛫 Understanding Aircraft Pressurization Systems

This paragraph introduces the pressurization control systems in modern airliners, focusing on the safety devices and indicating systems for crew awareness. It explains the need for pressurization at high altitudes to maintain a breathable environment for passengers and crew. The regulatory requirement to maintain cabin conditions equivalent to a maximum altitude of 8,000 feet is highlighted, along with the structural limits of the aircraft's pressurized hull. The normal operating maximum differential pressure is discussed, along with how it affects the aircraft's maximum operating altitude. The paragraph concludes with a practical example of how a change in differential pressure affects the maximum altitude an aircraft can safely fly.

05:10

🔧 Pressurization Control and Safety Devices

This section delves into the mechanics of cabin pressurization, detailing how a constant mass flow of air is managed through the use of air conditioning packs and outflow valves. It describes the automatic and manual control methods for the outflow valve, which调节s cabin pressure and altitude. The paragraph outlines various safety devices, including the safety valve to prevent over-pressurization, inward relief valves to handle negative pressure differentials, and their requirements for dual installations. It also discusses the dump valve for emergency depressurization, blow out panels to equalize pressure between compartments, and the ditching control to minimize water ingress during a water landing. The importance of warnings for the crew when cabin altitude exceeds safe limits is also emphasized.

10:10

✈️ Maximizing Aircraft Altitude and Safety

The final paragraph summarizes the key learnings from the lesson, reinforcing the importance of understanding the maximum permitted cabin altitude and positive differential pressure. It instructs on how to calculate the aircraft's maximum operating altitude using a barometric pressure table and the aircraft's maximum differential pressure. The paragraph also reviews the purpose of the pressurization system's safety devices, such as pressure relief valves, dump valves, and blow out panels, ensuring that the crew is aware of their functions in maintaining both the aircraft's structural integrity and the safety of its occupants.

Keywords

💡Pressurization Control Systems

Pressurization control systems are integral to modern airliners, ensuring that the aircraft's cabin is maintained at a safe and comfortable pressure for passengers and crew. These systems are designed to simulate conditions equivalent to an altitude of no more than 8,000 feet, which is sufficient for normal human functioning without the need for supplemental oxygen. The script mentions that these systems are fitted with various safety devices and indicating systems to ensure the crew is aware of the cabin's pressure status.

💡Cabin Pressurization

Cabin pressurization refers to the process by which the interior of an aircraft is pressurized to maintain a breathable atmosphere at high altitudes. The script explains that up to an altitude of 10,000 feet, the air pressure is sufficient for humans, but beyond that, pressurization is necessary. Modern aircraft are pressurized to simulate conditions equivalent to a maximum of 8,000 feet to prevent oxygen deprivation.

💡Differential Pressure

Differential pressure is the difference in pressure between the inside of the aircraft's cabin and the outside atmospheric pressure. The script discusses how the aircraft's structure must be strong enough to withstand these pressures and how the maximum differential pressure is set by the manufacturer to ensure safety. It also explains how changes in differential pressure can affect the aircraft's maximum operating altitude.

💡Safety Valves

Safety valves are critical components of the pressurization system, designed to relieve excessive pressure within the cabin. The script specifies that there must be two safety valves fitted to the system, one for positive pressure relief to prevent structural limits from being exceeded and another for negative pressure relief to protect against inward crushing forces.

💡Outflow Valve

The outflow valve is a key component in controlling cabin pressure. It regulates the rate at which air is discharged from the cabin to the atmosphere. The script describes how the position of the outflow valve can be controlled automatically or manually by the flight crew, with closing the valve increasing cabin pressure and opening it decreasing it.

💡Maximum Operating Altitude

The maximum operating altitude of an aircraft is determined by the maximum differential pressure it can safely withstand. The script provides a calculation example using the barometric pressure table, showing how an aircraft with a maximum differential pressure limit of 8.2 psi would have a maximum operating altitude of 40,000 feet.

💡Barometric Pressure Table

A barometric pressure table is a reference tool used to determine air pressure at various altitudes. In the context of the script, it is used to calculate the aircraft's maximum operating altitude based on the maximum differential pressure. The table shows that at 8,000 feet, the air pressure is 10.91 psi, which is a reference point for these calculations.

💡Cargo Compartments

Cargo compartments are areas of the aircraft that are typically not pressurized. The script mentions that in the event of a cargo door opening in flight, blow out panels are designed to equalize the pressure between the passenger and cargo compartments to prevent structural damage.

💡Blow Out Panels

Blow out panels are safety devices fitted in the floor between passenger and cargo compartments. As described in the script, these panels are designed to open in response to a pressure differential, equalizing the pressure and preventing damage to the aircraft's structure.

💡Ditching Control

Ditching control is a feature of some pressurization systems that allows the crew to close all discharge valves in the event of a forced landing on water. The script explains that this control reduces the flow of water into the cabin, which is a critical safety measure during emergency landings.

💡Cabin Altitude Warning

A cabin altitude warning system is designed to alert the crew if the cabin altitude exceeds 10,000 feet. The script mentions that this warning can be oral, such as a horn, or visual, like a red light, ensuring that the crew is aware of the cabin's pressure status and can take appropriate action if necessary.

Highlights

Modern airliners are equipped with pressurization control systems to enhance efficiency at high altitudes.

Pressurization allows passengers and crew to function normally without additional oxygen at high altitudes.

Cabin pressurization systems are regulated to simulate a maximum altitude of 8,000 feet for safety.

At altitudes above 10,000 feet, the lack of oxygen can become a significant issue for humans.

The airframe structure must be strong enough to withstand the differential pressures generated by pressurization.

Aircraft manufacturers set a maximum differential pressure limit for the pressurized hull's structural integrity.

The normal operating maximum differential pressure for modern transport aircraft is between 8 and 9 psi.

The maximum operating altitude of an aircraft is determined by its maximum differential pressure and the barometric pressure at that altitude.

Safety devices like the safety valve are fitted to prevent exceeding the structural limit of the pressurized hull.

Inwards relief valves are used to prevent negative pressure differentials that could cause structural failure.

The dump valve allows the crew to manually depressurize the cabin for emergency situations.

Blow out panels equalize pressure between passenger and cargo compartments to prevent structural damage.

A ditching control system can close all discharge valves to reduce water ingress during a water landing.

Cabin altitude exceeding 10,000 feet triggers an oral or visual warning for the crew.

Pressurization is controlled by maintaining a constant mass flow of air and调节 its exit through outflow valves.

Various safety devices are fitted to the pressurization system to ensure the safety of the aircraft and its occupants.

The pressurization system's design and operation are critical for the well-being of passengers and crew at high altitudes.

Transcripts

In this lesson, we will examine the pressurization control systems fitted

modern airliners. We will look at the necessary safety devices fitted and the

indicating systems available to the crew.

Modern aircraft operate more efficiently at high altitudes and they have high

rates of climb and descent.

To take advantage of these properties the interior of an aircraft flying at

high altitude is pressurized to allow passengers and crew to function normally

without the need for additional oxygen.

Up to an altitude of 10,000 feet the air pressure and consequently the amount of

oxygen is sufficient for humans to operate without too many problems.

However, the effects of lack of oxygen can become apparent at altitudes above

this.

To prevent any risk of problems due to a lack of oxygen it is a regulatory

requirement that cabin pressurization systems are designed to produce

conditions equivalent to a maximum of 8,000 feet in the aircraft

cabin. This means that there is no need for oxygen equipment except for

emergency use by crew or passengers.

The difference in pressure between the pressurized hull and the atmosphere

produces stresses which are applied cyclically every time the aircraft is

pressurized and depressurized causing fatigue which can ultimately lead to

structural failure. The airframe structure must be strong

enough to withstand the differential pressures generated.

The aircraft manufacturer was set as a structural limit a maximum differential

pressure.

That is the difference between the pressure inside the pressurized

compartment

and the pressure of outside air which the pressurized hull can safely

withstand. The normal operating maximum will be slightly lower than this.

On modern transport aircraft the normal operating maximum differential pressure

is typically between eight and nine pounds per square inch or psi or between

552 and 621 hectopascals.

This maximum differential pressure along with the maximum permitted cabin

altitude of 8,000 feet will set a maximum for the altitude at which the

aircraft can operate. As you can see from this barometric

pressure table the air pressure at 8,000 feet is 10.91 psi.

So if the manufacturer has set the aircraft's maximum differential pressure

at 8.2 psi.

Then the minimum outside pressure is 10.91 minus 8.2

which equals 2.71.

We can see from the chart that this equates to just a touch over 40,000 feet.

Thus an aircraft with a maximum differential pressure limit of 8.2 psi

will have a maximum operating altitude of 40,000 feet. This is of course the

pressurization limit the aircraft's maximum altitude may be further limited

by other factors.

If the maximum permitted differential pressure is reduced by an aircraft

defect, for instance, a cracked cockpit window, the maximum aircraft altitude

will also be reduced by the need to maintain the maximum cabin altitude at

8,000 feet. For instance, if the maximum differential

pressure is reduced to 6 psi then the minimum outside pressure will now be the

pressure at 8,000 feet which is 10.91 psi minus 6 which

equals 4.91 psi. From the table the altitude equal to 4.9

1 psi is approximately 27,000 feet. This is the maximum altitude limit with a

differential pressure of 6 psi.

The passenger cabin

flight deck

and cargo compartments are normally pressurized.

The landing gear bays

radome

and the tailore nose cones are unpressurized.

Cabin pressurization is achieved and controlled by having a constant mass

flow of air entering the cabin from the conditioning system and then varying the

rate at which it is discharged to atmosphere. The constant mass flow of air

is supplied by the air conditioning packs fire their mass flow controllers

and is discharged atmosphere through the discharge or outflow valve or valves.

The position of the outflow valve can be controlled either automatically by an

automatic pressurization controller

or manually by the flight crew.

Closing the outflow valve reduces the outflow and increases the cabin pressure

causing the cabin altitude to descend.

Opening the valve has the opposite effect increasing the outflow reducing

the cabin pressure and causing the cabin to climb.

There are a number of safety devices which must be fitted to any cabin

pressurization system. The safety valve is a simple mechanical outwards pressure

relief valve fitted to relieve positive pressure in the cabin when the normal

maximum pressure differential allowed for the aircraft type is exceeded,

preventing the structural limit from being exceeded.

This valve is totally independent of all other control systems and will open if

the cabin differential pressure rises to approximately 0.25 psi above the normal

maximum. The regulations stipulate that there must be two safety valves fitted.

The fuselage is designed to withstand the positive differential pressure

produced by the pressurization system. However, it is not able to withstand the

crashing forces that a negative pressure differential will produce.

To prevent this problem simple mechanical inwards relief valves are fitted.

They will open if the pressure outside the aircraft exceeds that inside by 0.5

to 1.0 psi there must also be two of these valves. The inwards and

outwards safety valves may be combined together in one unit or maybe completely

separate components. They are positioned above the aircraft floatation line so

that in the event of a landing on water they will not allow the water to flow

into the aircraft.

The dump valve is a manually operated component it enables the crew to reduce

the cabin pressure to zero for emergency depressurization. This valve may in some

systems also be used as the air outlet during manual operation of the

pressurization system.

Blow out panels are fitted in the floor between passenger and cargo compartments.

In order to prevent excessive differences in pressure occurring

between these areas in the event of, for example a cargo door opening in flight,

the panels are normally fitted under passenger seats and they are held in

place by springs.

If a cargo door should open then the pressure differential between the

passenger and cargo compartments will overcome the springs and the panels

will open. Equalizing the pressure between the compartments before the

floor structure is damaged.

Some systems are fitted with a ditching control which will close all the

discharge valves in the event of a forced landing on water, this will reduce

the flow of water into the cabin.

There will be an oral or visual warning if the cabin altitude exceeds 10,000

feet the oral warning may take the form of a horn and the visual warning a red

light in prominent view of the pilot. There is normally a horn cutout button

which can be depressed to cancel the horn.

That is the end of the lesson you should now know that the maximum permitted

cabin altitude is 8,000 feet and that the normal maximum positive

differential pressure is between 8 & 9 psi you should understand that the

aircraft's maximum operating altitude is dependent upon the maximum differential

pressure and given a barometric pressure table you should be able to calculate

this maximum altitude.

You should also know that the pressurization is controlled by having a

constant mass flow of air into the fuselage and controlling its exit using

out flow valves.

You should understand the purpose of the various safety devices fitted to a

pressurization system, the positive outward opening pressure relief valves

prevent the structurally limiting maximum differential pressure from being

exceeded

and negative inward opening relief valves prevent the pressure inside the cabin

becoming less than that outside. The dump valve is used to completely depressurize

the aircraft and the ditching control is used to

close all outflow valves prior to the aircraft landing on water.

Blow out panels are fitted in the floor between passenger and cargo compartments

in order to prevent excessive differences in pressure occurring

between these areas and finally an oral or visual warning

will be given to the crew when the cabin altitude exceeds 10,000 feet.