Bleed Air Systems - Pneumatics - Airframes & Aircraft Systems #38

Summary

This lesson delves into the bleed air systems of modern jet airliners, which are crucial for air conditioning, pressurization, and powering various aircraft components. The system draws air from engine compressors, with backup from the Auxiliary Power Unit (APU) and ground carts. It's designed to handle pressure variations across different engine speeds, using both low-pressure (LP) and high-pressure (HP) air. The air is cooled and regulated to maintain optimal temperature and pressure for various uses. Safety features include duplication of critical components, isolation valves for engine independence, and monitors to prevent over-temperature and over-pressure. The lesson highlights the system's redundancy and safety measures to ensure flight safety.

Takeaways

• 🚀 Modern jet airliners utilize high-volume, low-pressure air systems for various purposes including air conditioning, pressurization, ice protection, and engine starting.
• ✈️ Bleed air, sourced from engine compressors, is the primary supply for these systems, with auxiliary power unit (APU) and ground air supply carts as backups.
• 🔥 The engine bleed air system comprises the engine compressor as the power source, and control devices to regulate temperature and pressure.
• 🔧 The system must maintain a reasonable air supply at low RPM and restrict excessive pressure at high RPM, often by drawing air from two compressor stages.
• 🔄 High-pressure (HP) air is controlled by a pressure-sensitive, pneumatically operated high-pressure shutoff valve (HP SOV) to ensure adequate flow.
• 🌡️ Bleed air from the compressor is too hot for direct use, so it passes through a pre-cooler where engine fan air cools it down to around 175 degrees Celsius.
• 💧 Engine bleed air control valves manage the flow into the pneumatic system, modulating to maintain a maximum duct pressure of approximately 55 psi.
• 🔒 Regulatory requirements mandate that any single component failure in cabin air conditioning or pressurization systems must not compromise safety, leading to system duplications.
• 🛠️ Multi-engine aircraft keep engine supplies separate, with isolation valves that can be opened if one engine supply fails to ensure continued operation.
• ⚠️ The system is equipped with safety measures such as temperature and pressure monitors, with automatic closure of bleed air valves in case of excessive conditions.
• 🔄 In the event of faults, pilots can restore system functionality by opening isolation valves to allow air from one engine to operate the entire system.

Q & A

What is the primary source of air for the high volume low pressure air systems in modern jet airliners?

The primary source of air for these systems is bleed air from the engine compressors.

What is the role of the Auxiliary Power Unit (APU) in the bleed air system?

The APU serves as a backup for the bleed air system, providing air when needed.

How does the bleed air system contribute to the aircraft's air conditioning and pressurization?

The bleed air system is used for air conditioning and pressurization by supplying conditioned air to the cabin.

What are the different uses of bleed air mentioned in the script?

Bleed air is used for ice protection, engine starting, driving hydraulic pumps, operating air-driven flaps and slats, pressurizing hydraulic reservoirs, potable water tanks, and cargo compartment heating.

How does the bleed air system maintain a reasonable supply of air during varying engine speeds?

The system takes air from two compressor stages to maintain a reasonable pressure band at all engine speeds.

What is the function of the high-pressure shutoff valve (HP SOV) in the bleed air system?

The HP SOV is pressure-sensitive and pneumatically operated, opening when there is insufficient air pressure from the LP system to maintain the required flow.

How is the temperature of the bleed air controlled in the system?

The air from the compressor is passed through a pre-cooler, and a valve modulates the amount of fan air passing through the cooler to control the temperature of the bleed air.

What is the purpose of the engine bleed air control valves?

These valves control the flow of bleed air into the pneumatic system manifold and are controlled electrically by switches on the flight deck.

Why is it a regulatory requirement to duplicate certain components in aircraft cabin air conditioning or pressurization systems?

Duplicating components ensures that the failure of any single element does not jeopardize the safety of the aircraft or its occupants.

What happens if there is a failure in the bleed air system that could cause excessive temperature or pressure?

If the temperature or pressure becomes excessive, the bleed air valve will automatically close, and a warning light in the cockpit will come on.

How can a pilot restore the bleed air system in the event of a fault?

The pilot can restore the system by opening the isolation valve, allowing air from one engine to operate the entire system.

Outlines

00:00

🛫 High-Volume Low-Pressure Air Systems in Jet Airliners

This paragraph discusses the bleed air systems in modern jet airliners, which are primarily supplied by air from engine compressors and can be supplemented by the Auxiliary Power Unit (APU) or external air supply carts. The bleed air is utilized for various purposes including air conditioning, pressurization, ice protection, and powering air turbine motors for engine starting and hydraulic pumps. The system is designed to maintain a consistent pressure and temperature, with air taken from different compressor stages to manage pressure variations. The paragraph also explains the operation of high-pressure shutoff valves and non-return valves, as well as the use of pre-coolers to regulate the temperature of the bleed air. The bleed air control valves are electrically controlled and are designed to maintain a maximum duct pressure. The system is also equipped with safety features to handle over-temperature and over-pressure situations, ensuring the safety of the aircraft and its occupants.

05:01

🔧 Bleed Air System Safety and Backup Mechanisms

The second paragraph delves into the safety and backup mechanisms of the bleed air system. It explains the regulatory requirement for duplicating critical components in aircraft systems to ensure safety. The system is designed such that each engine supplies its own services, with isolation valves that can be opened in case one engine supply fails. The paragraph also discusses the use of a two-pointer duct pressure gauge to monitor pressure in both left and right ducts, and the presence of engine bleed air valve position indicators. It highlights the system's ability to automatically close the bleed air valve in response to excessive temperature or pressure, activating warning lights in the cockpit. In case of faults, pilots can restore the system by opening the isolation valve. The paragraph concludes by mentioning the role of the Auxiliary Power Unit and external air connections in supplying air to the bleed air manifold, and the temperature monitors that prevent damage from excessive temperatures due to leaks.

Keywords

💡Bleed Air

Bleed air refers to the hot, compressed air extracted from the compressor stages of a jet engine. In the context of the video, bleed air is crucial for various systems on modern jet airliners, including air conditioning, pressurization, ice protection, and powering auxiliary systems. The script mentions that bleed air is normally supplied from engine compressors and can be backed up by the Auxiliary Power Unit (APU) or ground air supply carts. It is essential for maintaining a comfortable and safe environment within the aircraft.

💡Auxiliary Power Unit (APU)

The Auxiliary Power Unit is a small turbine engine on an aircraft that provides power for systems when the main engines are not running, such as during ground operations or when starting the main engines. The script explains that the APU can supply bleed air to the aircraft's systems, ensuring that there is a backup source of power and air for essential functions like air conditioning and pressurization.

💡Pressurization

Pressurization is the process of maintaining a higher internal pressure in the aircraft cabin compared to the external atmospheric pressure, especially at high altitudes. The video script discusses how bleed air is used for pressurization, which is vital for passenger comfort and safety by simulating a lower altitude environment within the cabin.

💡Ice Protection

Ice protection is a system designed to prevent the buildup of ice on critical aircraft surfaces, such as wings and engines. The script mentions that bleed air is used for ice protection, indicating that the hot air can be directed to areas prone to icing to prevent it from forming or to remove it if it does.

💡Air Turbine Motors

Air turbine motors are devices that use the flow of air to generate mechanical power. In the video, it is mentioned that bleed air is used as a power source for air turbine motors, which are essential for engine starting, driving hydraulic pumps, and operating air-driven flaps and slats.

💡Hydraulic Reservoirs

Hydraulic reservoirs are containers that hold hydraulic fluid, which is used to transmit power in various aircraft systems. The script explains that bleed air is used to pressurize hydraulic reservoirs, ensuring that the hydraulic systems have the necessary pressure to function correctly.

💡High-Pressure Shutoff Valve (HP SOV)

The High-Pressure Shutoff Valve is a component that controls the flow of high-pressure bleed air. As described in the script, this valve is pressure-sensitive and pneumatically operated, opening when there is insufficient air pressure from the low-pressure system to maintain the required flow. It plays a critical role in managing the pressure and temperature of the bleed air system.

💡Pre-cooler

A pre-cooler is a device that reduces the temperature of the air before it is used for various purposes within the aircraft. The script mentions that the air from the compressor is too hot for its required purpose, so it is passed through a pre-cooler. This cooling process typically uses engine fan air as a medium, and the temperature is regulated to a predetermined level, usually around 175 degrees Celsius.

💡Non-Return Valve (NRV)

A non-return valve is a valve that allows fluid flow in only one direction, preventing backflow. In the context of the video, when the high-pressure shutoff valve opens, it closes the non-return valve, shutting off the low-pressure supply. This mechanism ensures that the aircraft's systems receive the appropriate pressure of bleed air.

💡Isolation Valve

An isolation valve is used to shut off the flow of air or fluid in a system. The script explains that multi-engine aircraft keep the supplying engines or sides separate, with each engine supplying its own user services. Isolation valves are used to keep these systems independent and can be opened if an engine supply is lost to feed the other side services, ensuring the aircraft's systems remain operational.

💡Duct Pressure Gauge

A duct pressure gauge is an instrument used to measure the pressure within the aircraft's air ducts. The video script describes a two-pointer duct pressure gauge, where one pointer displays the pressure in the left duct and the other in the right duct. This gauge is essential for monitoring the pressure in the bleed air system and ensuring it remains within safe operating limits.

Highlights

Modern jet airliners use high-volume low-pressure air systems supplied by engine compressors or auxiliary power units.

Bleed air is utilized for air conditioning, pressurization, ice protection, and powering various aircraft systems.

The engine bleed air system includes the engine compressor, control devices, and temperature and pressure regulation mechanisms.

Air from different compressor stages is used to maintain a reasonable pressure band at all engine speeds.

The high-pressure shutoff valve (HP SOV) controls the high-pressure source and is pressure-sensitive and pneumatically operated.

The non-return valve (NRV) shuts off the low-pressure (LP) supply when the HP SOV is open.

As engine speed increases, the LP air pressure rises, eventually closing the HP shutoff valve.

Bleed air from the compressor is too hot for direct use and must pass through a pre-cooler.

A cooling matrix with engine fan air as the cooling medium is used to reduce the temperature of the bleed air.

Engine bleed air control valves are controlled electrically and act as pressure regulators for the pneumatic system.

A regulatory requirement ensures that any single component failure in cabin systems does not compromise safety.

Multi-engine aircraft keep supplying engines or sides separate, with each engine supplying its own user services.

Isolation valves can be opened to feed other side services if an engine supply is lost.

The system is equipped with a two-pointer duct pressure gauge to monitor pressure in both left and right ducts.

Excessive temperature or pressure in the system triggers automatic closure of the bleed air valve and a warning light.

Pilots can restore the system by opening the isolation valve in case of faults.

Temperature monitors are mounted along the bleed air manifold to detect leaks and prevent damage.

The auxiliary power unit air is fed into the left duct, and the isolation valve must be open to supply all services.

External air connections feed air into the right duct, requiring the isolation valve to be open for full service supply.

Transcripts

in this lesson we will discuss the high

volume low pressure air systems used by

modern jet airliners

the system is normally supplied with

bleed air from the engine compressors

this can be backed up with air from the

auxiliary power unit APU

ground by external air supply carts

this air is used for air conditioning

and pressurization

ice protection

and is a power source for the air

turbine motors used for engine starting

driving hydraulic pumps and air driven

flaps and slats

it is also used for pressurizing

hydraulic reservoirs and potable water

tanks

and for cargo compartment heating

the engine bleed air system consists of

the power source the engine compressor

and control devices the temperature and

pressure regulation during operation

because of the great variation of air

output pressure from a gas turbine

engine between idle and maximum rpm

there is a need to maintain a reasonable

supply of air during low rpm as well as

restricting excessive pressure when the

engine is at high rpm

this is usually done by taking air from

two compressor stages to maintain a

reasonable pressure band at all engine

speeds

here we show a typical bleed air system

low pressure LP air is taken from an

early compressor stage

and high-pressure HP air is taken from a

later stage

you

the high-pressure source is controlled

by the high-pressure shutoff valve HP

sov this valve is pressure sensitive and

pneumatically operated it is open when

there is insufficient air pressure from

the LP system to maintain the required

flow

when this valve is open HP pressure will

close the non-return valve NRV shutting

off the LP supply

as the engine speeds up the LP air

pressure will increase until it closes

the high-pressure shutoff valve so that

in most stages of flight bleed air will

come from the LP stage

the air from the compressor is too hot

for its required purpose so it is passed

through a pre-cooler in order to reduce

its temperature

this usually takes the form of a cooling

matrix with engine fan air being used as

the cooling medium

valve modulates to vary the amount of

fan air passing through the cooler and

thus control the temperature of the

bleed air at a predetermined level

usually about 175 degrees Celsius

the engine bleed air control valves

control the flow of bleed air into the

pneumatic system manifold

they are controlled electrically by

switches on the flight deck

when selected open the bleed air valve

acts as a pressure regulator the valve

position is automatically modulated to

maintain a maximum duct pressure of

approximately 55 pounds per square inch

if the output pressure is below this the

valve will be fully open

some aircraft have separate bleed air

and pressure regulating valves

however they function in the same manner

as the combined valve

it is a regulatory requirement that any

single component in an aircraft's cabin

air conditioning or pressurization

systems the failure of which could

jeopardize the safety of the aircraft or

the people in it must be duplicated

to this end most multi-engine aircraft

keep the supplying engines or sides

separate with each engine supplying its

own user services

these are kept independent by isolation

valves which are normally closed but

which may be opened if an engine supply

is lost to feed the other side services

the system is fitted with a two-pointer

duct pressure gage one pointer displays

the pressure in the left duct and the

other the right

some systems have engine bleed air valve

position indicators however in the

system shown here the valve position is

shown by the switch position unless a

warning light is indicating a problem

if the temperature of the air downstream

of the bleed air valve becomes excessive

the valve will automatically close

a warning light in the cockpit which is

monitoring the system for both excessive

temperature and pressure will come on

excessive pressure in the system can be

caused by failure of the high pressure

shutoff valve to close when required a

pressure sensor monitors duct pressure

upstream of the bleed air valve in the

event of excessive pressure the bleed

air valve will once again automatically

close and the warning light will come on

in the event of either of these faults

the pilot can restore the system by

opening the isolation valve allowing the

air from one engine to operate the

entire system

the air in the bleed air manifold is hot

in excess of 180 degrees Celsius so any

air leaking from the manifold could

cause a great deal of damage

there are temperature monitors mounted

adjacent to the manifold all along its

length

if any of these monitors senses an

excessive temperature caused by a bleed

air leak the wing duct overheat light

for the affected side will illuminate

the pilot can isolate the leaking area

by closing the appropriate bleed valve

and ensuring that the isolation valve is

closed

in the system shown here the auxiliary

power unit air is fed into the left duct

so in order for it to supply all

services the isolation valve has to be

opened

similarly the external air connection

feeds air from an external source into

the right duct so again the isolation

valve needs to be open for it to supply

air to all of the services

that is the end of the lesson in this

lesson you have learned that most modern

gas turbine engine aircraft have a bleed

air system supplied with air tapped from

the engines compressors

the air is used by a number of systems

as shown here

the air supply is backed up by the

auxiliary power unit and on the ground

by external air supply carts

the system is duplicated

tected from over temperature and

overpressure

you