Aeroplane Air-Mixing Valve

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Questions and Answers

What is the primary function of the air-mixing valve?

To separate water from the air

To control the flow of cold air to the pack

To regulate the temperature of the cabin

To control the proportion of hot and cold air to the pack and distribution system (correct)

How is the common disc shaft of the air-mixing valve driven?

By a manual control switch

By a temperature controller

By a 24-V DC motor

By a 115-V AC motor (correct)

What is indicated by the visual position indicator on the air-mixing valve?

The temperature of the pack

The speed of the motor

The pressure of the air

The position of the valve in the cold or hot range (correct)

What happens to the air when it goes through the cooling pack?

It gets cooled down Signup and view all the answers

What is the purpose of the water separator?

To condense water from air Signup and view all the answers

What controls the air-mixing valve?

The cabin temperature controller or the manual control switch Signup and view all the answers

What is the function of the position transmitter in the air-mixing valve?

To indicate the valve position Signup and view all the answers

What is the configuration of the air-mixing valve?

Two disc plates mounted on a common shaft 90° opposed Signup and view all the answers

What is the primary function of the temperature control valve?

To regulate the pack outlet temperature Signup and view all the answers

What happens when electrical power is applied to the temperature control valve?

The bleed port in the pressure chamber closes Signup and view all the answers

What is the purpose of the downstream temperature-sensing element?

To control the temperature control valve Signup and view all the answers

What happens to the valve in case of failure?

The valve moves to a spring-loaded closed position Signup and view all the answers

What is the function of the air mix valve in the air-cycle machine?

To bypass hot bleed air from the engines Signup and view all the answers

What is the result of repositioning the temperature control valve?

The pressure in the pressure chamber changes Signup and view all the answers

What is the type of control used in the temperature control valve?

Electrical and pneumatic control Signup and view all the answers

Where is the outlet air from the inlet typically routed to?

To the mix chamber Signup and view all the answers

What is the primary function of the Flow Control Shut-Off Valve in an air cycle system?

To control and adjust the flow of air into the air conditioning pack Signup and view all the answers

What is required to open the Flow Control Shut-Off Valve?

Both electrical power and upstream pneumatic pressure Signup and view all the answers

What happens when the dump valve solenoid is energized?

The valve opens to vent control pressure to ambient Signup and view all the answers

What is the failsafe position of the Flow Control Shut-Off Valve in the event of electrical power failure?

Fully open Signup and view all the answers

What happens to the secondary piston when the downstream pressure is too high?

It moves to open the bleed orifice Signup and view all the answers

What acts on the primary piston to open the Flow Control Shut-Off Valve?

Upstream pneumatic pressure Signup and view all the answers

What is the function of the lever mechanism in the Flow Control Shut-Off Valve?

To open the bleed orifice Signup and view all the answers

What is the location of the Flow Control Shut-Off Valve in an air cycle system?

In the air conditioning compartment Signup and view all the answers

What is the purpose of the overheat sensors in the supply duct?

To provide indication and stop operation when the temperature is out of limits Signup and view all the answers

What happens when the imbalance across the Wheatstone bridge is amplified?

The mixer valve varies temperature to rebalance the bridge Signup and view all the answers

What is the function of the duct temperature anticipator sensor?

To read the rate of change in the duct air temperature Signup and view all the answers

What happens when the temperature continues to climb despite the lower overheat sensor signaling the mixing valve to the full cold position?

The pack valve closes Signup and view all the answers

How does the resistance of the duct temperature anticipator sensor change as temperature increases?

It decreases Signup and view all the answers

What is the purpose of the actuator control in the temperature control system?

To move the mix-valve so that cabin temperature changes slowly and without raising duct temperature above limits Signup and view all the answers

What is the result of the system resetting after the temperature has cooled to below trip temperature?

The duct overheat warning light turns off Signup and view all the answers

Why are large aircraft often divided into zones for air distribution?

To provide more efficient temperature control Signup and view all the answers

What happens when the temperature of the cold bleed air in the water separator decreases towards freezing?

The low limit system adds hot bleed air to cold bleed air via the low limit valve Signup and view all the answers

What is the purpose of the temperature control valve in the air conditioning system?

To mix hot and cold air to regulate temperature throughout the cabin Signup and view all the answers

What signal is received by the temperature controllers from various sensors?

A signal to open or close the temperature control valves Signup and view all the answers

What is the function of the standby temperature control valve?

To provide backup control for the discharge temperature of the air conditioning pack Signup and view all the answers

What happens to the air in the mix chamber?

Hot air from the air mixing valve is added to cold air Signup and view all the answers

What is the purpose of the air-mixing valve in the air conditioning system?

To mix hot and cold air to regulate temperature throughout the cabin Signup and view all the answers

What is the primary function of the low limit system?

To prevent ice formation in the water separator Signup and view all the answers

What happens when the normal temperature control system fails?

The standby temperature control valve takes over Signup and view all the answers

What is the purpose of the gold coating on some pneumatic manifold ducts?

To reduce heat transfer Signup and view all the answers

What is the function of the isolation valve in the pneumatic manifold?

To divide the manifold into two separate systems Signup and view all the answers

What is the typical pressure range of air stored in metal bottles for high-pressure systems?

1000 to 3000 psi Signup and view all the answers

What is the purpose of the control valve in a high-pressure storage cylinder?

To act as a shutoff valve Signup and view all the answers

What is the purpose of overheat detectors in pneumatic ducts?

To detect high temperatures Signup and view all the answers

What is the advantage of using corrosion-resistant tubing for pneumatic manifold ducts?

It protects against corrosion Signup and view all the answers

What is the purpose of insulation blankets on some pneumatic ducts?

To decrease heat transfer Signup and view all the answers

What is a disadvantage of high-pressure storage cylinders?

They have a definite disadvantage Signup and view all the answers

What is the primary advantage of using compressed air over hydraulic or electrical systems?

It is universally available in an inexhaustible supply. Signup and view all the answers

In which range of pressure does a medium-pressure system operate?

50 to 150 psi Signup and view all the answers

What is the primary function of the venturi system in early light aircraft?

To provide a vacuum source. Signup and view all the answers

What is a potential problem that can be prevented by proper maintenance of filter systems?

Component failure and system contamination. Signup and view all the answers

What is one of the advantages of compressed air systems compared to hydraulic systems?

They are lighter. Signup and view all the answers

What is the range of pressure for high-pressure systems?

1000 to 3000 psi Signup and view all the answers

What is the primary limitation of using a pneumatic system for continuous operation of systems such as landing gear or brakes?

Insufficient air pressure Signup and view all the answers

What is the purpose of permanently installed air compressors on some aircraft?

To recharge air bottles whenever pressure is used Signup and view all the answers

What is the purpose of the filter in the venturi system?

To exclude dirt and moisture. Signup and view all the answers

What is the primary source of pneumatic power on the ground?

Mobile starter units Signup and view all the answers

What happens to the air in the venturi system as it accelerates through the narrowing section?

Its velocity increases. Signup and view all the answers

What is the typical location of low-stage bleed ports on a turbine engine?

Around stages five and ten Signup and view all the answers

What is the primary use of high-pressure bleed air?

During slow flight or descent Signup and view all the answers

What is the purpose of adding multiple air-pressurising units to an aircraft?

To increase system reliability Signup and view all the answers

What is the primary advantage of using an Airborne Auxiliary Power Unit (APU) as a pneumatic source?

Alternate source of air pressurization Signup and view all the answers

What is a limitation of using engine compressors as a pneumatic source?

Limited air supply during idle engine conditions Signup and view all the answers

What is the primary source of pneumatic air in some aircraft?

Bleed air taken from a gas turbine engine compressor Signup and view all the answers

What type of compressor is the Roots-type blower an example of?

Positive displacement compressor Signup and view all the answers

How is pressure controlled in a cabin system?

By using outflow valves Signup and view all the answers

What is the purpose of the check valves in a multi-stage compressor?

To allow air to flow in only one direction Signup and view all the answers

Why are independent cabin compressors designed to overcome the limitations of bleed air sources?

Because bleed air sources have many disadvantages Signup and view all the answers

What is the purpose of the fins on the Roots-type blower housing?

To provide some cooling and allow outside air to pass through Signup and view all the answers

What is the result of each rotation in a Roots-type blower?

Pressure increases Signup and view all the answers

What type of compressor can be driven by an accessory drive gearing or powered by bleed air from an engine compressor?

Independent cabin compressor Signup and view all the answers

What is the typical pressure range of a medium-pressure pneumatic system?

100-150 psi Signup and view all the answers

Where is the bleed air typically taken from in a turbine engine?

From the turbine engine compressor section Signup and view all the answers

What is the primary use of compressor bleed air in a turbine engine?

All of the above Signup and view all the answers

What is a limitation of high-pressure pneumatic systems?

They cannot be recharged in flight Signup and view all the answers

What is the purpose of the pressure-controlling unit in a medium-pressure pneumatic system?

To reduce the pressure of the bleed air Signup and view all the answers

What is the typical pressure range of air stored in metal bottles for high-pressure pneumatic systems?

1000-3000 psi Signup and view all the answers

What is the function of the control valve in a high-pressure pneumatic system air bottle?

To act as a shutoff valve Signup and view all the answers

What is the benefit of using bleed air from turbojet engines for medium-pressure pneumatic systems?

It is a readily available source of air Signup and view all the answers

Study Notes

Air-Mixing Valve

Controls pack output temperature by directing airflow through the cooling pack or around the cooling pack to the mix chamber
Dual housing assembly with two disc plates mounted on a common shaft 90° opposed
Driven by a 115-V AC motor, controlled by the cabin temperature controller or manual control switch on the flight deck
Incorporates a visual position indicator showing the valve in the cold or hot range
Sends signals to the air-mixing valve position indicator on the cabin temperature control panel in the flight deck

Temperature Control Valve

Used to control the pack outlet temperature
Electrically controlled and pneumatically operated
Relies on a signal from the downstream temperature-sensing element, controlled through the temperature control system
Fails safe closed in case of electrical power failure

Air Cycle System Components

Flow Control Shut-Off Valve (FCSOV) controls and adjusts the flow of air into the air conditioning pack
Normally situated in the air conditioning compartment
Electrically controlled and pneumatically actuated
Regulates downstream pressure and flow

FCSOV Valve Operation

Upstream bleed air pressure opens the valve
Downstream overpressure releases control pressure and modulates the valve to the closed position
Failsafe is in the open position in case of electrical power failure, maintaining air supply for pressurization

Overheat Sensors

Located in the supply duct
Give indication and can stop operation when temperature is out of limits
Lower overheat sensor signals the mixing valve to the full cold position
Higher overheat sensor causes the pack valve to close

Anticipator

Duct temperature anticipator sensor is part of a bridge circuit in the temperature controller
Reads the rate of change in the duct air temperature
Adjusts the air mix valve position to ensure cabin temperature changes slowly and without raising duct temperature above limits

Separate Zone Control

Large aircraft may be divided into zones for air distribution
Low limit system adds partially cooled bleed air to cold bleed air to prevent ice in the water separator
Mix chamber adds hot bleed air from the hot side of the air mixing valve
Various cabin sensors give signals back to position the mixing valve, regulating temperature throughout the cabin

Pneumatic Systems

In-flight pneumatic sources include engine compressors and an Airborne Auxiliary Power Unit (APU) as an alternate source.
Ground pneumatic sources include mobile starter units, ground pneumatic networks, APUs, and engine compressors.

Engine Bleed Air Supply

Pressurized air is taken from the engine compressor in turbine engines (jet or turboprop).
Air is bled off the compressor at one or two places, usually around stages five and ten.
Low-pressure air (from the fifth stage) is used during take-off, climb, and cruise conditions.
High-pressure bleed air (from the tenth stage) is used during slow flight or descent, where engine power is reduced.

Bleed Air Distribution

Pneumatic systems that use bleed air include engine start, air conditioning, pressurization, engine inlet cowl anti-ice, wing thermal anti-ice, water tank pressurization, and total air temperature probe aspiration.

Ducts

Pneumatic manifold ducts are made of strong, corrosion-resistant, lightweight tubing.
Some ducts have a gold coating to protect against hydrocarbon contamination and decrease heat transfer.
Insulation blankets cover some ducts to reduce heat transfer.
Overheat detectors are located near the pneumatic ducts to detect leaking ducts.

High-Pressure Distribution System

High-pressure systems store air in metal bottles at pressures ranging from 1000 to 3000 psi.
The bottles have a charging valve and a control valve (shutoff valve).
The system is limited by the small supply of bottled air, as it cannot be recharged in flight.

Compressors

Independent cabin compressors are designed to overcome the disadvantages of bleed air sources.
Compressors can be driven by accessory drive gearing or powered by bleed air from an engine compressor.
Types of compressors include positive displacement compressors (Roots-type blower) and centrifugal compressors.
Positive displacement compressors work like a gear pump, taking a predetermined volume of air, compressing it, and delivering it to the cabin duct.

Medium-Pressure Systems

Medium-pressure pneumatic systems (100–150 psi) do not include an air bottle.
Compressed air is taken from the turbine engine compressor section or a separate compressor device.
The bleed air is routed to a pressure-controlling unit to reduce the pressure to about 40 psi and then distributed to the operating units.

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Description

This quiz covers the air-mixing valve in aeroplane systems, including its function and installation. It is part of the CASA Part 66 training materials.