6 - Pneumatics

Question 1

Advantages of pneumatic systems

Answer 1

• Unlimited supply of air
• Lightweight/simple system
• Compressed air is lightweight
• Relatively free from temp problems
• No fire hazard, minimal explosive hazard with good system design

Question 2

Disadvantages of pneumatic systems

Answer 2

• Must be compressed significantly before it could actuate a heavy mechanical system
• The compressibility of air (opposed to liquid) leads to a lag in response and lack of control
• Difficult to provide air tight seals/find leaks

Question 3

What are high pressure systems used for?

Answer 3

Full pneumatic systems (3000 psi)

Back up pneumatic systems (1000-3000 psi)
- metal bottles, air or nitrogen, two valves (charging and control), emergency operation of systems, cannot be recharged on ground

Question 4

What are medium pressure systems used for?

Answer 4

Anti/de icing, engine starting, cabin pressurisation, cabin air conditioning (100-150 psi)

Question 5

Where do medium pressure systems source air from?

Answer 5

Engine driven compressor or
Bleed air from turbocharger

Question 6

What are low pressure systems used for?

Answer 6

Vacuum/pressure instruments (gyros)

Question 7

What is a shuttle valve used for?

Answer 7

Directs either normal or emergency air pressure into the system (two sources for one purpose)

Question 8

What is the purpose of a relief valve?

Answer 8

Limits pressure, preventing burst pipes/seals

Question 9

What is the purpose of a check valve?

Answer 9

Prevents back flow/one way flow control

Question 10

What is the purpose of a restrictor?

Answer 10

Reduces the rate of airflow and therefore the speed of actuation

Question 11

What is the purpose of a control valve?

Answer 11

Allows the selection of pneumatics to alternate system

control valve closed - emergency system vented and blocked from main system
control valve open - main pneumatics routed to emergency system and vent closed

Question 12

What kind of filters are typically used for pneumatic systems? (2) What are their purpose?

Answer 12

Replaceable cellulose cartridge
Permanent wire screen

Prevent dirt/particles from entering system

Question 13

Why is moisture an issue in pneumatic systems?

Answer 13

As the pressure of the air is decreased during actuation any moisture will condense and freeze (reduced temp with alt increases likelihood)

Question 14

What moisture control system are applied? (2)

Answer 14

Moisture separator (98% of moisture)
- removes moisture downstream of compressor

Desiccant/chemical dryer
- removes remaining moisture
- replaceable cartridge of dehydrating agent (blue when dry)