Pneumatic Systems Flashcards
Pneumatics System Uses on light and large AC
Light:
Power gyroscopes
Cabin heating and inflate de-icing boots
Large:
Cabin conditioning
Pressurisation
Ice protection
Bleed air to power pneumatic systems (reduces thrust)
Advantages of Pneumatic Systems
+ Air readily available and light
+ easy to generate compressed air using engine drive compressor
+ gas turbine have supply of bleed air system
Disadvantages of pneumatic systems
Bleed air reduces thrust and not efficient
Designers moving away from bleed air
Electrical systems replacing pneumatic systems
Light A/C pneumatic sources
Engine driven vacuum pump
Pump driven by the engine via quill drive shaft
Quill Shaft
Used on light A/C pneumatic systems to generate compressed air
Designed to shear if torque becomes excessive
Large A/C pneumatics air supplies
Bleed air from main engines
Compressed air from APU
Compressed air from ground power unit
GPU
Ground Power unit
Used to provide conditioning air before APU started
Can start engines with air starter mechanism
+ Save running APU and burning fuel
APU
Small turbojet at the rear of fuselage
Provide electrical power and compressed air when engines not running
Drives a compressor to generate air
Bleed Air
Generate large amounts of compressed air by tapping it from engine compressor
Problems with Bleed Air
Very high temp and pressure variations must be cooled before use
The Bleed Tapping
High pressure/low pressure pipelines from engine drawn through system.
High pressure line passes through high pressure shut off valve (HPSOV)
Route:
400psi (HPSOV) 100psi (PRSOV) 40psi
Then goes to the pre cooler
High pressure reducing shut off valve - pneumatics
Used to reduce high pressure aired from rear of engine from 400 psi down to 100 psi then onto the PRSOV
Pressure reducing shut off valve
Used to reduce pressure further from high pressure line and low pressure line from 100psi to 40 psi for pre cooler
How is air distributed around A/C
Ducts and shut off valves to control supply to different systems
Shut off Valves operations
Pneumatically operated and electrically controlled
Sensor detect position of valves and pressure and relay to flight deck
Duct Construction
Stainless steel for high temp/pressure ducts
Light alloy for mid temp/pressure ducts
Plastic/fibreglass for low temp/pressure ducts
Overheat detection and warning in pneumatic systems
Overheat detection loops location in all sensitive areas
Can be operated with one loop inoperative for limited time
Must detect a hot gas leak within 0.3 metres
Zones/Loops in Pneumatic system
Used for heat detection systems
Two loops for every zone
One loop needed for flight ops minimum
Light AC air conditioning
Hot air form heat exchanger around exhaust mixed with cold air ram air system
Air conditioning requirements CS25
1lb of fresh air per minute per seat
0.5lb of fresh air per minute per set in emergency
Temp between 18/24 degrees
Co2 not to exceed 1/20000
ECS
Environmental control system collective name for components required for conditioning and pressurisation
Bootstrap system in operation
Air moves from engine compressor to pre cooler
Passed onto the cooling pacts
Ducted to pacts via isolation valve
Passed to primary heat exchanger in ram air duct
Move toward temperature control valve (bypasses or goes through compressor)
Compressor to secondary heat exchanger
Condensed air extracted to ram air
Passed through turbine for further cooling then warmed with bypass air
Off to the cabin
Mixing Valve Role
Controls hot bleed air allowed to bypass the cold air unit
Temperature control valve or mixing valve
Bootstrap system - Cold Air Unit
Combined compressor/turbine in the conditioning pack
Water Seperator
Located downstream and extracts moisture from the air and dumps it in ram air duct
Ram Inlet role
Control airflow to heat exchanger
NACA ducts on the sides of aircraft’s
Pressure Ducts role
Located under floor and circulate air up the side of fuselage and up above then flows back down and picked up by recirculation fan
Why do we have pressurisation controls
Control minimum cabin pressure
Control max differential pressure
Control max rate of change in cabin pressure
What is cabin altitude
The cabin pressure usually limited to 8000ft
Negative relief valve
Let’s pressure into the AC if outside has a higher pressure
Role of pressure controller
Maintains required differential pressure (we set destination QNH)
Outflow Valves
Allow pressure to be past to atmosphere from the inside
Safety Valves
Protect from over pressurisation if outflow valves do not operate
0.5psi above max differential pressure they will open
Dump Valve
Manually operated valve to dump pressure in emergency
Important for emergency landings to get the doors open
Ground Pressurisation
Slight pressurisation before take off to smooth out variation and help the pressurisation controller
Isobaric Range
Constant maintained pressure between discharge from outflow and the inflow of air
3 types of decompression
Normal - 6 to 10 seconds
Rapid - 4 to 6 seconds
Explosive - 0 to 3 seconds
Excess cabin altitude warnings x3
10,000ft - visual and aural warning
14,000ft - oxygen masks drop
Rate of change of cabin pressure is known as
Cabin vertical speed
Ground cooling fan
Draw air through the ram air duct when the aircraft is stationary to ensure heat exchangers are kept cool
