Pneumatic Systems Flashcards
What are the pros and cons of using pneumatics in an aircraft?
Pros -
1. Pneumatic systems are much lighter than hydraulic systems
2. They need little maintenance
3. They require no special fluids.
4. The air is freely available from outside the plane, so no reservoir is needed to store it.
Con
1. Pneumatics use compressed air. Air is a gas, and all gases are compressible. Because it’s compressible, pneumatic power is far less efficient than hydraulic power for heavy-duty jobs.
Where does the air for the pneumatic system come from?
On a piston plane, pressure or vacuum pumps are driven mechanically off the engines.
However, on a turbine aircraft, the air comes from the compressor section of the engines.
Turbine engines are essentially giant pneumatic pumps.
What is the difference between high and low pressure bleed air?
High and low pressure bleed air come from different places on the engine and have different temperatures and pressures. The higher the pressure of the bleed, the higher the temperature of the air.
The low pressure bleed air comes from an early stage of the compressor section, and the high pressure bleed air comes from the high-pressure compressor section.
When the plane is operating at high power, the low-pressure bleed air usually has enough pressure to maintain the air conditioning and pressurization system.
However, when the plane is operating at low power, the pressure will fall, so the high pressure bleed valve opens to make sure there’s enough pressure and airflow.
The high-pressure bleed valve will also open when anti-ice air is turned on, because the system needs a lot of hot air.
How is pneumatic air used to start the aircraft?
Typically for engine starting, the aircraft APU is started electrically to provide bleed pneumatic power, or else a ground pneumatic power source is used.
When the captain calls for starting, a start valve is opened, sending compressed air to spin up a small turbine in the pneumatic starter. This, in turn, spins up the engine core compressor.
Fuel is introduced, and when N2 / high-pressure compressor RPM reaches a predetermined value, the start valve is closed, and the starter turbine is disengaged from the engine’s compressor.
What is high-pressure air used for?
- Engine and wing anti-ice
- Cabin heat
- Cooling
- Pressurization
It provides cabin heat, cooling, and pressurization by using some combination of air cycle machines (ACMs, or Packs) and heat exchangers.
What is low-pressure air used for?
Low-pressure air in turbine aircraft drives air gyros and pneumatic deicing systems, if installed. Low- pressure air may also be used for mechanical control functions such as valve actuation (pneumatic and/or hydraulic), control of pressurization outflow valves, and inflation of door seals to sustain pressurization
What kind of systems typically use pneumatics in an aircraft?
Air conditioning and heat
Pressurization
Engine and wing anti-ice
Cabin door acoustic seal
Rudder bias heat
Baggage compartment heat
What is an ACM?
An ACM is an air cycle machine. It takes hot bleed air from the engines to produce cold air. This conditioned air serves as the primary source of pressurization.
It consists of…
- Primary and secondary heat exchangers
- Water separator
- Over-temperature switch
What is the bleed air system?
The aircraft will take compressed air from the APU or engines and distribute it to other systems on the plane.
What supplies bleed air and when?
If both engines are running, they both supply bleed air.
If just one of them are running, a crossbleed valve will open that allows the air from one engine to supply air for all the systems on the aircraft.
The crossbleed valve will also be open if the APU is running and supplying air.
How will using bleed air impact aircraft performance?
You’ll have less thrust and the aircraft will be less efficient.
When air is bled off from the engine, it reduces the air mass flow through the engine. That will reduce the engine’s thrust. This will be reflected on the engine pressure ratio gauge / EPR.
Reducing the air mass flow through the engine also means there will be less cold air available to cool the combustion chamber. Thus, your exhaust gas temperature / EGT will be higher.
On the flip side, bleeding air from the compressor reduces the load on the turbine. This will increase the engine RPM for a given fuel flow, and fuel economy will decrease.
So whenever an engine bleed is open, the aircraft will burn more fuel than if the bleed is closed.
What are some hazards of bleed air?
- Use of bleed air for thermal wing anti-ice is either not permitted for takeoff or not allowed to be on for very long.
Not only do the bleeds sap lots of engine power, but on the ground, the 600°–800°F bleed temperatures can weaken the aluminum wing leading edges.
- Due to the very high temperatures involved, fire can result from unattended bleed air leaks within the aircraft. Therefore, bleed systems are normally monitored in the cockpit.
What is the difference between a check valve and an isolation valve in the pneumatic system?
Check valves, like the ones in our reference water system, are used to restrict pneumatic flow to one direction.
Isolation valves, which can be automatic or manual, allow separation of interconnected bleed systems in the event of leaks or other failures.
Normally, if one bleed is shut off at its source, most systems can be operated on the remaining bleed air source or sources except under very high-draw situations. However, if a serious leak occurs somewhere in the pneumatic system, requiring use of an isolation valve, some bleed-powered items will likely become inoperative.
