SOURCES OF PNEUMATIC AND CLASSES OF PNEUMATIC SYSTEM

1.4 SOURCES OF PNEUMATIC AND CLASSES OF PNEUMATIC SYSTEM

As said in previous Articles, hydraulic/pneumatic system can be used to exploit two major advantages over other types of mechanical systems. One is the ease with which force can be transmitted over large distances and into and out of sealed compartments. The other is the large gain in mechan­ical advantage made possible by varying the size of pistons.

Aircraft is provided with fluid system (hydraulic and pneumatic system). Normally, modern aircraft uses hydraulic system for transmission of pressure and produce forces to a remote actuating cylinders for movement of flight control surfaces, applying brakes, extending/retracting landing gears and doors with mechanical advantages. In the same way, pneumatic system may be used for the same purpose, in the same way with a little difference in the system designs, for example, hydraulic system uses return system so that same fluid is used again and again, but pneumatic system need not have return lines leaving the used pneumatic from the system to outside. Using the pneumatic system for transmission of pressure and actuating cylinders has got some advantage over the hydraulic system as mentioned in earlier Articles.

Aircraft pneumatic system is a complete system designed with:

•   Pneumatic supply sources and storage bottles/manifolds
• Plumbing lines (pipes and hoses)
•   Controlling valves, fittings, filters, driers and so on
•   Pumps
• Water separators
• Actuators

And other system components for indicating system status in cockpit along with cockpit controls.

This and subsequent Articles/weeks will illustrate system layouts of aircraft 

pneumatic system, discuss essential features including supply sources and storage systems, plumbing lines and fittings, controls and components, indication and warning systems, essential maintenance activities and so on.

Air source of a pneumatic system may be:

v     High pressure storage bottle: 1000 to 3000 psi (High pressure pneumatic system) 

v     Engine Bleed: 100 to 150 psi (Medium Pressure pneumatic system)

v     Air pumps: 100 psi (Low pressure pneumatic system)

High-pressure bottle may be:

v     Pre-charged on ground only: disadvantage of discontinuous use permitting only for limited/emergency services

v     Re-charged during flight: advantage of continuous use; recharging being done from air compressors driven from engine gearbox.

Engine bleed is the pneumatic source in most of the wide-bodied modern aircraft that employs a pneumatic system manifold pressurized by bleed air from running engines or APU or a running ground pneumatic compressor. This is in the class of medium pressure pneumatic system.

Air pumps are the sources of pneumatic in many piston-engined aircraft that employs engine driven vane type pumps those supplies pneumatic at low pressure. This is in the class of Low pressure pneumatic system.