1. GROUND SERVICING OF VARIOUS SUB SYSTEMS

2. Aircraft air conditioning system
Aircraft-cooling systems, also called air-conditioning systems, are used to reduce the temperature inside an aircraft for crew and passenger comfort
Types
vapor-cycle machine
air-cycle machine

3. Air-Cycle Cooling
Modern large turbine powered aircraft make use of air-cycle machines to adjust the temperature of the air directed into the passenger and crew compartments of these large aircraft.
the cabin can also be heated and pressurized by the use of an air-cycle machine
These large aircraft utilize air-cycle cooling because of its simplicity, freedom from troubles.

4. Conti.. Refrigerant used – Air.
The principle of cooling by means of a gas is simple. When a gas (air) is compressed, it becomes heated, and when the pressure is reduced, the gas becomes cooled.
If a pressure cylinder is connected to an air compressor and compressed air is forced into the cylinder, one can observe that the cylinder becomes warm or even hot, depending upon the level of compression and the rate at which the air is compressed.

5. Conti..
If the cylinder filled with highly compressed air is then allowed to cool to ambient temperature, the pressure in the cylinder will be reduced to a certain degree as the air temperature is reduced.
If a valve is then opened and the air is allowed to escape from the cylinder, the temperature of the escaping air will be much lower than the ambient temperature due to the air expanding as its pressure returns to the ambient value. This cold air can then be used as a cooling agent.
In an air-cycle system, the air is continuously compressed and then cooled by means of heat exchangers through which ram air is passed; then the pressure is reduced by passing the air through an expansion turbine

7. Conti..
The air leaving the expansion turbine is at low pressure and low temperature. The cooled air is directed through ducting with control valves to regulate the amount of cooling air needed to produce the desired cabin temperature.
The turbine-compressor unit by which air is cooled is called an air-cycle machine (ACM).
Hot compressed air from the compressor of one of the turbine engines flows through the primary heat exchanger. The heat exchanger is exposed to ram air, which removes heat from the air.

8. Conti..
The cooled but still compressed air is then ducted to the compressor inlet of the ACM. The compressor further compresses the air and causes it to rise in temperature. This air is directed to the secondary heat exchanger, which, being exposed to ram air, removes heat from the compressed air.
The compressed air is then directed to the expansion turbine. The expansion turbine absorbs energy from the air and utilizes this energy to drive the compressor.
As the air exits the expansion turbine, it enters a large chamber, which allows the air to expand and causes a further reduction in the air temperature.

9. Conti..
Thus the air leaving the turbine is cooled by the loss of heat energy and by the expansion that takes place. The great reduction in temperature causes the moisture in the air to condense, and this moisture is removed by means of a water separator.
The dried, cold air is then routed to ducting to be utilized as required to provide the desired temperature in the cabin.

10. AIR CONDITIONING SYSTEM MAINTENANCE
Procedure depends on the particular aircraft model
Maintenance and manufacturer manual must be referred for correct procedure
It is consists of inspections, servicing, removing, and installing components, performing operational checks, and troubleshooting for the isolation and correction of troubles within the system

11. Inspections
Periodically inspect the system for component security and visible defects.
Particular attention should be paid to the heat exchangers for signs of structural fatigue adjacent to welds
The ducting should be securely attached and adequately supported.
Insulating blankets must be in good repair and secured around the ducting.

12. Servicing
Each refrigeration unit contains Freon for absorbing heat, plus oil mixed with the Freon for lubricating the compressor motor bearings.
insufficient Freon system incapable of absorbing heat.
insufficient oil, the motor bearings will overheat and cause unsatisfactory compressor operation.
System Freon loop contains quantities of both liquid and vapor. So it is unpredictable exactly where in the system the liquid will be at any one instant makes it difficult to check the quantity of Freon in the system.
a standard set of conditions should be obtained when checking the Freon level. These conditions are specified by the manufacturer

13. Conti..
To check the Freon level, it is necessary to operate the refrigeration unit for approximately 5 min to reach a stable condition
If the system uses a sight glass, observe the flow of Freon through the sight glass. A steady flow indicates that a sufficient charge is present If the Freon charge is low, bubbles will appear in the sight glass.
When adding Freon to a system, add as much oil as is felt was lost with the Freon being replaced. It is impossible to determine accurately the amount of oil left in a Freon system after partial or complete loss of the Freon charge

14. Conti..
Usually one-fourth-once of oil is added for each pound of Freon added to the system
The oil used is a special highly refined mineral oil free from wax, water and sulfur. Always use the oil specified in the manufacturer's maintenance manual for a specific system.