1. 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

2. 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.

3. 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

4. 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

5. 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.