AIRCRAFT CABIN PRESSURISATION SYSTEM MAINTENANCES

INTRODUCTION

This week discusses activities during removal/installation of components, tests to be performed on pressurisation system and other allied maintenances.

3.2 COMPONENT INSTALLATION

Before installing any component of a cabin pressure control system, an inspection should be made to ensure that no deterioration of rubber components, corrosion of metal parts or other damage has occurred during storage or transit. The security of pipe connections, electrical connections, actuators, etc., should also be checked and, where specified, pre-installation functioning tests carried out.

Some aircraft pressure controllers' discharge valves and safety valves operate as preset units and must only be used in specific combinations. When renewing or replacing such units it is therefore essential to ensure that their part numbers are correct for the installation.

Pressure Controllers

It is recommended that where facilities are available, controllers should be bench tested before installation to ensure that the controlling mechanism has not been disturbed during storage or transit. Details of tests peculiar to particular types of controller should be obtained from the relevant Aircraft Maintenance Manual.

When assembling static pressure and reference pressure sensing pipelines, care is necessary to ensure that no obstruction or leakage can occur. Particular care is necessary to ensure that all pipes are correctly assembled. Cabin pressure must not leak into low pressure pipes or chambers as this would unbalance the control characteristics of the system. Where vents to cabin pressure form part of the system, they should not be obstructed by loose trimming or soundproofing materials.

After the installation is completed, functioning tests should be made to prove the correct connection or adjustment of electrical circuits, micro-switches, altitude selectors, rate of change control mechanisms and the pipeline system. Details of tests applicable to particular aircraft installations should be obtained from the relevant Aircraft Maintenance Manual. General guidance on the testing of pressurization systems is provided in this discussion.

Discharge Valves and Ground Automatic Relief Valves

Before installation the valve faces and seating surfaces of discharge valves should be cleaned with a soft lint-free cloth. High pressure air blasts should not be used on discharge valves of the diaphragm-controlled type as damage may be caused to the diaphragms.

To ensure that the joint between discharge valves and appropriate adaptors on the pressure bulkhead or fuselage skin is pressure tight, new mounting gaskets or 0-rings should be fitted. Where specified, mounting surfaces should be lightly smeared with the appropriate lubricant as specified in the relevant Aircraft Maintenance Manual.

Pipelines and valve unions should be checked to ensure that they are in correct alignment. Plug and socket connections to electrically actuated discharge valves should be secure.

If ditching cocks are fitted, the ditching control run in the aircraft should be checked before making the connections and locking the cocks to the appropriate discharge valve. Some discharge valves may be closed for ditching purposes by a directly mounted hand control wheel. In such cases the opening and closing of a valve should be checked by rotating the wheel through the requisite number of turns.

After installation of a discharge valve, functioning tests should be carried out to prove the correct connection of pressure sensing pipelines and, where appropriate, the connection of electrical cables to valve actuators. Details of the tests applicable to particular aircraft installations should be obtained from the relevant Aircraft Maintenance Manual.