FUEL SYSTEM SERVICING

General: Aircraft fuelling is an extremely important operation for a service facility, but it is often assigned to a person just getting started into the aviation service business. It is important from a business standpoint because the line service per­sonnel are the direct contact between the facility and the customer. And, economically, line servic­ing is important because it takes only one impro­perly fuelled aircraft to generate enough lawsuits to wipe out most businesses.

Checking Fuel for Contaminants: When servicing an aircraft fuel system, you must be extremely sure that you are furnishing the correct grade of fuel for the aircraft and that the fuel is free from contamination. Some of the business aircraft flight crews will demand a check -of the fuel before it is pumped into their aircraft. This is a good policy as it assures that the fuel is free from water and other types of contamination that could cause engine failure

. Types of contaminants:

a.   Solid particles: Sand blown into the storage tanks or into the aircraft tank during the fuelling operation or rust from unclean storage tanks are solid particles which can clog strainers and restrict the flow of fuel.

b.                                    Surficant:                These are partially soluble compounds which are by-products of the fuel processing, or they may come from fuel additives. They have the tendency to adhere to other contaminants and cause them to drop out of the fuel and settle to the bottom of the fuel tank as sludge.

c. Water:            Although it has always been present in avia­tion fuel, water is now considered to be a major source of contamination since aircraft fly at alti­tudes; where the temperature is low enough to cause water entrained, or “dissolved,” in the fuel to precipitate out and form free water. This free, water can freeze and clog the fuel screens.

d.                                    Micro organisms: Tiny micro organisms grow from airborne bacteria and gather in the fuel. They lie dormant until they can come into contact with free water, but then they grow at a prodigious rate as they live in the water and feed on the hydrocarbon fuel, and on some of the surficant contaminants. The scum which they form holds water against the walls of the fuel tanks and causes corrosion.

2. Detection of contaminants:

Draining a sample of fuel from the main strainers of an aircraft has in the past been con­sidered an acceptable method of assuring that the fuel in the system is clean. But the fuel cleanli­ness requirements for modern aircraft engines re­quire more extensive testing than this.

One test recommended by the FAA for check­ing for fuel contaminants is to drain about a quart of fuel into a spotlessly clean ten-quart white enamel bucket. Stir the fuel with a clean paddle and swirl it in the form of a vortex cone. All of the contaminants will gather at the centre of the vortex and can be easily seen. A few drops of food colouring will dye any water that is in the fuel so it will be readily visible, but the food colouring will not colour the fuel.

A commercial water test kit is used far more often than the white-bucket test. This kit con­tains a small glass jar and a supply of capsules containing a grayish-white powder, and a 100-cc sample of fuel is taken from the truck and put in­to the jar. Then one of the test capsules is dumped into this fuel, the lid is screwed on, and the contents are shaken for about ten seconds. If the powder changes its colour from gray-white to pink or purple, the fuel has more than 30 parts per million of water, and it is not considered safe for use. It may be pumped through the water traps and filters in the truck and another sample taken. This test is fail-safe, because any error in performing the test will cause it to give an unsafe indication.

If the fuel sample is taken from the aircraft, the fuel should be drawn from the main strainer and some fuel should be allowed to flow from every one of the tanks. Drawing fuel from the main strainer when the tank selector is in the both position will not necessarily get fuel from all of the tanks.

Fire Hazards: when Fuelling and De fuelling Aviation fuels are both highly flammable and volatile, and special care must be exercised when transferring them into or out of an aircraft. Be sure that the proper type of fire extinguisher is available at the aircraft and that it has been pro­perly serviced and has not been used, even par­tially, since it was last serviced.

Never service an aircraft with fuel inside a hangar or in any other closed area. If fuel is stored in containers other than the fuel service truck or the aircraft fuel tanks, be sure that the containers are closed, not only to prevent the en­try of contaminants, but also to prevent the re­lease of fuel vapours.

If any fuel is spilled, wipe it up immediately; or, if too much has been spilled to wipe up, wash it away with water or cover it with sand.

Be sure that there are no open fires, gasoline motor exhausts, or electrical equipment opera­ting in the vicinity of fuelling or defuelling opera­tions, and be sure that all electrical power on the aircraft except that required for the fuelling opera­tion is turned off.

Wear only a type of shoes that will not cause sparks and clothing that will not produce sparks from static electricity. And, be sure that the air­craft and fuel truck or hydrant are properly bond­ed electrically.

Static electricity builds up when fuel flows through the fuel lines and its discharge must be guarded against.) About fifteen minutes is needed for a tank full of fuel, especially turbine fuel, to relax itself of the charges of static electricity after the tank has been filled.

When fuelling or refuelling an aircraft, first connect the fuelling vehicle or fuel cabinet to the aircraft with a static ground wire, and then con­nect the nozzle to the grounding point at the tank filler opening. Only after this has been done should the filler cap be removed.

If a fire does occur, always fight it from up­wind and use a steady sweeping motion at the leading edge of the flames.;. Flammable liquids have a tendency to flash back if they are not suffi­ciently cooled after the flame has been extin­guished.

 Fuelling Procedures: If you are to fuel an aircraft from a fuel truck, approach the aircraft from the front, parallel to the wings, and park the truck in front of the air­craft in such a direction that if it should roll, it cannot possibly roll into the aircraft being ser­viced, or into any other aircraft.

Be sure that only the proper grade of fuel is used. This is extremely important, as servicing an aircraft with the incorrect grade of fuel can cause serious damage to the engine.

Remove the fire extinguisher from the fuel truck and place it where it is immediately accessi­ble. Then connect the static bonding wire be­tween the truck and the aircraft.

If a ladder is used to reach the top of the wing, be sure it is padded so it cannot damage the finish on the wing or the de-icer boots if they are installed. Put a wing mat in place over the wing and connect the bonding wire between the fuel nozzle and the tank, and then remove the fuel tank cap. This is called over-wing fuelling.

De fuelling: When fuel is removed from an aircraft, follow all of the safety procedures required for fuelling, and be sure that all of the fuel removed from the aircraft tanks is returned to the proper fuel truck, or, if it is to be stored in drums or other con­tainers, that these are clean. After they are filled and properly closed, they should be stored in a designated storage area.

Fuel Leakage

When leakage or spillage of fuel has occurred, care must be taken to ensure that all traces of fuel and vapour are removed.    Where lagging has become contaminated with fuel in areas adjacent to passenger cabins and crew com­partments, the lagging should be removed and cleaned, and any residual fuel should be mopped up. Where fuel has leaked into a compartment which is vented and drained, the venting and drainage arrangements should be checked to ensure that they are functioning correctly and that there is a flow of air through the compartment. It is sometimes specified that a check of the venting system of such a compartment should be carried out with the cabin pressurised. In the event of a gross leakage, consideration should be given to the effects that fuel may have on other materials and components, such as cable insulation, seals, transparencies and bearings.

Aircraft FUEL SYSTEM SERVICING AND MAINTENANCE

  INTRODUCTION

The fuel system is very important to the safe and efficient operation of an aircraft, and particular care must be taken to ensure that the instructions and precautions contained in the relevant manuals, schedules, and servicing instructions, are properly carried out.

  Safety Precautions

The flammability of a fuel depends to a large extent on its flash point, and the different types of fuel vary considerably in this respect. Kerosene is far safer to handle than gasoline, but, regardless of the type of fuel used in a particular system, it is essential that precautions are taken to prevent the combustion of fuel vapours during servicing operations. In addition, precautions must be taken to prevent the harmful effects to health which may result from handling fuel or inhaling fuel vapour. The following general precautions should be observed whenever the fuel system is being worked on, and the relevant manuals should be consulted for any requirements which are applicable to a particular aircraft or fuel system. The special safety precautions necessary when entry into a tank is to be made.

(a)           The aircraft should be electrically earthed, and any ground equipment or containers should be earthed to the aircraft.

(b)           Suitable and adequately manned fire-fighting equipment should be available, and suitable warning notices should be prominently displayed.

(c)            Aircraft electric supplies should be switched off, and no live electrical cables should be left disconnected.

(d)            Only spark-proof electrical equipment should be operated in the vicinity of the aircraft.

(e)           Explosion-proof lamps and torches should be used.

(f)            When draining fuel, any precautions detailed in the relevant Maintenance Manual regarding centre-of-gravity movement or maximum permitted jack loads, must be observed.

(g)          To prevent undue spillage of fuel, tanks and pipes should be drained or isolated as appropriate, before breaking a connection or removing a component.

(h)            Air-fed respirators should be worn in areas of high vapour concentration, e.g. near an open tank access hole.

Aircraft TESTING OF PRESSURISATION SYSTEMS

TESTING OF PRESSURISATION SYSTEMS

Pressurisation systems must be tested to ensure that there are no serious leaks and that pressure control equipment and pressure limiting devices function correctly to maintain the cabin differential pressure within the limits appropriate to the aircraft type. The periods at which functional tests and leak tests should be made are specified in the approved Aircraft Maintenance Schedules. Tests may also be necessary after repairs or modifications which affect the structural strength of a cabin (e.g. Proof Pressure Tests), or after suspected damage to the fuselage. The procedures for carrying out the proof pressure test and precautions to be observed, are also detailed in the relevant approved Structural Repair Manual.

NOTE: On the repair of metal aircraft, it is important to have attention to the accuracy required in skin joints and seams, the necessity for the skin to be free from waves and buckles, and the importance of cleanliness when making airtight joints.

The precise method of carrying out the required tests depends on the type of aircraft and on the nature of its air conditioning and pressurisation system. It is, therefore, essential to make reference to the relevant Aircraft Maintenance Manuals for full details. There are however, certain recommendations, precautions to be observed, and aspects of testing procedures which are of a general nature, and these are summarized for guidance in the following paragraphs.

Test Preparation

The aircraft structure must be complete and fit for flight before attempting to carry out any ground test.

It is recommended that those personnel participating in a pressure test who are stationed within the pressurised area, be certified medically fit. This would include freedom from colds and sinus troubles.

NOTE: Where the pressure differential between the working environment and ambient exceeds 10 p.s.i, medical supervision must be sought.

A minimum of two test operators should be inside the pressurised area during any pressure test where an external ground test unit is used as the air supply source. When using engines for the air supply a third operator to run the engines is required.

When using an external ground test unit as the supply source, intercommunication between test personnel inside the pressurised area and those operating the test unit must be established. (A socket for the connection of an interphone system is normally provided in aircraft for this purpose and is located in an area such as a nose-gear bay.)

NOTE: Warning placards should also be positioned around the aircraft indicating that such testing is being carried out, and only the test personnel should be within, or in the vicinity of the aircraft during testing.

It is necessary to ensure that static pressure and pitot pressure pipelines, within the pressurised area, are complete and connected to their relevant instruments and components such as autopilot coupling units and height lock units. Failure to observe this precaution will result in damage to instruments or units during a pressure test.

NOTES:

(1)    If it is not possible for a connection to be made, the relevant instrument or unit should be removed and the pipelines blanked off.

(2)    Some pressure tests require certain instruments to be removed.

All doors, clear vision windows, emergency exits, etc., should be free to operate, after closing checked for security. If an unusual force is necessary to close any of these items, the cause should be investigated and rectified before the cabin is pressurised.

Where sandwich type windows are fitted, a check for security should be made and, where applicable, services for window de-misting purposes should also be checked to ensure freedom from leaks and obstructions, and for correct venting, i.e. to atmosphere or to the pressurised area dependent upon the design.

During testing, the maximum cabin differential rate of change must not exceed the values specified in the relevant Aircraft Maintenance Manual.

Manometers and other portable test indicators e.g. pressure gauges and vertical speed indicators, required for testing must be checked and calibrated at regular intervals.

Where any disturbance of cabin air ducting has occurred, checks should be made for correct alignment, security and freedom from foreign matter. As necessary, air tightness of the ducting should be checked by blanking local sections and subjecting them to pressure tests.

Any seals, glands or expansion joints should be checked for correct fitting, and where controls pass through the aforementioned they should be lubricated as necessary and in the manner specified in the relevant Aircraft Maintenance Manual. NOTE: Detachable blanking plates used when testing should not be sealed with jointing compound.

Following the satisfactory completion of tests, the operation of all windows, doors and hatches (including those of galley units) should be checked.

NOTE: Following the conclusion of tests, it must be ensured that cabin pressure has been reduced to the prevailing ambient conditions before attempting to open any doors, windows or hatches.

Functional Tests

To perform a full or partial functional test of the pressurisation system reference should be made to the relevant Aircraft Maintenance Manual. Where it is required that the pressurisation system is pressurised, this can be achieved by one of the following methods:­

(a)       Running the engines, utilising the bleed air or engine-driven compressors, as appropriate.

(b)       Connecting a ground supply unit to the ground service connection point (where fitted)

(c)       Employing bleed air supplied from the auxiliary power unit (APU).

It is however, recommended that functional tests are carried out by running the engines and utilising bleed air or air supplied from engine-driven blowers, as this enables all components to be tested simultaneously.

When carrying out tests, additional test instruments and equipment may be required and reference should be made to the relevant Aircraft Maintenance Manual for precise details of the type and method of connection into the pressurization system. Generally, a portable vertical speed indicator and mercury manometer or pressure gauge are required, together with a stop watch and a pitot-static test set. The test set is normally used for checking for leaks from pressure controllers, pressure signal and static pressure pipelines, and also for checking the function of discharge valves in response to selected pressure signal settings from pressure controllers.

Preparation: Unless otherwise specified in the Aircraft Maintenance Manual, all internal doors or hatches within the pressurised area of the fuselage should be secured in the open position. In all cases, the doors of equipment which could be damaged by differential pressures, e.g. galley cupboards, ovens, should be opened. Unpressurised areas adjacent to the pressure cabin should be vented to atmosphere.

After entering the aircraft, the entrance doors, emergency exits and hatches, toilet servicing connections, sliding and direct vision windows in the crew compartment should all be closed. Where specified in the Aircraft Maintenance Manual, other apertures such as toilet ventilation bleed outlets should be blanked off.

NOTE: Care must be taken to ensure that certain specified fuselage and compartment drains are not obstructed as allowance is made in the leak rates permissible during pressure tests. Reference must always be made to the relevant Aircraft Maintenance Manual for details of drain locations.

Test: Electrical power should be switched on and the controls of the appropriate cabin air temperature control system and pressurisation system units selected to the setting specified in the Aircraft Maintenance Manual for functional testing.

When introducing the air supply, the cabin pressure should be controlled in the manner appropriate to the system to ensure that the rate of pressure change (normally given in feet per minute) does not exceed the maximum values specified in the Aircraft Maintenance Manual.

The cabin pressure should be allowed to increase until it stabilizes at the maximum working differential pressure for the aircraft type, and a check should be made to ascertain that the pressure remains constant with a temporary increase in air supply. If the differential pressure stabilizes at a figure above or below the maximum value, the pressurization control system should be investigated and rectified as necessary after conclusion of the test. After such rectification, a further test should be made.

Where multiple pressure control units are provided, each unit should be selected in turn and checks made to ensure that the differential pressure builds up and stabilizes at the relevant maximum value.

NOTE: Whilst the cabin is pressurised, it may be required that all flying controls are operated to test the efficiency of cable seals etc, therefore reference should be made to the relevant Aircraft Maintenance Manual.

The automatic action of safety valves should also be checked during pressurization system tests, with the discharge valves isolated from pressure signal sources. Air should be supplied to the cabin at the specified controlled rate and a check made on the pressure at which the valves open. The cabin should then be allowed to depressurize slowly until the valves close and the corresponding pressure noted. The pressures at which valves open and close should be within the limits specified in the relevant Aircraft Maintenance Manual.

NOTE: When checking the operation of cabin safety valves which are set to relieve at the maximum differential pressures permissible for the aircraft type, control of the air supply must be carried out with extreme care to ensure that the pressure never exceeds the maximum value.

If during a pressure test the leak rate incre

ases unduly, as denoted by both a sudden fall in differential pressure and a sudden ascent indication on the cabin vertical speed indicator, the pressure must be released and the fuselage examined for the cause before continuing the test.

When all functional tests are concluded, the air supply should be cut off and the cabin pressure then allowed to fall gradually at a controlled rate. If the pressure is released too rapidly moisture precipitation may occur damaging electrical cables and cabin installations.

Leak Rate Tests

Leak rate tests are necessary at specified periods to ensure that no marked deterioration in the sealing standard of the aircraft fuselage has occurred. The tests should also be carried out whenever a component affecting the pressurised area is renewed, refitted or modified, and after a proof pressure test. Before testing, adequate time should be allowed for the drying of any freshly applied sealants. On certain aircraft, leak rate tests may be combined with functional tests; in other cases the tests should be carried out separately. The periods at which the tests and tests methods are to be carried out, are provided respectively in the relevant Approved Maintenance Schedules and Aircraft Maintenance Manuals.

As in the case of functional tests, observers are required inside the aircraft. It is preferable to supply air to the cabin from a ground air supply unit thus eliminating the danger from propellers or jet engine intakes and exhausts to personnel inspecting the outside of the fuselage.

The instructions given in the relevant Aircraft Maintenance Manual for leak rate testing should be closely followed. It is the practice on some aircraft to render the pressure controller inoperative by disconnecting it from the discharge valves, in which case the cabin pressure obtained is at maximum determined by the safety valves. On other aircraft the delivery rate of the air supply is controlled and the air is shut off when the pressure reaches a specified value lower than maximum.

A check should be made on permanent fuselage drain holes, battery compartment vents, hydraulic system reservoir bleeds etc., to ensure that they are unobstructed.

The air should be introduced to the cabin gradually until the pressure stabilises. In some cases manufacturers recommend that the pressure is raised slightly above the specified values and then allowed to fall to this value before checking the leak rate.

After the pressure has stabilised, the air supply should be shut off and the pressure allowed to fall by normal fuselage leakage. The time taken for the pressure to fall over the range appropriate to the aircraft type must not be less than that quoted in the relevant Aircraft Maintenance Manual.

If the leak rate is excessive, an inspection of the fuselage pressurised area should be carried out with the cabin pressure held to the value specified for the aircraft type. Escaping air may usually be detected by sound, or touch, but a soapy water solution may be used to trace certain leaks and this should be cleaned off after testing. When inspecting the outside of the aircraft for leaks, inspection personnel should exercise caution when entering nose-gear bays or similar breaks in the main pressurised area.

The sealing standard of the fuselage should be improved as necessary and in the manner detailed for the aircraft type, until the leakage rate is within limits.

At the conclusion of the tests the air supply should be shut off and the cabin depressurized ensuring that the rate of pressure change does not exceed the specified value. Before opening doors, windows, or hatches, it must be ensured that cabin pressure has been reduced to prevailing ambient conditions.

Electrical power should be switched off and all blanks and plugs used during tests should be removed.

Where pressure control system components have been removed or isolated for purposes of leak rate tests, they should be restored to their normal operating condition. Leak tests of the system should be carried out with the aid of a pitot-static test set and in the manner detailed in the relevant Aircraft Maintenance Manual.

The fuselage should be examined for obvious damage or distortion, particular attention being paid to the pressure bulkheads, cabin floor members, window and windscreen frames and panels, and suppressed antenna covers. The transparencies should be examined for signs of crazing. All doors, hatches and windows which are intended to open should be fully opened and then closed, to check for free movement and absence of deformation.

MAINTENANCE

General:      Details of the operations necessary for the inspection and maintenance of pressurisation system components will be found in the relevant Aircraft Maintenance Manuals and Approved Maintenance Schedules, and reference must at all times be made to such documents. The information given in the following paragraphs is intended only as a general guide to the checks normally required on the principal components covered in this literature.

Pressure Controllers

Functioning tests of the pressure controller, should be made when defective operation of the pressurisation control system is suspected, and at all other times specified in the relevant Approved Maintenance Schedule. Further checks can be carried out where electronic pressure controllers are equipped with Built In Test Equipment (BITE). Adjustments and rectifications which may be made in situ are limited, and therefore the relevant Aircraft Maintenance Manuals appropriate to the type of aircraft and controller should be referred to before any adjustments or tests are carried out. If the results of the functioning tests are unsatisfactory and the pressure controller is found to be defective, it should be removed from the aircraft and tested according to the manufacturer's recommended test schedule and as appropriate, repaired, overhauled or replaced.

At certain specified intervals, some pneumatic pressure controllers are required to be lightly lubricated using only lubricants recommended by the manufacturers. Checks should also be made for security, corrosion or damage, and the associated electrical circuits should be tested as necessary for continuity and insulation resistance.

Discharge Valves and Ground Automatic Relief Valves

Discharge Valves and Ground Automatic Relief Valves

At intervals specified in the Aircraft Maintenance Schedule, the pressure controlling function should be tested. These tests are normally done in situ and in conjunction with the associated pressure controller. The security and functioning of ditching system controls, where fitted, should also be carried out.

Valve faces and seats should be inspected for damage and deposits of dust and nicotine tar which should be removed in the manner specified in the relevant Aircraft Maintenance Manuals. The cleaning fluids used should be of the type recommended by the manufacturers, and on completion of a cleaning operation all traces of fluid should be removed and all surfaces cleaned using a dry, soft, lint-free cloth. High pressure air blasts should not be used to dry the seating surfaces of diaphragm ­controlled discharge valves as damage may be caused to the diaphragms. Bonding leads and their attachment points should be inspected for security of attachment and checked for electrical continuity.

At specified periods, discharge valves should be removed, inspected, and leak tested to ensure that the leak rate is within specified permissible limits. After reinstatement or replacement of a discharge valve a full functional check should be carried out.

NOTE: On some aircraft types, shims are installed to aerodynamically align the discharge valve, with the fuselage. When removing the valve from the aircraft these shims should be retained in order to maintain that alignment when replacing the valve.

Safety Valves and Inward Relief Valves

Safety Valves and Inward Relief Valves

Mountings, pipe unions, electrical actuator where fitted, (and its connections) should be checked for signs of damage, deterioration and security.

Leak tests and functioning tests should be made after installation, when the serviceability of valves is suspect, during cabin pressure testing, and at the periods specified in the Approved Maintenance Schedule. The requirements for functional testing of inward relief valves is normally restricted to checking freedom of movement.

Valve faces and seats should be inspected and deposits of dust and nicotine tar removed in the same manner as that specified for discharge valves. After cleaning, a check should be made to ensure that valve faces and seats make good contact.

Electrical and manual override controls should be checked for security and tested for correct operation, particular attention being paid to the settings of actuator limit switches, lost motion in linkages, cable tension and static friction. Where specified, moveable parts should be lightly lubricated using only the specified lubricants.

Certain types of safety valves can be adjusted in situ, but before any attempt is made to alter the relief pressure setting it should be established that any error in the pressure at which the valve relieves is not due to a defect in the valve mechanism. After making adjustments to a valve its operation must be checked by repeating the appropriate cabin pressure and functional test.

Aircraft Filters and Air Driers

Filters and Air Driers

The element of cartridge type filter units should be removed and replaced by anew element when necessary. Before fitting the element the filter casing and connecting union orifice should be cleaned with the recommended cleaning agent as prescribed in the relevant Aircraft Maintenance Manual.

Checks should be made on the condition of Silica Gel Crystals and the appropriate air drier containers recharged as necessary. The condition of sealing rings should also be checked.

Baffle type air driers should be checked for security paying particular attention to the condition of the sealant. The filter gauze which is also provided must be free from corrosion and cleaned with the recommended cleaning agent.

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.

Safety Valves and Inward Relief Valves

Safety Valves and Inward Relief Valves

Valves should be cleaned and installed in a similar manner to discharge valves. Where facilities allow, bellows-type safety valves should be subjected to a leak test before installation in the manner described in the Aircraft or Component Manufacturer’s Maintenance Manuals.

In some installations, safety valves are provided with electrical and manual override controls, in such cases functioning checks on the controls must be made after installation (e.g. limit switches of electrical actuators which may need to be adjusted to cut off the power supply when the valve is at the fully open and fully closed positions). Manual controls should be correctly rigged and checked to ensure that the controls and valve move in phase and operate freely over their full range of travel.

NOTES:

(1)    Limit Switch adjustment is normally carried out when the component is workshop tested.

(2)    After carrying out override control system checks, the appropriate controls should be reset to their normal positions.

Some valves incorporate an electrically-operated position transmitter which signals the valve position to an indicator in the crew compartment; checks should therefore also be made to ensure that the indicated positions correspond.

 Filters and Air Driers

Before installing a cartridge type of filter unit the cartridge and casing should be inspected for signs of damage and also to ensure freedom from dirt and other foreign matter.

In some unit designs the case is made of rubber and for this reason it may have been stored in French Chalk. Before fitting the cartridge into this type of case, all traces of chalk must be removed by lightly brushing the rubber with a suitable cleaning agent as recommended in the relevant Aircraft Maintenance Manual.

In some installations, air driers are of the type which utilises Silica Gel Crystals as drying media. Before installation, the units should be detached and charged with a fresh quantity of Silica Gel Crystals. After installation, a check should be made to ensure that the inspection windows, where provided, are readily visible.

When installing baffle-type air driers, it must be ensured that the sealant necessary for fixing is of the correct type and is correctly applied to the appropriate area of the fuselage skin and air drier casing.

After installation, the connections between the filters, air driers and all appropriate pressure sensing lines must be securely made and leak-tested in the manner described in the relevant Aircraft Maintenance Manual.