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 increases 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.