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.
