AIRCRAFT PRESSURE TESTS

PRESSURE TESTS          

General: Pressure tests are normally required at regular intervals, after repairs, modifications, and replacement of components, and whenever leakage is suspected. In those vent systems which utilise part of the wing structure (e.g. top hat sections) to form the vent duct, vent pressure tests may also be required after structural repairs.

The tests required will be specified in the relevant Maintenance Manual, and should be carefully carried out. Test rigs, capable of supplying fuel or air under pressure, are required, and should include an accurate pressure gauge, a relief valve, and, in the case of a fuel pumping rig, a flow meter.     All test rigs should be clearly identified with the certification (or re­certification) date. In addition, special blanks, plugs, cover plates, and dummy com­ponents may be required. The vent, feed, and transfer systems are usually tested separately since different test pressures are generally prescribed.

Vent System Pressure Test: For this test, the vent system on each side of the aircraft should normally be tested separately. All vent^- openings should be blanked, and it will often be necessary to gag float-operated valves, or to replace them with dummy components. Alternative means of venting the tanks during the tests should be provided. Air pressure should be applied to the system either through a water drain valve or through an adaptor fitted to one of the blanks, and the pressure should be slowly raised to the test pressure quoted in the relevant Maintenance Manual. When the air pressure supply cock is turned off, any decrease in pressure will indicate leakage, and the drop in pressure over a prescribed time should be noted. The source of any leakage in excess of that permitted should be traced and rectification action should be taken.

Feed System Pressure Test: The feed system from a tank to its associated engine should be tested individually. Cross-feed and inter-engine valves should be closed, and the low-pressure cock should be opened. On some aircraft the feed systems are pres­surized by switching on both pumps in the tank concerned, whilst on others the booster pumps are replaced by dummy components, and fuel pressure is applied by means of an external test rig. In some systems there will be flow through the bleed hole in the suction valve, and this must be within prescribed limits. Rates of flow indicated on the test rig flow meter, which are in excess of these limits, will be indicative of either an internal or external fuel leak. All pipes, connections, and valves should be checked visually for signs of leakage under pressure; no leakage is normally permitted.

NOTE: In systems in which drip shields or heat shields are fitted to some couplings, the test pressure must be applied for a sufficient length of time to enable any leakage to collect and flow through the drain. Alternatively, a separate pressure test of the drip shield may be specified, or the shield may be required to be removed for the test.

Transfer System Pressure Test: The pipes and couplings in the fuel transfer system may be pressure tested in a similar manner to the feed system. Pipes should be dis­connected and blanked at the positions specified in the relevant Maintenance Manual, and fuel pressure should be applied by means of the transfer pump, or by use of an external test rig, supplying fuel through a dummy pump. No leaks should be evident, and no fuel flow should be recorded on the test rig flow meter.

Additional Pressure Tests: A number of other pressure tests may be specified, in order to ensure that there is no leakage which could prove hazardous, or prevent proper operation of the fuel system. One example is the pressure testing of conduits which pass through the fuel tanks, and house electrical cables. These conduits are usually sealed by means of a pressure bung or pressure seal, and are tested by applying air pressure to the inside, through a drain pipe, or special adaptor. When the air supply is shut off, there should be no drop in pressure over a prescribed period of time. If leakage is evident at the pressure bung, it is usually permissible to apply sealant to seal the bung and the holes through which the cables pass.