AIRCRAFT FLOW TESTS

FLOW TESTS

Flow tests should be carried out in accordance with the relevant Maintenance Manual, as and when required by the approved Maintenance Schedule, or when necessitated by repairs, replacements or modifications. The tests are designed to ensure that the system will provide a fuel flow to each engine which is in excess of the requirements of the engine when it is operating at maximum power, and at a pressure suitable for proper operation of the carburettor or engine-driven pump, as appropriate. For all tests the aircraft should be levelled laterally and longitudinally, and the fuel tanks should contain the minimum quantity of fuel (i.e. unusable fuel plus sufficient for the test only); tank vents should be clear, and over wing filler caps should be fitted. All equipment used should be bonded and electrically earthed.

Full Flow Test: A full flow test is normally only required after initial installation or major breakdown of the system. Fuel flow test rigs are required for the test, and should be located adjacent to each engine, with the test rig pump at the same level as the engine-driven pump. The rig inlet hose is usually connected to a self-sealing coupling on the engine bulkhead, and the outlet directed to a suitable container.  An external electrical supply should be connected to the aircraft, in order to operate the fuel system valves and to check operation of the associated warning lamps and indicators. The test includes suction feed operation (using the test rig pump), pressure feed operation (using the aircraft booster pumps), and all possible combinations of cross-feeding, to ensure that fuel flow is satisfactory under all flight conditions. The schedule of test operations, and the flow rates and pressures which should be achieved, are detailed in the relevant Maintenance Manual.

For the suction test, the test rig pump is used to draw fuel from the tanks. Valve selections should be made according to the test schedule, and the flow rates and pressures obtained at each stage of the test should be recorded. These results should be within the limitations prescribed for the suction test.

For the pressure test, the aircraft booster pumps should be used to pump fuel from the tank. The test rig pump is switched off, and its by-pass opened. Selections of pumps and valves should be made in accordance with the test schedule, and the flow rates and pressures obtained at each stage of the test should be recorded. These results should be within the limitations prescribed for the pressure test.

Limited Flow Test: A limited flow test is often considered as a satisfactory method of checking a fuel system after a component has been changed; only that part of the system affected by the component change needs to be tested. The fuel feed pipe is dis­connected at the engine, or, in some instances, a drain pipe is connected to a special drain valve at the engine, and a suitable container is positioned to catch the drained fuel.

The appropriate low pressure cock should be turned on, and the flow rates should be checked with the associated pumps operating separately and together. For each part of the test, when the fuel flow is free from bubbles, it should be directed into a calibrated container, and the time taken to pump a given quantity of fuel should be recorded. These figures should be converted to flow rates, which should not be less than the minimum flow rates specified in the relevant Maintenance Manual.

Gravity Feed Test: To check a gravity feed system such as is fitted to some light aircraft, the feed pipe should be disconnected at the carburettor, and a suitable container should be positioned below the engine. With the fuel outlet positioned at the same height as the carburettor, and the fuel valve turned on, the fuel should be checked for freedom from bubbles and for full-bore flow, and then directed into a calibrated container. The time taken to drain a given quantity of fuel should be recorded, and the equivalent flow rate should not be less than the minimum flow rate specified in the relevant Main­tenance Manual.