Aircraft-Instrument-Maintenance - PROCEDURE OF REPLACEMENT...

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PROCEDURE OF REPLACEMENT OF AIRCRAFT INSTRUMENTS FIELD MAINTENANCE & FUNCTIONAL TESTS Maintenance of Pitot – Static systems : The following paragraph detail the general maintenance necessary on pitot – static systems and their major components and should be read in conjunction with the Maintenance manuals for the type of aircraft and equipment concerned. Pitot tube Pitot & Static tube Pressure heads : Pressure head and their supporting tubes or masts should be inspected for security of mounting and signs of distortion. Where specifically required, checks on the lateral and longitudinal alignment with respect to the aircraft datum must be carried out to ensure that they are within the limits given in the aircraft Maintenance Manual. Checks should also be made that electrical connections at all appropriate points are secure and the insulation of cables does not show signs of cracking or chafing. The pitot pressure entry hole drain hole and if applicable the static holes should be inspected to ensure that they are unobstructed. Cross sectional view of a typical Pitot-Static tube with heater circuit. 1
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Note:- The size of static holes and drain holes and drain holes is aerodynamically critical and they must never be cleared of obstruction with tools likely to cause enlargement or burring. In the absence of special clearing tools, which may be locally manufactured from drawing given in manufacturer’s manuals, the use of a stiff non-metallic brush is recommended, taking care not to displace debris into the system and cause obstruction. Electrical Checks: Heating elements should be checked for functioning by ensuring that pressure head casing and where applicable support masts and static vent plates, commence to warm up when switched on or, if an ammeter is fitted in the heating circuit, by a current consumption check. An insulation check on the resistance between the electrical leads and the appropriate component should be made by connecting a voltage supply appropriate to the rating of the heating elements, switching on the supply and allowing the component to warm up until it is just too hot to cold with the bare hand. The current should then be switched off and, while the component is hot, the insulation resistance measured, using the specified tester. The component should then be allowed to cool and the insulation resistance measured in this condition. The resistance values thus obtained should not be less than those quoted in Maintenance Manuals; a typical value is 3 mega ohms. Leakage Tests: The method of checking pressure heads for leaks differs with the various types and in each instance the procedures given in the relevant Manual should be followed. Note:- All pressure heads are sealed on manufacture and no attempt should be made dismantle or tamper with them in any way. If a pressure head fails a leakage test it should be renewed.
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