Once in service, the cables will be inspected regularly for a variety of possible faults, whilst the swaged end fittings will require minimal inspection. In some installations, red paint is applied at the junction of the end fitting where the cable emerges, leading to a gap showing if the cable has slipped within the swaged end fitting during normal service. Some individual ferrules, fitted to non-critical cables, may be inspected for signs of cracking whilst in service.
It is rare for cables to be removed from service to have a scheduled proof load test. If there is any doubt to the possibility of the cables lasting a long time in service, they will be either checked for stretch by measuring their length under load, or they will be given a finite life and replaced when that life is reached.
Cable systems have to receive regular inspections due to their being subject to a wide variety of environmental conditions and wear. Their degradation, due to wear, can take the form of wire/strand breakage (which is fairly easy to detect), or may exist as less visible (internal) wear, or as corrosion and distortion.
12.2.1 Cable Wear
Critical areas for strand breakage are where the cable passes over pulleys or through fairleads. Examination of cables will normally involve passing a cloth along the length of the cable, which will both clean any dirt from it and detect broken strands if the cloth ‘snags’ on the projecting wires.
radius, which may show up broken internal strands that would not be visible when installed and tensioned.
External wear (refer to Fig. 1) will extend along the cable, equal to the distance the cable moves at that location and may occur on one side of the cable or over its entire circumference. The limits of permitted wear will be found in the AMM.
Internal wear occurs in similar places in the wire to external wear, around pulleys and fairleads and is much more difficult to detect. Separating the strands, after removing the cable, is the only way to detect internal wear and this only permits limited inspection.
Generally any signs of internal wear within a cable will mean its replacement. Broken strands on a cable at a location not adjacent to a pulley or fairlead, could be an indication that the breakage was due to corrosion.
The inspection of a cable for internal corrosion should be done off aircraft, and will involve rejection of the cable if corrosion is found.
External Cable Wear Fig. 1
Plan View Side View
Plan View Side View
based, it is vital that cables, passing through high risk areas such as battery bays, toilets and galleys, receive regular rust preventative treatments in addition to visual inspections.
Most cables have external corrosion preventative compounds applied in varying amounts, whilst internally they will have been soaked in some form of thin grease or low-temperature oil to resist the formation of the difficult to detect internal corrosion.
Normally in dry and desert atmospheres, the application of certain compounds to cables is not permitted. This is because the adhesive properties of these compounds will cause the sand and dust to stick to the cable and, thus, cause extremely high rates of wear.
All controls will be monitored, by the flight deck crew, on a day-to-day basis but, during maintenance, more subjective tests must be completed. The tension of the cables will be measured, as will the rigging of the complete runs, to ensure that the controls remain accurate and precise in their operation.
Whilst it is not usual to find faults on the cable end fittings, these should all be checked for any signs of damage, corrosion and stressing of the cable at the end fitting. Items checked will include turnbuckles and ball end fittings, to ensure that the cable is operating at the designed angle, tension and over the correct range. 12.2.2 Bowden and Teleflex Cable Systems
A typical Bowden cable control might be a brake lever on the control column operating a remote brake control valve.
Maintenance of Bowden cable systems is usually restricted to cleaning and lubrication of the inner cable at regular intervals and adjustment of the outer conduit (e.g. if the brakes needed adjustment). The lubrication would keep moisture out of the cable to prevent it freezing at low temperatures.
The Teleflex cable system is more complex than the Bowden cable system in that the operating cable, within the conduit, is actually a number of spirally wound cables which surround a core tension cable, giving it support. This allows the cable to transmit a push force as easily as a pull force, doing away with the need for any form of return spring.
A typical use of a Teleflex system might be a throttle lever to engine fuel control system connection.
it is vital that the inner cables are regularly removed, cleaned and lubricated with low temperature grease. It is also important that the conduits are thoroughly cleaned using a form of ‘pull-through’, prior to the inner cable being installed. At longer intervals, it might become necessary to inspect the outer conduit for signs of damage or kinking; which can cause the control to become tight or ‘notchy’.