Control Cables – Bowden Control Systems

This type of control cable is used to operate remote control systems on the aircraft. A remote control system is a system where the controls are located in another area of the aircraft in relation to the item to be controlled. For example Landing gear emergency extension.

Bowden control cables are constructed from stainless steel cable, Then coiled wire is placed around the cable which is covered by a cotton braiding which in turn is housed in a conduit to keep out moisture and contaminants. Then a cap is placed on the end of the conduit (not cable) to stop the coiled wire from unravelling and help to seal the end of the conduit.On the end of the the cable a nipple is either soldered or swaged on (see maintenance manual and airworthiness notices to see which one to use). to transmit the control load. Depending on the type of fitting and control there are three types of nipples available, Plain, spherical or Trunnion (Refer to the maintenance manual for the correct one to use). It must be noted that this system is designed to operate lightly loaded components/systems.

This type of control will only operate in one direction like a normal cable and therefore some means of moving the control back must be provided, This is achieved by the two methods, either by two cables, This gives control in both directions , or a cable and return spring.

This type of control , like any cable can become slack over its service life. This problem is overcome by the addition of one of two little devices. These two devices are adjustable stop or double ended stop. The adjustable stop is the standard stop consisting of a tee-barrel, locknut and hollow threaded portion fixed on the conduit at the receiving end of the cable which is fitted through the tee-barrel which is mounted to the structure of the aircraft. A lock nut is fitted to stop the adjuster moving.

The second type of adjuster is the double ended stop. This type is used when the system does not permit appropriate access to the ends of the cable to permit the use of the standard adjustable stop. This is located at an accessible portion of the cable and is easily adjusted in the same manner as the adjustable stop with the exception that the tee-barrel is replaced by an internal female threaded hollow tube which is fitted with a witness hole to inspect for serviceability.

Sometimes it is necessary to connect Bowden control cables to other types of control cables and this is achieved by a cable connector. When only a one to one joining is needed, you can use a cable connecting barrel. When it is necessary to connect one cable to more than one cable, a junction box may be used.

Operating hand levers are located at the transmitting end of the cable and connect to a cable nipple to transmit the control load onto the cable. The most common types are Thumb, Ratchet and plain type. The ratchet type and plain type both have a three and a half inch throw in radial motion. The ratchet type operates on a ratchet mechanism so that more than one pull of cable is possible to allow more cable movement whereas the plain type can only use one pull on the cable thus limiting the movement on the other end of the cable.

The thumb type of hand lever works on the same mechanism as the plain type of hand lever but has a shorter lever thus giving a shorter throw of only one and a half inches.

Axial alignment of cable assemblies must be such that the centre line of the cable within the conduit is in a straight line through the mid position of the rise and fall of the arc of travel of the lever. This can be more simply explained by moving the control lever to three quarters of total travel one way, check that the cable is in a straight line with the conduit. Now move the control lever to three quarters of total travel in the opposite sense (the mirror position of earlier) and check that the centre line is straight with the rest of the conduit as before. The reason why this is so important is that when a cable is moved through the angles of the control it can rub and chafe against the the casing of the control lever thus giving accelerated wear and possibly impairing flight safety in the future.

The inspection of a Bowden control system is called upon at regular intervals along with the maintenance and lubrication. As with a normal cable system you would follow the service manual for the correct methods and lubricants but in general you would inspect for:

  •             Corrosion
  •             Deformation, bends, kinks
  •             Chafing
  •             Tension
  •             Cleanliness
  •             Broken strands or wires
  •             Other forms of damage or cause for concern.

 

Remember you are going to be a professional, DO NOT EVER TURN A BLIND EYE, If you are unsure always fail until you can prove otherwise by further research not the other way round. Remember at 36000ft there are no second chances.

 

 

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