Saturday, March 05, 2011

Aircraft Control Cable - What is it?

The standard for fight-critical aircraft control-cable is MIL-DTL 83420. It is estimated (Defense Daily Network July 27, 2005) that less than 2% of "aircraft control cable sold in the world today meets MIL-DTL-83420. Most of it is what you would find in your local hardware store.  Tests performed on non-MIL-DTL-83420 cable concluded that the fatigue strength requirements were rarely met. If your log book entry or sales receipt uses the term "aircraft control cable" then you might be implying that the cable is MIL-DTL-83420 when it is not.

There are two easy identification methods that may help you identify aircraft control cable:

  1. All MIL-DTL-83420 contains a two-color tracer filament emended within the cable that identifies the manufacturer,
  2. All MIL-DTL-83420 cable sold on a shipping real must contain the identification number of the manufacturing reel. (All MIL-DTL-83420 cable is lubricated with a corrosion inhibitor.)
Aircraft Control Cable Wear


Aircraft devices are designed based on:

  1. Strength
  2. Endurance

Often we focus only on the strength aspect. "How strong is it? or How many "G's"? One should also ask "For how long? This is called "endurance." Fatigue strength gives us endurance. The principle difference between aircraft and non-aircraft control cable is endurance (fatigue strength). How many times will the cable bend over the pulley before it starts to break (frays)? Fatigue strength is measured in number of cycles at a given load.

One may think incorrectly that fatigue strength is not so important for a lightly loaded aircraft control cable. An interesting example of fatigue strength importance is on the Eagle-Aircraft where the control cables were fraying between 400 and 900 hours in service. In models X-TS150 and 150B, Australian-Airworthiness-Directive-AC/XT-S/2 and CASA #0008 was issued along with Service Bulletins from the aircraft manufacturer to inspect cables for fraying at the pulley. In this instance the fraying was attributed to the small size of the cable-pulley. Although not mentioned in the report, cable fatigue strength is also a factor in cable fatigue failure (fraying).

Another possible cause is the use of stainless steel cable instead of galvanized steel. Stainless steel has high friction and the individual wires can gall as they rub against one another. Galvanizing acts as a lubricant and keeps the steel cable from wearing. Consequently, wear rates on stainless steel cable used where the wires may move - such as rounding a pulley - are far greater with stainless steel. More frequent inspections are required. As of 2004, Boeing uses practically no stainless steel cables. They use the Tin over Zinc variety of carbon steel cable in their primary flight control cables.

A review of Malfunction and Defect reports from several countries seems to show that premature cable fraying is not an isolated event. A fatal Twin-Otter-crash because of worn stainless steel elevator cables in Tahiti prompted BAE, Transports Canada and the European-Agency-for-Air-Safety to ask owners of these aircraft to inspect elevator cables. This is not a new problem. In our relatively lightly loaded control systems, cable fatigue strength and wear rate might be more important than ultimate strength.

See also Inspecting Aircraft Control Cable

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