Wire rope used as flight-critical aircraft control cable must meet MIL-DTL 83420 (latest revision). 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 embeded 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 inhibiter.)
Non-Military Specifications
There are many other Aircraft Control Cable specifications besides the US Military MIl-DTL-83420 and MIL-DTL-87161.
For example, if you are working on a Cessna Aircraft, then control cable meeting CM3862 may be required. One must be careful and check the aircraft maintenance manual for the type or specification required for that aircraft.
Discussion:
Aircraft devices are designed based on two principles:
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?
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 pully before it starts to break (frayes)? 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 pully. 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 insteand 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 pully - are far greater with stainless steel. More frequent inspections are required.
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.
NONMAGNETIC Wire Rope for Aircraft Control
There is another specification for aircraft control besides MIL-DTL-83420 and that is MIL-DTL-18375H, WIRE ROPE, FLEXIBLE, CORROSION-RESISTING, NONMAGNETIC FOR AIRCRAFT CONTROL. This is a special purpose wire rope that you may never see.
Specification Description
BMS 7-265 Boeing Material Specification, "Wire Rope for Aircraft Controls - Flexible"
DMS 2187 Douglas Material Specification "Wire Rope, Aircraft 7X31 Warrington-Seale, IWRC"
DMS2192 Douglas Material Specification "Wire Rope, Aircraft 7X31 Warrington-Seale, IWRC"
BAMS 511-011 Bombardier Aerospace Material Specification Wire Cable for Aircraft Control Systems"
BA24917 Raytheon Aircraft "Wire Rope, Steel, Flexible for Aircraft Flight Control Systems"
CM3862 Cessna Aircraft "Control Cable-Corrosion Resistant"
CMS 511-01 Bombardier (Canadair Div.) "Wire Cable for Aircraft Control Systems"
P100 General Dynamics "Wire Rope, Flexible, CRES, Coated and Uncoated"
P101 General Dynamics "Wire Rope, Flexible, Carbon Steel, Plastic Coated and Uncoated"
PS 60043 New Piper Aircraft "Limited Relaxation Control Cable"
ISO 2020 International Standartds Org. "Preformed Flexible Steel Wire Rope for Aircraft Controls"
BS 2W 12:1989 British Aerospace Standard "Preformed Zinc Coated Carbon Steel Wire Rope for Aerospace Applications"
