Saturday, March 05, 2011

Aircraft Control Cable - Stainless or Galvanized?

Aircraft Control Cable
Galvanized on top and Stainless on bottom
"A general service history has shown the use of stainless steel cables in aircraft control systems results in premature wear and has been a factor in minor incidences as well as catastrophic failure. A current trend is underway in the aviation industry to move away from the use of stainless steel cable for primary flight control applications, except where marine operations are performed." FAA Special Airworthiness Information Bulletin CE-12-01 Dated October 24, 2011.

 Frayed stainless steel control cable is suspect in a Twin-Otter crash killing 14 passengers in Tahiti in August of 2007. 

General aviation aircraft generally use control cable made from either stainless steel or galvanized steel.  Each type has its advantages and disadvantages. Generally;
  • Galvanized rope is stronger.
  • Galvanized rope has greater fatigue strength.
  • Galvanized rope has less wear.
  • Galvanized rope is easier to inspect for corrosion damage.
  • Per FAA CE-12-01:
  • Stainless steel is more corrosion resistant.
  • Stainless steel has considerably less service life due to high wear.
  • Stainless steel cannot be inspected for corrosion damage.
  • Stainless steel is stiffer and has lower bending fatigue resistance - important in flight control systems
  • Stainless steel has a higher friction coefficient that results in increased wear every time the cable is flexed.
  • Stainless steel becomes more stiff, leading to increased abrasion wear in the inside as well as the outside of the cable

The poor wear resistance of stainless steel rope has resulted in aircraft control problems. More frequent inspections are required. For more information on this subject reference: Special-Airworthiness-Information-Bulletin:-SAIB CE-01-30, July 11, 2001.  For 172S airplanes see FAA SAIB: CE-11-3. also  CE-11-36, Piper Service Bulletin 1048.

There are several reasons why stainless wears more than galvanized steel when used on flight controls:
The bending of a wire rope causes the individual wire stands to not only bend but to rub against one another. Galvanizing is a natural lubricant. For example, galvanized threads have a lower friction (K) factor then plain steel. The individual wires can easily move about with very little friction and wear. Stainless steel on the other hand has high friction and has a reputation for seizing and galling when rubbed together. Every time the wire rope is flexed, the stainless wires rub together. High friction creates high wear.

There are several methods of reducing wear and increasing fatigue resistance in a wire rope. Wear resistance can be increased by changing how the wire stands are wound. In the picture above the individual wires are horizontal (parallel to the axis of the rope). This is called "right regular lay" and is the standard lay.  Another method of winding the wire stands is so that they form an angle to the axis of the rope. This is called "lang lay". Lang lay increases fatigue strength and abrasion resistance without any decrease in ultimate strength.  Another method of changing the wire characteristics of fatigue strength,. abrasion resistance, and flexibility  is to use wires of different diameters. For example, Douglas-Specification DMS2192 calls for a Warrington Seal (IWRC) construction. This type of wire rope has larger wires on the outside and and smaller wires on the inside.

There are other wire rope designs that the engineer can call for to optimize specific performance goals. This is why when we replace wire rope we should make sure that the replacement meets the original manufacturer's specifications.

Galvanized Vs Stainless - who uses what?
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.

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