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.
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.
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. Frayed stainless steel control cable is suspect in a Twin Otter crash killing 14 passengers in Tahiti in August of 2007.
There are several reasons why stainless wears more than galvanized steel when used on flight controls:
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.
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.
Three inspection suggestions:
1. Broken wire check - release tension if necessary, bend cable with hands to expose, check near pulley. Displacing the control cable system full travel in each direction for complete inspection. Rub cloth in both directions along wire. Replace when 4 or more wires in a 12 inch length or 6 broken wires in a total cable run. Wires break most frequently where cables go through fairlead areas or around pulleys. Examine these areas carefully.
2. External wear check - look where pulley OD contacts cable - blind side of cable. Replace cable where one strand has worn wires where one wire cross section is decreased by 40% or more.
3. Internal wear check - compare OD. Replace when reduced by 30%. Be advised that stainless steel wire wears much more internally than galvanized wire.
Several control cable failures have occurred even though the cable was inspected according to the book. Aileron control cable failure on a Boeing 737-3TO on takeoff at Seattle, September 27, 1997 just six weeks after the cable was inspected for wear. The inspection consisted of checking for visible wear (external wire wear). However, the NTSB found that the internal wires were 90% worn. Most notably was the loss of aileron control on another Boeing 737-100, Flight 1659. The NTSB found that existing inspection methods could not detect the breakage of 98 of the 133 strands in the cable!
The broken strands were not detected using the prescribed method of drawing a cloth rag over the cable. Only until tension was released from the cable were the broken strands detectable. Thus the need to release cable tension to better detect broken strands.
The other Boeing standard at the time was to replace a cable when the the diameter of any single wire was reduced by 40%. This is called an "external wear" inspection. However, what the NTSB found in Flight 1659 was that cables wear internally as the individual wires slide past one another. This internal wear is greater on stainless steel cables than on galvanized cables because the galvanizing acts as a lubricant and stainless steel is noted for galling. Therefore, a maximum allowable reduction in cable diameter specification needs to be specified in the maintenance manual.
In the 737-3TO incident illustrates the need for a cable diameter specification. the "NTSB found that several locations where the overall diameter of the cable had been reduced without damage to the exterior cable surface, which the NTSB metallurgist characterized as indicative of internal cable wear. In some locations, the cable diameter was reduced by as much as 0.03 inches (corresponding to approximately a 30% reduction in cable cross-sectional area for a nominal 3/16 inch diameter cable.)"
