Aircraft typically use control cable made from either corrosion resistant "stainless steel" (composition A) or galvanized carbon steel (Composition B) per MIL-DTL-8342. Each type has its advantages and disadvantages:
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. 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.
Why does stainless wear more than galvanized steel?
Galvanizing is a natural lubricant. The individual wires can easily move about with very little friction and wear. Stainless steel has high friction and galls when rubbed together. Every time the wire cable is flexed, the stainless wires rub together resulting in wear. This wear is not visible from the outside as it occurs where one wire rubs against another.
Wear resistance is improved by changing how the wire stands are wound. In the picture above the individual wires are horizontal (parallel to the axis of the cable). 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 cable. This is called "lang lay". Lang lay increases fatigue strength and abrasion resistance without any decrease in ultimate strength.
Wear resistance, abrasion resistance, and fatigue strength can be improved by using wires of different diameters. For example, Douglas Specification DMS2192 calls for a Warrington Seal (IWRC) construction. This type of wire cable has larger wires on the outside and and smaller wires on the inside. There are other wire cable designs that the engineer can use to optimize specific performance goals. This is why we should make sure that the replacement control cable meets the original manufacturer's specifications. Not all wire cable is the same!
Cable Inspection Issues with Stainless Steel
Control cable failures have occurred even though the cable was inspected according to the book. An 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. A Boeing 737-100, Flight 169 lost aileron control when the cable broke.
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. 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)."
Influence of Pulley Size
A small pulley causes more rubbing of the individual wires in the control cable and more wear, especially when stainless steel cable is used.
"Several instances of rudder control cable fraying have been detected on some Eagle aircraft between 400 and 900 hours of service. The source of the defect (cable fraying) is attributed to the small size of the cable pulley." Source link.
Inspection and Replacement Summary
The type of inspection, and the inspection interval should be different for galvanized and stainless control cable. Tension release to detect broken wire strands and a cable diameter inspection criteria are necessary for stainless cable.
Cable that does not meet MIL-DTL-8342 has been found through tests to have significantly shorter fatigue and wear life. Much of the "aircraft control cable" sold does not meet MIL-DTL-8342.
Substitution of one cable type (stainless for galvanized) should not be made without approval and consideration of the different inspection requirements and service life.
Always use the cable specified in the maintenance manual. Many manufacturer's have their own cable specification.
Mechanic's Toolbox Software a listing of cable specifications, and just about everything you ever wanted to know about aircraft control cable.
- Galvanized steel is slightly stronger.
- Galvanized steel has less wear.
- Galvanized steel is easier to inspect for corrosion damage.
- Galvanized steel has a short life due to corrosion in high-salt areas.
- Stainless steel is more corrosion resistant.
- Stainless steel cannot be inspected for corrosion damage.
- Stainless steel has considerably less service life in bending applications due to high wear.
- Stainless steel is hard to inspect for wear (wears internally).
Stainless steel cable wears faster and has resulted in aircraft control problems:
"This Special Airworthiness Information Bulletin (SAIB) alerts you, an owner or operator of certain Piper models listed below, about possible corrosion, fraying and/or failure, and subsequent loss of control authority that may occur due to a considerably shorter life of the stainless steel cables compared to galvanized cables."