Sunday, January 02, 2011

Inspecting Aircraft Control Cable

Is this aircraft cable cable wear acceptable?

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. Must have been a failed inspection -- yes?  Not so quick, the inspection was performed "by the book." The inspection technique and process was at fault, not the mechanics.

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! Did you detect the broken strands in the picture above? Here is another picture with the tension removed.
Same cable with tension released

The NTSB investigation found that using professional FAA approved maintenance inspection at the most professional level will not detect dangerous control cable conditions. 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.

What about measuring the external diameter? 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.

Notice also that stainless steel "the galling steel" wears faster than galvanized steel. Hmm, maybe stainless isn't so good after all. 

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.)"

Same cable tension released and bent

And then there is the Twin-Otter crash killing 14 passengers in Tahiti in August of 2007 from frayed stainless steel control cables. The poor wear resistance of stainless steel rope has resulted in death and destruction. More frequent inspections are required for stainless steel flight control cables. For more information on this subject reference: Special-Airworthiness-Information-Bulletin:-SAIB CE-01-30, July 11, 2001.

There certainly has been enough time for the airframe manufacturers to update their maintenance inspection processes for flight control cables to reflect the lessons learned by the NTSB.

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