This page created 2/23/99
(Click on photo for full-sized view)
Rich and Judith Tweedie of San Jose, California have nearly completed a thorough restoration of their Stinson 108-2, N175C. Rich reports that some Stinsons have a very serious problem with misalignment of the flap cable. If this situation exists, the cable can cause critical damage to the elevator push-pull tube. In fact, he found the flap cable on N175C had "sawed" almost 1/3 through the elevator push-pull tube. A recent inspection of three other Stinsons at Rich's home field at Frazier Lake, California, revealed that all three had some evidence of the cable sawing on the push-pull tube. In addition, this misalignment has, in at least one case, contributed to an elevator control jammed in the full down position during a control check before take off. Here's a description of the problem and an easy solution, all courtesy of Rich Tweedie.
The problem is caused by a slack flap cable dropping below the flap cable "wear strip" and getting "trapped" under under the bottom of the wear strip. The wear strip is a piece of paper phenolic about a half inch wide by about three inches long located under the left rear portion of the co-pilot's seat, just forward of the rear cabin air vent. It is held in place with two small screws. To inspect, remove the co-pilot's seat and the floorboard under the co-pilot's seat. Here's a photo showing the location of the wear strip:
In this view (above), you see two cables. One is an aileron control cable and the second is the control flap cable. The brown vertical strip is the phenolic wear strip. If the cable falls below the strip, it can become trapped. The cable can become slack enough to get trapped during an ordinary pre-flight inspection that includes pulling down the flaps as well as by a fairly strong breeze from the rear of the airplane. Below is a diagram showing the cable in the "normal" and "trapped" conditions. This view is from the passenger side of the airplane.
To prevent the cable from being trapped, simply bevel the bottom of the wear strip to allow the cable to slip up from the bottom when the slack condition is removed. Lengthening the strip so that it extends all of the way to the bottom of the bracket it is attached to will work as well. The goal is to eliminate the "edge" that can trap the cable. The diagram below shows the modification. This view is from the front of the airplane looking aft:
Originally the wear strip was made of paper phenolic, but Rich suggests using the much superior linen phenolic for its replacement (and for any other wear strips and cable fairleads that need replacing). The linen phenolic is available in sheets from Aircraft Spruce, among others (listed under the plastics section of the catalog).