PFA were concerned about buckling in the long elevator and aileron push rods. As Vance has recently said the diameter of the elevator push rod is marginal.

To prevent the risk of buckling in the elevator push rod and to reduce the tendency to vibrate we decided to divide it into two lengths with an intermediate idler.

The aileron push rod has to be in one piece and we were made to proof load it. We had to comply with JAR-VLA which recognises the situation where the aileron may seize and the pilot and co-pilot together exert force to try and move it. In our case this resulted in a theoretical sideways load on the stick of 100lbs. Via the stick linkage to the aileron push rod a load of 232lbs would be transmitted. The aileron push rod has to withstand that load without buckling.
This was demonstrated by placing one end of the tube on the floor and the upper end beneath a 6"x2" pivoted beam. A plumb line ensured the rod was vertical.
The weight of the unloaded beam on the
rod was measured on a weighing scale
at 15lbs
A 56lb weight was suspended from the beam three feet from its fulcrum so that the total force was 183lbs
There was some bowing of the tube at that weight of about 3" at its centre point,
The tube gave the impression that it would buckle if any more weight were added and so at that point we stopped.

In the wing the tube runs in the D-box which is approximately 3"x3½".  This restrains the tube as it bends and so prevents buckling The test was repeated but on the second occasion the tube was placed inside a 3" diameter plastic drain pipe. The 56lb weight was moved to four feet from the fulcrum and the tube supported a total weight of 239lbs. Sufficient to satisfy PFA

We may have to carry out a similar exercise on the rudder cables to show that the idler is not pulled out of the rear bulkhead when two hefty pilots stamp on the pedals. How we are going to do that in the confines of the completed fuselage I do not yet know – maybe PFA will forget if I don’t remind them!