Maintaining proper cable tension on your aircraft's control cable is vital for flight safety. The National Transportation Safety Board (NTSB) Safety Letter A09-30-37, dated April 14, 2009, addresses a series of in-flight structural failures of the Zodiac CH-601XL aircraft. The NTSB Safety Letter also states that there is substantial evidence that flutter, thought to be caused by improperly tensioned aileron cables, was involved in some of (if not all) the accidents.
Obtain the maintenance manual for your aircraft. In the manual, you will find the procedures for measuring your aircraft's cable tensions. For example, flight controls may need to be locked in a neutral position. Follow the procedures completely. The maintenance manual will also show the locations of access panels to gain access to the control cables.
If you cannot identify a specific cable, move the appropriate flight control and see if the cable moves. The maintenance manual also contains a graph that displays the proper cable tension. The graph will be plotted as cable tension vs. temperature. The temperature of the aircraft is important because the steel cables and the aluminum airframe have different coefficients of expansion; the cable tension will change with temperature. The graph will also give you a range for the proper cable tension for each temperature.
Obtain a cable tensiometer. A tensiometer can be expensive to buy, but you may be able to borrow one from your local aircraft maintenance mechanic or local Experimental Aircraft Association chapter. Another option is to rent one from an aircraft tool rental website, such as Hangartoolbox.com (link in Resources). . Once you acquire the tensiometer, read the instructions; each tensiometer model is a little different.
Move the aircraft to an area with a stable temperature. If you are bringing your aircraft in from the snow into a heated hangar, give the aircraft time to warm up to the room temperature of the hangar. While waiting for the temperature of the aircraft to stabilize, remove the panels that allow access to the aircraft's control cables.
Take the temperature of the aircraft. Simply put, note the temperature in the hangar given that the plane has acclimated to the hangar temperature. If you want to be more precise, use a remote laser thermometer and aim it at the fuselage. Record the temperature and use it for reading the cable tension graphs in the maintenance manual.
Measure the diameter of the cable that needs its tension measured. On some cable tensiometers, you set the cable diameter on the dial before making a tension measurement. On other tensiometers, you need the cable diameter to use the calibration chart provided with the tensiometer. Hold up a short machinist ruler to the cable, and measure the diameter.
Measure the tension of each aircraft cable. Record the temperature, the diameter of the cable, the tension reading and the function of the cable (rudder, aileron, etc.).
Calculate the final tension reading. Some tensiometers have a calibration chart. For example, the chart may tell you that when the tensiometer dial reads 20 pounds tension for a one-eighth inch cable, the actual tension is 22 pounds. Your final tension reading should be the actual tension given by the calibration chart.
See if your measured cable tensions are within aircraft specifications. Compare your tensions at the measured temperature with the graphed tensions from the maintenance manual. All measured cable tensions should be within the range given in the graph(s).
If the tension is too tight or too loose, the tension will need to be adjusted. This cable tension adjustment can be performed by the owner of an experimental aircraft. However, adjustments on certificated (commercially built) aircraft must be done by a licensed aircraft mechanic. If a cable tension is out of specifications, thoroughly inspect the aircraft to find the cause. Your inspections may find a damaged or worn part.
