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Torque vs. FrictionAge causes more than wrinkles by Jim Irwin, Vice President, Products The aircraft fleet is aging. Why is such an obvious statement important? Unlike cars, aircraft require frequent inspection and repair to assure airworthiness, therefore, the age of an aircraft really doesn't matter. True? Not quite! While this conventional aviation concept is widely expressed, it is incorrect regarding the flight control system, that part of the airplane that is critical to normal autopilot operation. Unbelievable as it may seem, the FARs, and specifically Part 43, do not require any specific control system maintenance or checks during an annual, 100-hour inspection, or at any time interval, for that matter. Couple this fact with the well established reliability of the typical control system and the stage is set for gradual and significant deterioration in the system. Deterioration in autopilot performance will follow. In many cases, the autopilot is often the alert that calls attention to a control system in need of maintenance. Recent years have seen the number of autopilot performance problems caused by control system friction increasing as the fleet ages. While all aircraft are subject to friction, some aircraft are more susceptible than others. High wing aircraft are vulnerable because of the numerous 90°ree; turns the cables negotiate between the control wheel and the aileron bellcrank. Some aircraft with push-pull rod systems are awkward to inspect due to the location and availability of inspection holes. Aircraft owners and their shops often ignore both situations.
Symptoms indicative of high control-system friction are: Most of the associated problems are in roll, because the roll control mechanical system is more complex. Longer cable runs and more places for friction to develop are the culprits. Very often, the friction build up is unnoticeable at the control yoke due to the large mechanical advantage at that point in the control system. Typically, this problem occurs in only a few forms: loose or slack control cables stretched over time; control cables that are too tight, increasing friction; frozen, dry, or rusted pulley bearings; or stiff, dry, or rusted cables and/or rod ends. If the rigging is not correct or out of tolerance or if the system has stiff and/or rusty, aileron, elevator, or trim tab hinges, you can expect problems as well. In most cases, these frictions add up slowly, until the friction equals the AP clutch torque. At that point, the autopilot is nearly useless and complaints start. During the autopilot STC process, the aircraft control system is inspected and repaired or lubricated as necessary to assure that the friction of a standard system is what we are working with. The integrity of the certification clutch torque is dependent on a nominal system. During the STC process, the forces required to override the servo clutch in all conditions of flight are measured to determine that they are, with maximum clutch torque settings, below the allowed stick force limits. As the control-system friction increases, the apparent over power force goes down in relation to the static force required to move the controls. One way to diagnose a control-system friction problem is to measure the override force and compare it to the STC data. If the STC data indicated 20 pounds override at high torque and nominal torque is 60 percent of that value (12 pounds) and you measure six pounds, then either the clutch torque does not meet specifications, or the system has lost 50 percent of the system authority to friction. (The S-TEC Flight Engineering Department can help you with these tests.) Unfortunately, you cannot determine the condition of the control system based upon the overall condition or appearance of the aircraft. Some otherwise excellent aircraft that show great care may have control system deterioration. Even careful owners may not be aware of the condition. Remember that detailed inspections of the control systems are not required.
When lack of autopilot authority is suspected of causing performance problems, the mechanical systems checks should include: Excessively high bridal cable tension can add significant system friction in those installations where both bridal cable halves depart the capstan the same direction, since the tension side loads the capstan shaft. Given the average age of the fleet, it would be a good idea to inspect and fly an aircraft with the owner prior to starting the installation of a new autopilot. During the ground inspection, check for obvious friction when moving the control yoke. Also, move the surfaces by hand from outside the aircraft looking for "sticky" spots in the control travel. During the flight, be alert to the trimmability of the aircraft. Will it fly level hands off without an immediate bank starting? Look for apparent stiffness in the controls in cruise, indicated by a reluctance to return to center if rapidly displaced and released. Do the controls feel stiff, have excessive play in neutral, or is the elevator trim wheel hard to rotate? (The latter is important for electric trim installations.) Anything you learn about the aircraft in this test flight may help eliminate autopilot performance problems before they start. The bottom line: no amount of electronic box work or evaluation, or gain setting changes will correct a mechanical problem. Mechanical problems do occur and they are occurring more often. Some aircraft in the fleet have had five or six engines, been painted and upholstered several times, and still have the original cable and pulley system. Grease does not last 30 to 40 years without deterioration. Correcting these control-system problems may mean a slightly higher bill for the customer, but one thing is for sure, not recognizing this problem will cost both you and the customer time and headaches. |