Appropriate maintenance procedures should always be consulted prior to making any adjustments. In the event a flight control cable should encounter an ice accumulation, perhaps at a pressure seal, it may prevent the auto-flight system from making a minor flight control adjustment as the clutch in the motor could slip. The end result may be an automatic disconnect and a perceived failure of the autopilot. In some cases, the auto-flight system may have multiple means of introducing an aircraft axis change. In the case of a pitch system, the initial change may be an elevator command with a follow up of a horizontal stabilizer until the elevators are faired with the stabilizer. Should a stabilizer trim motor run too fast or too slow, the auto-flight computer may have to continue making flight control adjustments, which can result in a porpoising (vertical oscillation) condition. Should the trim system be equipped with a DC motor using brushes, the resulting dust may accumulate inside the housing resulting in sluggish operation or, if contamination should get in the motor braking system, overrun could occur. Aircraft using cables operated flight control systems and operating at high speed and high altitudes require additional attention be paid to rigging and cable tension adjustment. These aircraft will often have an automatic tension compensator that will make corrections as temperature dramatically decreases at high altitude. As flight control cables are most frequently made of steel, and aircraft structure of various aluminum alloys, the different rates of expansion can result in drastic changes in control cable tension. This may result in an involuntary flight control deflection or possibly desensitize the control system. Overcompensation of the autopilot may result in an oscillation of the aircraft in the axis where the cable discrepancy is located. Once again, this airframe deficiency is often interpreted as an autopilot failure. In fact, should this situation occur in the aileron system, the roll-oscillation may be countered by the rudder Yaw Damping system.
This rolling phenomenon is referred to as "Dutch Roll" and can also be induced by an uneven airflow over the wingtips. If uncorrected, this rolling action could result in an aircraft getting into an inverted situation. Significant wing leading edge damage, missing or inappropriately installed airflow enhancing devices such as boundary layer energizers or vortex generators, may also affect this lateral oscillation.
Yaw Dampers are considered by many manufacturers to be a critical piece of equipment and in some cases if the system is not functioning properly the aircraft may be grounded. In other cases, aircraft speed or altitude may be severely limited until normal operation of the Yaw Damping system is achieved. Frequently, turbine-powered aircraft have adjustable engine mounts that will allow the thrust angle to be adjusted both vertically and horizontally. There is, of course, a specification on verifying a correct setting, but all too often, the adjustment may inadvertently get modified. It will only take a minor difference in thrust angle to create a discrepancy in both the flight and performance characteristics of the aircraft. Some yaw damp systems may try to correct this asymmetric thrust condition by adjusting the rudder position. As the majority of these systems are designed to correct an adverse yaw condition and then return to neutral, a situation like this may cause the Yaw Damp system to disconnect or even indicate a failure. Improper rigging in flight control systems such as flaps, slats, spoilers/airbrakes and even ailerons and elevators or rudder can generate a similar condition that may show itself as a failed yaw damp system. A basic autopilot may also try to "correct" for problems in aircraft flight control rigging.
One of the primary inputs for an auto-flight system is a gyro or some other inertia-sensitive device such as an Inertial Reference System (IRS). In fact, many Yaw Damping systems have their own Yaw Gyro. Frequently, compensation for normal aircraft deck angle has to be made for these rate sensors and is accomplished by installing a tapered shim under the gyro. Direction of installation or orientation to the aircraft nose is also essential. Many airframe manufacturers will elect to operate this gyro anytime the electrical system is energized. Even though these devices are intended to remain in service for a significant number of flight hours, extended use on the ground during maintenance operations will significantly reduce a gyro's useful life. It is generally advantageous to deactivate this type device by pulling circuit breakers.
Flight control systems are often operated by hydraulic power and when an autopilot computer sends a command for an aileron deflection, it expects to see some type of response. This may be an electrical signal response from the specific flight control servo or may even be a modified output from a gyro or Inertial system. If the auto-flight computer waits for a response due to an aircraft movement and the aircraft is not in flight, excessive flight control deflection may occur. If unexpected this could be detrimental should personnel be in the area or if gust locks are installed.
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