By Jim Sparks
Oil change, replacement of ignition plugs and checking the pressure of tires are some of the many things often associated with routine aircraft maintenance. Most of us will automatically inspect the wheel and repack the bearings at a tire change, but unfortunately, similar considerations are often not applied to auto-flight systems. Experience and familiarity will often make the technician more suspicious of certain areas of the main wheel where cracks or other defects are most prevalent. This same familiarity with auto-flight systems can make routine maintenance practices almost second nature.
Autopilot systems in different aircraft will rarely work the same. In fact, some specific types of aircraft may have several different autopilot systems certified for use. Prior to conducting maintenance on any auto-flight system, it is important to have a good understanding of how the system should work.
By the most basic definition, an autopilot is nothing more than a labor saving device and can be categorized as a single, dual or three-axis. Essentially what this means is that the autopilot is the muscle used to operate the aircraft flight control systems. For the pilots to have a successful trip, they need information available, such as how to get to the destination, aircraft speed, altitude and navigation data. For the autopilot to achieve the same result, it needs to be able to utilize the same information. Flight Guidance or Flight Director systems will observe all aircraft flight parameters and based on priorities established by the flight crew, will provide the information required by the autopilot. A basic system will use a gyro reference and serve as a wing leveler, while more advanced technology will allow the autopilot to control the aircraft through all phases of flight including landing. Many high performance turbine aircraft will depend on automatic flight technology to provide protection against the hazards of high speed and high altitude flight. This makes it imperative for those involved in return to service to be fully aware of the possible consequence of any action on these machines.
Causes of autopilot deviation
With the number of aircraft in operation increasing exponentially every few years, many of the highways in the sky have reached the saturation point. The only reasonable solution is to decrease the space between the lanes. In some areas, Reduced Vertical Separation Minimums (RVSM) are already in place. This is where the typical 2,000 feet vertical spacing between aircraft is now decreased to 1,000 feet.
Altitude indicating, as well as auto-flight systems, are now under a much closer scrutiny and have to maintain a very small margin of error. One example to consider would be an aircraft with static ports located behind a removable nose cone. If the nose is not properly secured, or the aerodynamic fit to the aircraft is marginal, the airflow over the static ports can be affected inducing an altitude error. The result is that the autopilot will not hold the assigned altitude. Such things as paint damage or protruding rivets may also distort airflow. Some cases of autopilot deviation have been attributed to replacement of a nose compartment key lock with one possessing better theft-resistant tendencies. Unfortunately, these new and improved type locks protrude into the airstream about three millimeters further than the original and disrupt the airflow over the static port. Degradation of a static port heater may be another cause of autopilot ill health.
Horizontal stabilizer trim actuator with electric motor.
In many aircraft, flight control systems are common to both crew and autopilot systems. Electric motors are a common means of converting autopilot commands into aircraft response. That is, these motors are often connected in parallel with the flight deck controls. The autopilot is frequently operating the same controls as the flight crew. In most cases, these electric motors include a clutch assembly that gives the flight crew the ability to override the automatic systems, providing the flight crewmember's input force exceeds the autopilot servomotor's force. This override torque can often be adjusted.
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