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Autopilot Concepts

in Automated Flight Control

An autopilot can be capable of many very time intensive tasks, helping the pilot focus on the overall status of the aircraft and flight. Good use of an autopilot helps automate the process of guiding and controlling the aircraft. Autopilots can automate tasks, such as maintaining an altitude, climbing or descending to an assigned altitude, turning to and maintaining an assigned heading, intercepting a course, guiding the aircraft between waypoints that make up a route programmed into an FMS, and flying a precision or nonprecision approach. You must accurately determine the installed options, type of installation, and basic and optional functions available in your specific aircraft.


Many advanced avionics installations really include two different, but integrated, systems. One is the autopilot system, which is the set of servo actuators that actually do the control movement and the control circuits to make the servo actuators move the correct amount for the selected task. The second is the flight director (FD) component. The FD is the brain of the autopilot system. Most autopilots can fly straight and level. When there are additional tasks of finding a selected course (intercepting), changing altitudes, and tracking navigation sources with cross winds, higher level calculations are required.

The FD is designed with the computational power to accomplish these tasks and usually displays the indications to the pilot for guidance as well. Most flight directors accept data input from the air data computer (ADC), Attitude Heading Reference System (AHRS), navigation sources, the pilot’s control panel, and the autopilot servo feedback, to name some examples. The downside is that you must program the FD to display what you are to do. If you do not preprogram the FD in time, or correctly, FD guidance may be inaccurate.

The programming of the FD increases the workload for the pilot. If that increased workload is offset by allowing the autopilot to control the aircraft, then the overall workload is decreased. However, if you elect to use the FD display, but manually fly the aircraft, then your workload is greatly increased.

In every instance, you must be absolutely sure what modes the FD/autopilot is in and include that indicator or annunciator in the crosscheck. You must know what that particular mode in that specific FD/autopilot system is programmed to accomplish, and what actions will cancel those modes. Due to numerous available options, two otherwise identical aircraft can have very different avionics and autopilot functional capabilities.

How To Use an Autopilot Function

The following steps are required to use an autopilot function:

  1. Specify desired track as defined by heading, course, series of waypoints, altitude, airspeed, and/or vertical speed.
  2. Engage the desired autopilot function(s) and verify that, in fact, the selected modes are engaged by monitoring the annunciator panel.
  3. Verify that desired track is being followed by the aircraft.
  4. Verify that the correct navigation source is selected to guide the autopilot’s track.
  5. Be ready to fly the aircraft manually to ensure proper course/clearance tracking in case of autopilot failure or misprogramming.
  6. Allow the FD/autopilot to accomplish the modes selected and programmed without interference, or disengage the unit. Do not attempt to “help” the autopilot perform a task. In some instances this has caused the autopilot to falsely sense adverse conditions and trim to the limit to accomplish its tasking. In more than a few events, this has resulted in a total loss of control and a crash.

Specification of Track and Altitude

A track is a specific goal, such as a heading or course. A goal can also be a level altitude, a selected airspeed, or a selected vertical speed to be achieved with the power at some setting. Every autopilot uses knobs, buttons, dials, or other controls that allow the pilot to specify goals. Figure 4-1 shows an autopilot combined with conventional navigation instruments. Most autopilots have indicators for the amount of servo travel or trim being used. These can be early indicators of adverse conditions, such as icing or power loss. Rarely will a trim indicator ever indicate full travel in normal operation. Consistently full or nearly full travel of the trim servos may be a sign of a trim servo failure, a shift in weight resulting in a balance problem, or airfoil problems such as icing or inadvertent control activation.

Figure 4-1. A simple autopilot.

Figure 4-1. A simple autopilot.

Primary flight displays (PFDs) often integrate all controls that allow modes to be entered for the autopilot. The PFD shown in Figure 4-2 offers knobs that allow you to enter modes without turning attention away from the primary flight instruments. Modes entered using the controls on a PFD are transferred to the autopilot.

Figure 4-2. Entering goals on a primary flight display.

Figure 4-2. Entering goals on a primary flight display. [click image to enlarge]

Engagement of Autopilot Function

Every autopilot offers a collection of buttons that allow you to choose and engage autopilot modes and functions. Buttons used to engage autopilot modes appear along the bottom of the autopilot shown in Figure 4-1. The system shown in Figure 4-3 does not use a separate device for autopilot controls; it integrates the autopilot function buttons into another cockpit display.

Figure 4-3. An integrated avionics system with an autopilot.

Figure 4-3. An integrated avionics system with an autopilot.

Verification of Autopilot Function Engagement

It is very important to verify that an autopilot mode has engaged, and the aircraft is tracking the intended flight profile. Every autopilot displays which autopilot modes are currently engaged, and most indicate an armed mode that activates when certain parameters are met, such as localizer interception. The autopilot shown in Figure 4-1 displays the active modes on the front of the unit, just above the controls. The integrated autopilot shown in Figure 4-4 displays the currently engaged autopilot mode along the top of the PFD.

Figure 4-4. Engaged autopilot modes shown at the top of a PFD.

Figure 4-4. Engaged autopilot modes shown at the top of a PFD.

How Autopilot Functions Work

Once an autopilot mode has been engaged, the autopilot:

  1. Determines which control movements are required to follow the flight profile entered by the pilot, and
  2. Moves the controls to affect tracking of the flight profile.

Determination of Control Movements Required To Achieve Goals

Suppose you wish to use the autopilot/FD to turn to an assigned heading of 270°. The heading knob is used to select the new heading. Before any control movements are made, the autopilot/FD must first determine which control movements are necessary (e.g., left or right turn). To do so, the FD/autopilot must first determine the aircraft’s current heading and bank angle, determine amount and direction of the turn, and then choose an appropriate bank angle, usually up to 30° or less. To make these determinations, the FD gathers and processes information from the aircraft’s ADC (airspeed and altitude), magnetic heading reference instrument, and navigation systems.

Carrying Out Control Movements

Once the FD/autopilot has determined which control movements are necessary to achieve the flight change, the autopilot has the task of carrying out those control movements. Every autopilot system features a collection of electromechanical devices, called servos, that actuate the aircraft control surfaces. These servos translate electrical commands into motion, the “muscle” that actually moves the control surfaces.

 

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