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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. 

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## 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. 

## Flight Director 

## Flight Director Functions 

An FD is an extremely useful aid that displays cues to guide pilot or autopilot control inputs along a selected and computed flightpath. [Figure 4-5] The flight director usually receives input from an ADC and a flight data computer. The ADC supplies altitude, airspeed and temperature data, heading data from magnetic sources such as flux valves, heading selected on the HSI (or PFD/multi-function display (MFD)/ electronic horizontal situation indicator (EHSI)), navigation data from FMS, very high frequency omnidirectional range (VOR)/distance measuring equipment (DME), and RNAV sources. The flight data computer integrates all of the data such as speed, position, closure, drift, track, desired course, and altitude into a command signal. 

The command signal is displayed on the attitude indicator in the form of command bars, which show the pitch and roll 

