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Depending on the FMS, the highly automated flight that results when the navigation mode is used to follow a published route from the database uses a different skill set from using the navigation mode to track discreetly tuned VOR radials. Learning how to select preprogrammed routes from the database of airways can be challenging. Programming or tuning discreet VORs en route in turbulent conditions presents different challenges. Either skill set can result in a greater sharing of duties between pilot and technology and an increase in safety. 

## Fly Heading 

The heading mode is used to steer the aircraft automatically along a pilot selected heading. Using the FD/autopilot to fly a heading is a simple matter of selecting the assigned heading and then engaging the heading function or, more commonly, accomplished by first engaging the heading mode and gently turning the heading selection knob to the new heading. Gently turning the knob with the mode already engaged allows you to make a smooth change from level to turning flight. Many 

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autopilots make an abrupt bank if engaged when there is a big change to be made in heading or track. The heading function is illustrated in Figure 4-8. 

You should note that, when using the heading mode, the FD/ autopilot ignores the pilot‑programmed route in the FMS or any VOR radials you set. When in heading mode, the FD/ autopilot will fly the selected heading until fuel starvation. 

## Maintain Altitude 

The autopilot’s altitude mode maintains an assigned barometric altitude. When the altitude mode is engaged, the autopilot seeks to maintain the same barometric pressure (altitude) that the aircraft was flying at the time that the altitude mode was engaged. Figure 4-9  shows how to engage the altitude mode for one manufacturer’s autopilot. 

In addition to determining and carrying out the pitch commands necessary to maintain the flight’s assigned altitude, most autopilots are also able to trim the aircraft. 

