|CHAPTER 4. Automated Flight Control
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 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.
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.
An autotrim system is capable of automatically making any needed adjustments to the pitch trim to maintain the aircraft at the desired altitude and in a properly trimmed condition. Pitch control pressure applied with the altitude hold mode engaged will cause the autopilot to trim against you.
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