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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. 

## Climbs and Descents 

## Vertical Speed 

The autopilot’s vertical speed mode allows you to perform constant‑rate climbs and descents. Figure 4-10 illustrates the use of the vertical speed mode for one autopilot that is integrated with a PFD. 

When you engage the vertical speed mode, the FD/autopilot will attempt to maintain the specified vertical speed until you choose a different setting in autopilot, the aircraft reaches an assigned altitude set into the assigned altitude selector/alerter, 

or the autopilot is disconnected. If an altitude selector is not installed or functioning, the pilot has the task of leveling off at the assigned altitude, which requires monitoring progress and manually engaging the autopilot’s altitude hold function once the aircraft reaches the desired altitude. You must be very careful to specify an appropriate vertical speed, as the aircraft will fly itself into a stall if you command the autopilot to climb at a rate greater than the aircraft’s powerplant(s) is/ are capable of supporting. You also need to monitor descent airspeeds diligently to ensure compliance with V$_{NE}$ /V$_{MO}$  and V$_{A}$ or turbulence penetration speeds if there is doubt about smooth air conditions. As discussed in the previous chapter, you should be cognizant of the powerplant temperatures reciprocating powered aircraft and bleed air requirements for turbine‑powered aircraft. 

## Vertical Speed With Altitude Capture 

Some FD/autopilots have an altitude select/capture feature. The altitude select/capture feature is illustrated in Figure 4-11. The altitude select/capture feature combines use of the activated vertical speed mode and an armed altitude hold mode. To use this feature, the vertical speed function is initially engaged. The altitude hold mode usually arms automatically when a different altitude is selected for capture and vertical speed is activated. With an altitude select/capture option or feature, the altitude hold mode disengages the vertical speed mode upon capture of the selected altitude once the vertical speed function completes the necessary climb or descent. Once the aircraft reaches the assigned altitude, the vertical speed function automatically disengages, and the altitude mode changes from armed to engaged. The change from vertical speed mode to altitude hold mode is the capture mode, or transition mode. Any changes made by the pilot during this short phase usually result in a cancellation of the capture action, allowing the aircraft to continue the climb or descent past the selected altitude. Again, be familiar with the aircraft’s equipment. Let the system complete programmed tasks, and understand what it will do if interrupted. 

Many FD/autopilot altitude selectors include an altitude alert feature, an auditory alert that sounds or chimes as the aircraft approaches or departs the selected altitude. 

## Catching Errors: Armed Modes Help Prevent Forgotten Mode Changes 

You have already seen how remembering to make a needed mode change in the future can be an error‑prone process. Not canceling the armed function allows the altitude select mode to relieve the pilot from needing to remember to engage the function manually once the aircraft has reached the selected altitude. Do not interrupt the altitude armed or capture mode, unless prepared to manually control the process. 

