Modern EFIS displays such as Garmin’s TXi series display the active and armed modes along with the selected altitude.
Key Takeaways:
Altitude preselect (ALT SEL) is a key safety feature in Automatic Flight Control Systems (AFCS) that automates altitude management and reduces pilot workload, having evolved significantly from early autopilot systems.
Modern AFCS, which often automatically arm ALT SEL, can create hazards, particularly when intercepting a glideslope from above. Setting the preselect altitude below the aircraft's current altitude *before* glideslope capture can lead to undesired leveling off or capturing a false glideslope.
A cardinal rule for ILS approaches, especially when intercepting the glideslope from above, is to never set the altitude preselect below the current altitude unless the glideslope has been captured.
Pilots must pay close attention to AFCS annunciations (active and armed modes) to understand autopilot behavior and ensure safe operation, preventing unexpected system actions.
Of all the features of the modern Automatic Flight Control System (AFCS), perhaps the most useful is altitude preselect. It improves safety by reducing altitude busts and deviations. However, it can sometimes get you into trouble on the approach.
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