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 valuable safety feature of modern Automatic Flight Control Systems (AFCS), but its automatic arming can create hazards, particularly when intercepting an approach glideslope from above.
A cardinal safety rule for ILS, LPV, or VNAV approaches when intercepting the glideslope from above is to never set the altitude preselect below the current altitude unless the glideslope has already been captured, to prevent unintended leveling or false glideslope interception.
Pilots must diligently monitor AFCS annunciations (active and armed modes) to understand the autopilot's intended actions and ensure it performs as desired.
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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