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 Automatic Flight Control System (AFCS) feature that enhances flight safety by automating altitude management and providing alerts.
Modern AFCS systems automatically arm ALT SEL, which simplifies operation but can lead to critical safety issues, especially when intercepting a glideslope from above.
A crucial safety rule for ILS and similar approaches when intercepting the glideslope from above is to never set the altitude preselect below the current aircraft altitude unless the glideslope has already been captured.
Pilots must diligently monitor AFCS mode annunciations (active in green, armed in white) to correctly understand and manage autopilot behavior during all phases of flight.
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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