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 highly useful Automatic Flight Control System (AFCS) feature that enhances safety by reducing altitude deviations.
However, modern AFCS automatically arming ALT SEL can pose a risk, particularly when intercepting an ILS glideslope from above, leading to unintended level-offs or the capture of a false glideslope.
A cardinal rule for ILS approaches when intercepting from above is to never set the altitude preselect below the current altitude unless the glideslope has already been captured.
Pilots must pay close attention to AFCS annunciations (active in green, armed in white) to understand autopilot modes and prevent incorrect operation during critical flight phases.
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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