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 assisting with altitude management and providing alerts.
Modern AFCS systems automatically arm ALT SEL, which, while often beneficial, poses a significant risk during instrument approaches, particularly when intercepting a glideslope or vertical path from above.
A crucial safety rule for approaches intercepting from above is never to set the preselect altitude below the aircraft's current altitude unless the glideslope (or vertical path) has already been captured, to prevent unintended level-offs or false glideslope intercepts.
Pilots must diligently monitor AFCS annunciations (active modes in green, armed in white) to understand the autopilot's behavior and avoid unexpected altitude captures, especially during critical 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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