False Glideslopes

Today’s autopilots can arm for glideslope capture to make life easier. But, it’s possible to intercept a false glideslope if you haven’t managed flight path and autopilot correctly.

BTeter

Monday, December 18, 2023 at 08:00 AM ET

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Depending on antenna type the secondary glideslopes provide differing incorrect guidance. (Image from Airbus Safety First, “Lining Up with the Correct Glide Slope”)

Key Takeaways:

While Instrument Landing Systems (ILS) remain a critical precision approach system, pilots must be aware of "false glideslopes" that exist above the desired path.
Modern digital autopilots are prone to inadvertently capturing these false glideslopes, which can lead to hazardous and incorrect flight control commands, especially when approaching from high altitudes or due to altitude preselect interference.
To prevent false glideslope incidents, pilots should meticulously check their position and altitude before arming the approach mode, ensure they are below calculated safe altitudes, and rigorously adhere to charted altitudes.

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While instrument landing systems (ILS) see far less use today with the increasing development of RNAV (GPS) approaches, ILS approaches are still common and form the backbone of precision approaches available today. With the bonus of possible added precision beyond what GPS can provide and immunity from GPS interference, these approaches will remain a staple for years to come. In this world of GPS as king, we should remain proficient in ILS use and be careful to avoid their few gotchas.

Fundamentals

A complete ILS installation consists of the localizer and the glideslope, each being related but separate systems. The localizer transmitter and antenna array were developed in the late 1930s which was quickly followed by the glideslope transmitter and its antenna array. Both the LOC and GS are ground-based navigation aids as opposed to satellite-based aids. (See sidebar for GNSS glidepath guidance). The ILS uses a radio signal whose beam is formed by an antenna array.