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

Verified

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:

Instrument Landing Systems (ILS) remain crucial for precision approaches and immunity to GPS interference, but pose a significant threat due to "false glideslopes" existing above the true glideslope.
Modern digital autopilots are particularly susceptible to capturing these false glideslopes if armed too early or when the aircraft is too high on an approach, leading to dangerous, incorrect flight control inputs.
To prevent false glideslope capture, pilots must carefully verify their aircraft's position before arming approach mode, ensure they are not excessively high, and strictly adhere to charted altitudes and descent profiles.

See a mistake? Contact us.

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.