Refer to figures.
FALSE ILS GLIDE SLOPES

False Glide Slopes refer to specific vertical paths in the vicinity of the runway's centerline, other than the established ILS glide path, where DDM is zero. These false glide slopes are actually side lobes. The presence of false glide slopes is attributed to various factors, including the presence of metallic structures near the transmitter and ground reflections, as well as the characteristics of the transmitting antenna. Pilots should remain mindful of their existence.

The first false glide slope typically occurs at an altitude approximately double the glide path angle, which is around 6 degrees for a standard 3-degree glide path. In regular flight procedures, pilots typically align with the localizer and intercept the glide slope from below. This ensures avoidance of intercepting false glide slopes.

An indication of a false glidepath could be seen in the rate of descent or when cross-checking the altitude. If the rate of descent required is higher than stated in the charts, for example 1500 fpm instead of 500 fpm, the aircraft will be on a false glidepath. If the altitude passing at the specific DME distance is not the altitude described in the charts, for example 3 000 ft at 4.5 DME instead of 1 500 ft, the aircraft will be on a false glidepath. There is no warning flag appearing on the CDI, and that is the reason why many incidents have happened when aircraft intercepted false glide paths and pilots did not realize.

CDI WARNING FLAG

Regarding the Warning Flag, it appears under the following circumstances:

1. Equipment failure on the aircraft's receiver.
2. Ground station failure.
3. Indicator malfunction.
4. Weak signals received or the aircraft is out of the beacon's range.