Glide Path/Glide Slope

The glide path (or glide slope) is the prescribed descent path for an aircraft approaching to land. On an ILS, the glide slope is the electronically-defined descent angle (typically 3.0°) transmitted by the glide slope antenna and displayed on the pilot’s CDI/HSI as a horizontal guidance bar.

ILS Glide Slope

The ILS glide slope transmits on a UHF frequency paired with the ILS localizer. It provides a precise 3° descent path from approximately 10 NM final to the touchdown zone. The glide slope indicator (a horizontal needle on the CDI) shows whether the aircraft is above, on, or below the glidepath. Full-scale deflection indicates approximately 0.7° above or below the glidepath.

PAPI and VASI

Precision Approach Path Indicator (PAPI) and Visual Approach Slope Indicator (VASI) are visual glide path aids that provide colour-coded guidance: all white means too high; all red means too low; two red and two white (for PAPI) means on the correct glidepath. They are used for both IFR and VFR approaches.

Glide Path Angles

Standard ILS glidepath: 3.0°. Some approaches have steeper angles (up to 4.5°) for obstacle clearance. For each degree of glidepath, the vertical speed required at a given groundspeed is approximately: VS (ft/min) = groundspeed (kts) × 100 × tan(glidepath angle). For 3° at 100 kts: approximately 500 ft/min.

Related Terms

See also: ILS, Decision Height, Final Approach.