What is the vertical mode used during an ILS approach?

During an Instrument Landing System (ILS) approach, the vertical mode used is the Glide Slope (GS). This mode is essential because it provides vertical guidance to the aircraft as it approaches the runway. The GS signal indicates the optimal descent path to maintain during the final approach phase. It is specifically designed to ensure that the aircraft descends at the correct angle (typically around 3 degrees) to arrive safely at the runway threshold.

Maintaining proper glide slope adherence is critical for achieving a safe landing, particularly in low visibility conditions where visual references are minimal. If the aircraft deviates from the GS, it may result in an unsafe approach, leading to either a too high or too low altitude during landing.

In contrast, the other options refer to different modes or functions. For example, the LOC (Localizer) mode is responsible for horizontal guidance, while VS (Vertical Speed) is not specifically tied to ILS approaches but indicates a rate of climb or descent. VPTH (Vertical Path) might indicate a vertical navigation function but is not correct in the context of a standard ILS approach. Thus, GS remains the fundamental vertical guidance component for ILS operations.