Which component is NOT part of the ILS?

The precision approach path indicator (PAPI) is not a component of the Instrument Landing System (ILS). The ILS consists primarily of the localizer, which provides lateral guidance, and the glideslope, which provides vertical guidance. Marker beacons can also be part of the system, offering additional navigational aid at certain points along the approach.

The PAPI, on the other hand, is an airport lighting system that helps pilots visually align with the runway during the final approach. While it is an important tool for visual approaches, it does not provide the electronic guidance necessary for an ILS approach, which is designed solely for instrument flight. Understanding this distinction is crucial for pilots using ILS, as it outlines the specific components that directly contribute to the instrument approach procedure.