This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
ILS
Instrument Landing System
The Instrument Landing System is a precision ground-based navigation aid that provides both horizontal and vertical guidance to pilots conducting an approach to a runway. It consists of two primary radio components: the localizer, which transmits a narrow beam centered on the runway centreline to provide lateral guidance, and the glideslope, which transmits an angled beam (typically 3 degrees above horizontal) to define the correct descent path. Marker beacons - outer, middle, and inner - were traditionally used to indicate distance from the threshold, though DME has largely replaced them in modern installations. When flying an ILS, the pilot follows both needles of the CDI/PFD cross-pointer to stay on course and glidepath simultaneously.
ILS approaches are classified into three categories based on the minimum visibility and decision height they support. A Category I ILS requires a decision height of no lower than 200 feet above the touchdown zone and a runway visual range of at least 550 metres. Category II reduces the decision height to as low as 100 feet with an RVR of 300 metres, requiring special crew qualification and aircraft equipment. Category III encompasses multiple sub-categories that can allow approaches to very low or even zero decision heights, used for autoland operations in very low visibility conditions. The airport, aircraft, and crew must all hold the appropriate certifications for Cat II and III operations.