visual approach slope indicator system

The visual approach slope indicator system (VASIS) and the precision approach path indicator (PAPI) are lighting systems that provide visual guidance to pilots during aircraft approaches. These systems use lights to indicate the aircraft's position relative to the proper glide slope. The lights appear white when the aircraft is above the glide slope and red when it is below it. These visual aids supplement radio navigational aids like the instrument landing system (ILS), which provides electronic guidance along the runway's center line and at a correct approach angle. These systems are crucial for safe landings, especially in conditions of poor visibility or darkness.12