Abstract: Methods and compositions for preventing opposing ILS systems on a single runway from becoming active at the same time. A physical interlock system employs a physical switch element that may activate a first ILS system or an opposing second ILS system, but is not capable of permitting, and may prevent, opposing ILS systems from being active simultaneously. Also included are methods for preventing opposing ILS systems on a single runway from becoming active, comprising the use of a physical switch preventing activating signals from being sent to opposing ILS systems at the same time.

Abstract: Precision approach path indicator systems (PAPIs) effective in providing approach slope guidance for aircraft approaching an airport runway are provided. Such PAPIs include a plurality of light assemblies positioned on or in proximity to an airport runway and structured or configured to be effective in providing approach slope guidance light signals to a pilot of an aircraft approaching a runway. Each light assembly includes a light source comprising light emitting diodes (LEDs), preferably a first array of LEDs and a second array of LEDs.