What type of lights are runway edge lights, except in the case of a displaced threshold?

Runway edge lights are primarily designed to outline the edges of the runway during periods of low visibility and at night, providing important visual cues for pilots during takeoff and landing operations. Typically, they are fixed lights that emit a white or yellow color, depending on their location and function.

In general scenarios, runway edge lights are white, ensuring visibility for approaching and departing aircraft. However, in the case of a displaced threshold, where the usable length of the runway is altered, the edge lights may change to yellow, indicating to pilots that they are approaching the point where the runway is not available for takeoff or landing.

The correct answer states that runway edge lights show variable white or yellow, which encompasses both the normal white lights and the yellow indication during instances of a displaced threshold. This understanding is crucial for pilots as it directly affects the safe operations of aircraft on approach and during landing or takeoff.