If a VASI shows four white lights, what does that mean?

When a Visual Approach Slope Indicator (VASI) displays four white lights, it indicates that the aircraft is above the glide path. The VASI system is designed to provide visual guidance to pilots during approach and landing phases. A traditional VASI typically has two light units that can show different combinations of red and white lights to communicate the aircraft's position relative to the glide path.

In the case of four white lights, which may be observed in some extended VASI systems, it signifies that the aircraft is well above the optimal glide angle necessary for a safe approach to the runway. This is critical information for pilots, as being above the glide path could lead to a need for descending adjustments before landing to ensure safe alignment with the approach corridor. The other light combinations would provide different indications, such as being on the glide path or below it, but four white lights clearly convey the message that altitude correction is needed because the aircraft is too high.