What is the exception for descending below 100ft above the touchdown zone elevation using approach lights?

The correct answer highlights that for a pilot to descend below 100 feet above the touchdown zone elevation while using approach lights, it is essential that the red terminating bars or the red side row bars are visible. This visibility provides crucial visual cues indicating that the aircraft is on the correct flight path and is safely aligned with the runway. The red bars are designed to signify the end of the runway and help ensure that the pilot has sufficient visual references to proceed with landing.

In regulatory terms, visual reference is a critical safety element, and specifically seeing these red bar indicators helps mitigate the risks associated with landing under low-visibility conditions. Their visibility assures pilots that they can continue their approach with a reasonable expectation of safety, thus enabling a descent below the determined altitude above the touchdown zone.