At night, the unit was initially found to be too bright in that, even though it was extremely noticeable, "blooming" of the individual lights obscured the configuration. Reducing the voltage to one-third (40V) at night resulted in the best compromise of acquisition and recognition. Results were then very consistent.
During both high-brightness day and full dark night testing, the unit was acquired as "something on the runway" at from five to eight miles distance during a straight-in approach. A bright pulsing light in the runway touchdown zone should cause any pilot to question the advisability of landing, but without exception, each pilot recognized the signal as an "X" while atleast 1-1/2 miles from the threshold, and received a definite "Do not land" signal in plenty of time to execute a safe missed approach (figure 5). The pulsing feature was noted by several pilots as effective in drawing attention to the device. As the white lights were quite visible against the background of a concrete runway, colored filters served only to decrease intensity, with no noticeable improvement in recognition.
Although a limited number of approaches were made with high-performance aircraft, subsequent sharing of information with personnel at San Francisco International Airport, where a similar device is in operational use, has reinforced results obtained at the Technical Center.
The device meets all criteria set forth, except criterium no. 2, that is, that it be visible, omnidirectionally, from any point ½ mile from the runway threshold. It was found that the required peak intensity could not be obtained from an omnidirectional source, and that rotation of a large system of lights is not practical. Since all of the subject pilots found the device to be adequate during a circling approach, it is felt that this requirement is not critical.
The device was not tested at the lowest limits of VFR conditions, but as the lamp used is the same one used in the Medium Intensity Approach Lighting System, it is felt that its visibility would be similar.