There have been instances of aircraft landing on runways temporarily closed for maintenance or snow clearing operations. The problem becomes particularly acute at night when runway edge lights must be on for electrical maintenance or for avoidance by snow plow operators. The solution was determined by Task Group 3- 1.6 of the National Airspace Review to be the development of a lighted visual aid to provide the closed runway warning. The performance criteria specified by the Federal Aviation Administration (FAA) Office of Airport Standards for evaluation of the visual aid were as follows:
1. The visual aid shall be conspicuous when viewed from the air and readily distinguishable from other visual devices used on airports. The runway edge lights and other visual aids may be on while the runway-closed light is operating.
2. It shall be visible from any point 1/2 mile from the runway threshold and shall have a vertical coverage adequate for both circling guidance and final approach areas.
3. It shall be suitable for night operations down to a visibility of 3 miles.
4. It shall be capable of being quickly set up and turned on.
These criteria were supplemented by the Technical Center Visual Guidance Section in that it was felt the visual aid should have the following characteristics:
5. The message presented by the aid should be intuitively understood.
6. The aid should be readily constructed using "off-the-shelf" components. This would allowit too be built and used at small airports , where lack of a control tower would make it especially desirable.
Criterium no. 2 suggests an omnidirectional visual aid. Red lighting was selected as intuitively suggesting prohibition. Several versions of single-point red light sources were selected for preliminary evaluation, but all failed criterium no. 1, in that they were not sufficiently conspicuous and/or could be confused with other visual aids (e.g. VASI, PLASI). In fact, a very bright red-light source, color-coded Runway End Identifier Lights (REIL), was evaluated in 1983 as a closed-runway indicator and was found to be confusing to pilots at best (FAA Technical Note No. DOT/FAA/CT-TN83/52).
A message can also be conveyed through lighting by the configuration of a point multi-light source. The configuration chosen for preliminary evaluation was that of the letter "X", which any pilot will recognize as an indication of a closed runway when it is laid out on the pavement. To meet criterium no. 4, the "X" was formed upright to project the signal into the approach area and mounted on a trailer (figure 1). This configuration would not be omnidirectional, but should still provide sufficient horizontal coverage to be effective. Obviously, the larger the device, the more effective it would be. Fourteen-foot arms were chosen as the longest still meeting portability requirements.
Several combinations of clear and yellow spotlights or fluorescent lights were evaluated, along with a strobe-lighted "X" developed for San Francisco Internationall Airport in a parallel effort. Having too many lights was found to be as detrimental as having too few. Pulsing lights were found to be far superior to steady-burning lights for early acquisition. Pulse rates were varied for selection of the optimum cycle. The configuration selected for final evaluation was an evenly spaced set of nine 150W clear spotlights (figure 2) pulsing at a rate of three seconds "on" and one second "off".
The unit was set up on the runway numbers, with the runway edge lights turned on. Subjects ranged in experience from low-time private pilots to Technical Center test pilots trained in evaluation of visual aids. They were not informed of the nature of the evaluation, other than that a lighting system was being tested. They were asked to indicate to an observer when the system was noticed, when a message was conveyed, and what the message was. Straight-in and circling approaches were made, in no particular order. The pilots were then asked if they had time to execute appropriate maneuvers during both types of approaches, as it was felt that recognition at ½ mile might not provide sufficient reaction time. Five types ofaircraft weree used , ranging from single-engine to a Boeing 727-100. A summary of pilot responses to specific questions is shown in figure 3. Pilot comments are summarized in figure 4.