The Takeoff Hold Light

The Federal Aviation Administration (FAA) is developing a system called the Runway Status Light (RSL) System that is designed to automatically warn pilots of the ‘status’ of the runway on which they are about to land, depart, or cross. The system accomplishes this task by illuminating a series of red, in-pavement warning lights that are placed at strategic locations on the runway itself, at runway intersections and also at entrance taxiways. It is expected that the implementation of this system will greatly enhance operational safety at many large airports in the United States.

The system, which uses a surveillance data source such as an Airport Surface Detection Equipment Model X (ASDE-X) or Airport Movement Area Safety System/Airport Surface Detection Equipment Model 3 (AMASS/ASDE-3), conveys the runway occupancy status to the user (pilots, ground vehicle operators etc.), indicating when a runway is unsafe to takeoff or to enter through the use of red, in-pavement warning lights. The two major visual guidance components of the RSL system, which are controlled by the RSL control system, are the Runway Entrance Lights (REL) and the Takeoff Hold Lights (THL). The REL is a series of in-pavement lights that are positioned on the centreline of taxiways that enter the runway. The THL is a series of in-pavement lights that are positioned along the centreline of the runway, near the departure threshold. The RELs illuminate to warn aircraft that are entering the runway that the runway is already occupied and that it is not safe to enter. Conversely, the THLs illuminate to warn pilots preparing for departure that the runway is unsafe for takeoff.

The FAA’s Airport Technology Research and Development Branch, based at the FAA’s William J. Hughes Technical Centre in Atlantic City, New Jersey, was assigned the task of developing the optimal configuration of red, in-pavement lights to perform the function of the THL. The research would include identifying an optimal THL spacing configuration in which the conspicuity of the THL lights is clearly distinct from the runway centreline lights, determine the lowest visibility in which the configuration would be useable and determine the correct spacing of the light fixtures to avoid the main landing gear tracks of common air carrier aircraft types when the aircraft is on the runway centreline.

To perform this evaluation, two distinct phases were undertaken. The first phase involved the performance of photometric measurements and calculations, taken at the Airport Safety Technology R&D Sections Photometric Laboratory. In the second phase, field testing was undertaken on a runway at the Atlantic City International Airport (KACY). Combined, these two phases would provide the mathematical and performance data needed to make the right selection for the optimal THL configuration. Prior to beginning the research effort, a series of six unique configuration constraints were identified that would need to be considered in the research. These constraints were based on earlier research and simulation conducted by the FAA during the early conceptual stages of the RSL project. First the THL configuration was positioned on the runway centreline such that the first fixture was 375 ft. from the runway threshold. This ensured that the system would be visible from the cockpit of a typical air carrier aircraft when it was in position on the runway threshold. Secondly, the fixtures of the THL were placed mid-way between the existing runway centreline lights. It was demonstrated during the early development of the system that the conspicuity of the RSL fixtures were greatly enhanced by being positioned in between the runway centreline fixture, versus being placed right next to them. The third constraint was that the FAA Type L-850A runway centreline fixture, fitted with a 105 watt lamp and red filter, will be used in the configuration. Photometric testing concluded that the 48 watt lamps typically found in the fixture would need to be replaced with a 105 watt lamp to provide sufficient intensity to perform as needed. The THLS are positioned in very close proximity to the very bright white runway centreline fixtures, so the additional intensity would be essential to make the system visible to pilots, especially in lower visibility conditions. The fourth constraint was that the THLs were to be operated at one intensity step above that of the runway centreline circuit. This was to ensure that the THL fixtures had sufficient intensity to overpower the runway centreline fixtures and catch the pilot’s attention in the cockpit. The fifth constraint was that the spacing of the THL fixtures would be 100 ft, to keep the same spacing as the runway centreline fixtures. The sixth constraint was that only configurations using two parallel rows of lights were to be evaluated. This was determined based on simulation evaluation results that showed pilots rejected configurations featuring a single row of lights. For the purposes of this evaluation, a single row version of the THL was also included to serve as a baseline configuration to which the double row versions could be compared.

To identify the optimal separation distance between the runway centreline fixtures and the THLs, research personnel conducted a quick evaluation to simulate various fixture spacing. To do this, an FAA Type L-850A runway centreline fixture with a 48 watt lamp and another FAA Type L-850A fitted with a red filter and 105 watt lamp were placed side by side for comparison. Researchers viewed the two lamps from 500 ft. away, at ground level, and began varying the distances between the fixtures. After performing this observation, a second red THL was placed 2 ft. on the other side of the white runway centreline fixture. Further observations were taken with the THLs at 4 and 6 ft. separation from the white runway centreline fixture. At all distances, the intensity of the lights was altered between lighting step level 3 and lighting step level 4 to identify any noticeable differences between the two settings. The lights were distinguishable from one another at two ft, but the further apart the lights were separated, the more effective the fixture became. Six feet separation between lights was determined to be the most optimum configuration. This finding was confirmed through additional evaluation from a height of 14 ft. above the ground, to simulate the height of a pilot inside a typical commercial aircraft cockpit.

With the proper lighting fixture and optimal spacing now identified, researchers needed to conduct a full scale evaluation of the proposed configurations at the Atlantic City International Airport to prove, in a realistic environment, that the results of the preliminary evaluations remained true. Over a series of night-time evaluations, researchers evaluated a total of four configurations to determine how they performed. After researchers were afforded an opportunity to view each configuration, they were asked to complete a questionnaire designed to capture their opinion of what they had seen. These questionnaires were later collected and analysed as part of the research effort.

The first configuration that was evaluated consisted of a single row of fifteen lights that were placed 2 ft. from the runway centreline, spaced longitudinally 100 ft. apart, as depicted in Figure 1. Observers noted that the THL lights and the runway centreline lights blurred together, and that they were not easily distinguished from each other. A few observers with pilot certificates and flying experience noted that while the THL were located at the beginning of the runway, the configuration did look very similar to the runway centreline configuration that would be expected to be seen on the last 3000 ft. of a usable runway. (This configuration is further described in FAA Advisory Circular 150/5340-30B).

In Configuration 2, the lighting setup was changed from the one row of lights parallel to the runway centreline spaced 100 ft. apart and 2 ft. from the centreline on the right side, to two rows; one on each side of centreline, spaced 100 ft. apart and 2 ft. from centreline, as shown in Figure 2. Thirty light fixtures were placed alongside the centreline, 15 on each side, at 2 ft. separation from centreline. Configuration 2 showed improved conspicuity over Configuration 1; however, researchers noted that the THL lights still blurred together and were not easily distinguished from the white runway centreline lights.

Configuration 3 involved an increase in lateral spacing of the fixtures to a distance of four feet each side of the runway centreline. Again, 30 light fixtures were placed alongside the centreline, 15 on each side; however separation from centerline was increased to 4 ft. from the runway centreline, as shown in Figure 3. Configuration 3 was considered an improvement over Configurations 1 and 2, however, the last few fixtures of Configuration 3 (furthest away from the observation point) tended to merge together with the centreline, again making them hard to differentiate from each other.

Configuration 4, the final configuration that was evaluated, involved an additional increase in the lateral distance of the fixtures to 6 ft. on each side of the runway centreline. Thirty light fixtures were placed alongside the centreline, 15 on each side, at 6 ft. lateral separation, as shown in Figure 4. Researchers concluded that Configuration 4 provided the clearest visual representation, and at no time did the red THL lights or the white runway centreline lights interfere, merge, or blend with one another.

The data collected in the questionnaires that were completed during the evaluation, echoed many of the comments received in the field. In a question that asked how easily each participant could detect the THLs, the results showed that configuration 4 was ranked the highest as compared to the other three. In the second question, which asked participants if the THLs were easy to discriminate from other lights on the airport; well over two- thirds of those polled selected configuration 4 as their best answer. Again, in the third question, which asked if the participant would clearly understand that the configuration indicated that the runway was unsafe, the majority of those polled selected configuration 4 as being the most clear. The final question, which asked how participants would react to each configuration if the THLs were turned on while they were beginning takeoff roll, also indicated that configuration 4 was the most favoured configuration. Researchers now had field data from the evaluation and the subjective data from the questionnaires to support moving forward with configuration 4 as the most promising selection.

Now that the best configuration was identified, researchers needed to determine if the proposed spacing of the THLs would have an adverse affect on the landing gear of passing aircraft. If the gear width of an aircraft were the same width of the two THL fixtures, which was 12 feet for configuration 4, the flight crew and passengers on passing aircraft would experience an uncomfortable ride quality, as the aircraft would essentially run over the fixtures with their main gear every 100 feet. In an effort to avoid this situation, pilots may deviate from the centreline of the runway to avoid running over the fixtures; a situation which this system should not create. After a careful investigation of the gear width for over 25 various aircraft models, which represented both corporate and commercial jets, it was determined that 96% of aircraft had gear configurations that were either narrower or wider than the 12 foot separation of the THLs. It appeared as though configuration 4 was the best of the four evaluated.

The results of this evaluation validated that the configuration with a double row of THLs spaced 6 feet from the runway centreline was more effective than the other three configurations. The spacing of six feet from the runway centreline created very distinct lines that at no time visually blended with the runway centreline fixtures. As a result, this created a more conspicuous signal that would be easier to acquire by pilots. The use of double row THLs at a spacing of 6 feet was found to be a significant improvement to the THL system, and offers sufficient intensity and spacing to be effective enough to meet Category II criteria.

The FAA is expecting to install a THL system, like the one described in configuration 4, at Dallas Fort Worth International Airport (KDFW) in 2007, as part of an ongoing research effort to develop the Runway Status Light System.

Information on the FAA’s Airport Technology R&D Branch can be found at www.airporttech.tc.faa.gov

Jim Patterson, Jr

Jim Patterson, Jr is an Airport Safety Specialist with the Federal Aviation Administration’s Airport Technology Research and Development Branch. He is based at the FAA’s William J. Hughes Technical Centre in Atlantic City, New Jersey, where he manages research projects in Visual Guidance, Aircraft Rescue and Firefighting, Airport Design Safety and Operation of New Large Aircraft. Jim has a Bachelor of Science degree in Airport Management – Flight Technology from Florida Tech, is an active volunteer firefighter, and is also a holder of a Commercial pilot and Certified Flight Instructor certificate. Jim has worked in Visual Guidance Research and Development for thirteen years.