Improving airport safety is one of the FAA’s fundamental missions, and as part of that effort it is conducting a series of tests to establish standardized frangibility test performance characteristics for airport structures–such as approach light system towers or other landing aids–in runway safety areas to assess the ease with which they break apart on impact. According to the agency there are several ways in which a structure may properly fail when struck, ways that will lessen the damage suffered by the aircraft and reduce the possibility of its changing direction. Special shearable bolts mounting the structures to the tarmac, and inherently breakable couplings used in their construction are two such examples.
In 2009 the agency issued Advisory Circular 150/5220-23, which states that frangible structures in runway and taxiway safety areas must be designed “to withstand the static and operational wind or jet blast loads with a suitable factor of safety, but they should break, distort or yield readily when subject to the sudden collision forces of a 6,600-pound (3,000 kg) aircraft moving on the ground at 31 mph (50 km/h or 27 kt) or airborne and traveling at 87 mph (140 km/h or 75 kt).” Those requirements, according to the document, “cover the minimum levels of safety for airfield safety areas” and are mandatory for all projects funded with federal grant money through the Airport Improvement Program.
Yet that document takes its basis for test guidelines initially instituted by the Federal Highway Administration, according to an FAA spokesman. “We want to make sure that the testing methodology we develop is based on the aviation industry and not on the automotive industry,” he told AIN.
In early May in the UK, impact testing continued on a series of 18 approach light system towers, six examples each from three manufacturers. While each of the currently approved structures has undergone some type of full-scale dynamic testing in the past, there were significant variations in how that testing was performed over the years, with some trials using a rigid impactor, some with soft impactors and some using actual wing sections. This variability is spurring the FAA to develop a more standardized approach to the testing that it hopes to publish as a guidebook next year when the program concludes. To foster future improvements, the agency is also conducting dynamic finite element modeling analysis to develop computer models it hopes will validate what is learned in the collision testing.