As with any equipment in operation near a runway, approach lighting, instrument or microwave landing systems and wind direction indicators can be hazards. Though there have thankfully been few incidents of major damage caused by aircraft striking these structures, the potential for problems during the crucial phases of take-off and landing are seen as serious enough to demand proactive management.
The precedents that do exist show that lighting systems can become dangerous obstacles for aircraft, and that a fresh look at their design is perhaps overdue. In 1972, for instance, a Boeing 747 making an incorrect take-off stuck in the approach lighting system, suffering the loss of two main landing gears. Parts of the lighting structure also ruptured the aircraft’s fuselage. The resultant danger to flight safety was, obviously, severe.
The lack of development around this problem may in part be due to the apparent conundrum that must be solved in order to design a lighting system that is not only safer, but which also meets fully the demands on its operational capabilities.
"These installations quite often need to be pointed in a specific direction within very narrow tolerances, even during severe weather circumstances. Therefore, their construction and their support must be stiff," explain JFM Wiggenraad, head of the Structures Technology Department at the National Aerospace Laboratory (NLR) in the Netherlands and DG Zimcik, head of the Aeroacoustics and Structural Dynamics Department for Canada’s National Research Council, in their report ‘Frangibility of Approach Lighting Structures at Airports’.
The report continues: "However, should aircraft stray into these installations during landing or take-off, these same installations become obstacles to flight safety. Since the resistance of these ‘obstacles’ may well determine the outcome of such a mishap, it must be minimal."
THE FRANGIBLE OPTION
Combining minimal resistance with sufficient rigidity to enable precise positioning, even in strong weather conditions, may seem problematic, as rigidity seems to imply greater resistance. However, the quality of frangibility unlocks this conundrum.
"Frangibility is defined as the property which allows an object to break, distort or yield at a certain impact load while absorbing minimal energy, so as to present the minimum hazard to aircraft," explains Wiggenraad.
Given the apparently contradictory operational requirements for lighting structures, the International Civil Aviation Authority (ICAO) initiated the Frangible Aids Study Group (FASG) back in 1981, with members from Canada, the Netherlands, Sweden, the UK (on behalf of the ACI) and the USA still contributing to its progress.
FASG’s goal has been to define the precise design parameters and implementation guidelines for frangible lighting solutions. It has, therefore, focused on the problems that arise when aircraft impact obstacles while taxiing, as well as during take-off and landing, having delineated precise performance criteria – namely that a small commuter aircraft of 3000kg should be able to continue safely after impacting such structures at 140kph during take-off.
Early on in the engineering phase of its investigation of frangible lighting solutions, FASG found that peak force during impact, the energy absorbed during contact and the duration of contact were key factors in defining the level of frangibility a structure needed to possess.
TARGETING THE TEST PROCEDURE
In order to explore the effects of changes to these parameters, testing has centred on impacting new designs of lighting structure with light wing sections – representing small commuter aircraft – at 140kph. Levels of acceptability are then judged on the amount of damage to the wing and the integrity of the front spar.
Wiggenraad proposes that this test procedure must be re-examined in order to accelerate the development of workable frangible systems. Apart from live testing, he suggests that greater use of numerical analysis is the way forward, given that computer simulations of air crashes have grown significantly during the many years since frangibility was first examined by the US Federal Aviation Authority.
"Presently, the frangibility of an approach lighting structure can only be shown with sufficient confidence by performing the full-scale test on representative structures," he remarks. "Such experiments are expensive to perform and can only show frangibility for the unique set of experimental and loading conditions specified."
"Due to the cost of such experiments, numerical simulation with analytical codes is becoming more acceptable," he continues. "However, even with access to more powerful computers and proven commercial codes, the confidence in numerical solutions for entirely new structural concepts is still limited."
Results from computer modelling currently still require corroboration with live testing. Were they free from this constraint, computer-generated models could be used to assess the damage caused by impact under different parameters. They provide a relatively quick and cost-effective alternative to live testing, and enable analysts to alter factors such as the aircraft’s speed, height, size and direction.
Additionally, these models could be easily adjusted to incorporate new designs of aircraft, different flight configurations and new lighting tower designs, which at the very least could be used to refine the conditions of live tests and make it more likely that designers can hit upon the right solution at the minimum cost and in the fastest time.
"The availability of a reliable computer code is also beneficial to the airworthiness authorities, who judge the frangibility of scaled versions of a previously certified design," believes Wiggenraad. "The use of a numerical method may be the only approach to judge the frangibility of larger structures, such as ILS glide path masts."
In the Netherlands, NLR is concentrating its efforts on the development of enhanced numerical simulations, with input from Twente University and the National Research Council Canada (NRCC). It is using a PC-based code that has long been in use for evaluating the crashworthiness of general aviation aircraft and helicopters.
Currently, MLR’s models simplify the wing section and lighting structures, representing them as mass points and non-linear beams, the parameters of which are based on laboratory tests on components. Computations for this simplified model can be performed very rapidly, and can easily incorporate different materials.
The NRCC, on the other hand, has adopted a finite element analysis, where properties of the wing and lighting structures are defined at the material level, and which has been proven as a successful model for structural analysis. 3D models that account for edge effects and local deformation give added detail in assessing levels of damage from potential impacts.
As their accuracy and reliability is increasingly verified, these numerical analysis techniques may come to play a larger part in the search for the right design. At present, data from live tests has been collected as the basis for proposals to ICAO on frangibility. The organisation will then look to bring initial guidelines before its member states for approval. In the meantime, work on improving the sophistication of and confidence in numerical modelling techniques will continue.
"Work aimed at the development of the capability to determine the required frangibility by numerical analysis has provided promising initial results for further consideration," confirms Wiggenraad.
Though frangibility is clearly the solution to the problem, progress towards practical, workable designs for approach lighting and other airside systems are just starting to accelerate.