On the afternoon of 30 July 1971, Pan Am Flight 845 taxied onto the runway at San Francisco International Airport. Minutes later, the jumbo jet, bound for Tokyo, struck an approach lighting structure at the end of the strip, significantly damaging the aircraft.
Having managed to take off, the breached Boeing 747 circled over the Pacific Ocean while jettisoning fuel, before safely touching back down shortly afterwards.
In the historical ledger of aviation accidents, these events from nearly half a century ago barely get a mention. There were no fatalities among the 218 passengers, although two sustained serious injuries at the hands of rods from the lighting structure which tore through the passenger compartment.
But the incident was deemed serious enough at the time by the US Federal Aviation Administration (FAA) to warrant the introduction of new rules around airport structures, designed to break, distort or yield on impact with an aircraft – thus minimising potential damage to life and property. Similar regulations were also adopted by the International Civil Aviation Organization (ICAO) in the early 1980s.
Today, such structures are most commonly described as ‘frangible’, meaning they are designed to break into fragments in the event of a collision, rather than bend. The concept of frangibility can be applied to a wide range of systems, such as approach lights, glide path systems, weather equipment, anemometers and radar systems. While some of these systems are installed on masts, others may constitute their own structure, or are surrounded with fences.
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In recent years, airports around the world have sought to install more frangible structures. Going on a recent citation by Boeing – which says the majority of deaths as a result of plane accidents between 2008 and 2017 occurred during the final descent and landing phase – they are right to push ahead with such installations.
Based in Finland, Exel Composites has supplied frangible masts and towers to airports across the world for the best part of 30 years. Compliant with FAA and ICAO standards, the firm’s technology is based around the idea that the lower the mass of the structure, the less energy it will absorb in the event of an accident.
If well-designed, frangible structures can improve airport safety standards, as Exel product business owner for airport masts Pertti Kainu explains below. Given the growth forecast for air travel – IATA claims the number of global air travellers will top eight billion by 2037 – runways are likely to become even busier, meaning occasional mishaps are inevitable. But they needn’t be costly.
Ross Davies: Could you provide a brief summary of what Exel Composites does?
Pertti Kainu: We first started looking at poles and lattice masts in the 1980s. The poles are for single lights, while the lattice masts can be used for single and multiple lights, as well as extra tall masts that are up to 35m in height.
The basic idea around our frangible structures is that in the event of a collision at 140km/h, there would be no major damage to the plane.
Today, we have masts in over 600 airports around the world, from airports here in Helsinki to bigger ones, such as Heathrow and Singapore Changi Airport. As these airports have been modified and upgraded, they have continued to use our masts, which we see as a good sign.
RD: Would you say the airports are more attuned to the concept of frangibility, and its benefits, compared to when you started out?
PK: I would like to say yes. They like fully-frangible constructions, as the lifetime expectation for them is much longer. For example, fibreglass masts have a lot of good properties – they are very light – which give customers a lot of benefits and minimal maintenance. They perform well over a long lifetime.
RD: According to Boeing, the majority of deaths as a result of plane accidents between 2008 and 2017 occurred during the final descent and landing phase. How might frangible structures help to improve safety?
PK: The fibreglass material with composite thin-tubed walls make for fully frangible, low-mass structures, as opposed to heavy-duty masts. This means that when the plane hits a mast, the mast breaks, but there are no big heavy parts flying around the plane, which can cause serious damage. Minimum mass should mean maximum safety.
RD: How is frangibility currently regulated?
PK: The ICAO regulation applies here in Europe and Asia, while North America follows the FAA regulations. Russia recently announced its own certification system, with which Exel is now fully compliant.
I’d say these are the three major standards towards designing these frangible structures.
RD: How can frangible airport structures that are resistant to harsh weather conditions help airports save money on maintenance costs?
PK: Structures that are resistant to harsh weather conditions offer big benefits to airports. For example, our fibreglass masts are able to retain their mechanical properties in temperatures ranging from -50°C to 70°C without any problem.
Frangible masts should be able to withstand the extremities, whether they are in Scandinavia or Central Africa.
RD: What are some of the challenges that need to be accounted for when installing a frangible structure?
PK: The most important thing is to design poles and lattice masts according to an airport’s specific needs. You also need to take into account the local surroundings, as most airports are situated close to big roads and railway lines.
When installing masts at airports, particularly military bases, it is also crucial that they don’t interfere with radio signals. The good thing about fibreglass is that is transparent to electromagnetic waves.