The LED revolution
Lighting plays a crucial role in day-to-day air traffic operations. After years of gradual improvements, AGL / AFL (airfield ground lighting or airfield lighting) is now on the threshold of a revolution with the introduction of LED (light-emitting diodes) fittings for an ever growing number of AGL installations.
Standards and recommended practices
At all times and in all conditions, particularly at night or in poor visibility, AGL is required to provide pilots with crucial information about an aircraft position in the air – its alignment or orientation during the approach, landing and takeoff phases – or on the ground as the aircraft taxis to or from aprons. It is also essential for users moving other vehicles on the airfield surface.
AGL requirements are set by the ICAO (International Civil Aviation Organization), in the form of annexes to the 1944 Chicago Convention on International Civil Aviation. These are applied by signatory countries to the air traffic and navigation services that fall under their control. The annexes contain SARPs (Standards and Recommended Practices) regarding AGL. Actual installations are dependent upon airfield type, operations and many other considerations. They may also vary, as some regions/countries with a highly-developed and sizeable civil aviation sector have introduced more rigorous standards than those set by the ICAO.
For AGL, all electrical installations and components must also meet standards and technical specifications set by the IEC. The IEC plays a central role in the international standardisation of all aspects of airfield lighting through two of its TCs (Technical Committees), TC 34: Lamps and related equipment, which "prepares International Standards regarding specifications for all lamps, including LEDs" and, specifically, TC 97: Electrical installations for lighting and beaconing of aerodromes, which "prepares International Standards for design, installation, operation and maintenance of aeronautical ground lighting of aerodromes".
Complex requirements and multiple installations
AGL involves multiple installations, each with its own set of requirements and technical specifications. They include both inset and elevated luminaires (light fittings) for:
- approach lights, with centreline, crossbars and supplementary approach lights
- runway lights, including runway guard, threshold, centreline, edge, touch down zone, runway end and stopway lights
- taxiway lights with centreline, edge, stop-bar and intermediate holding position lights
- apron lights
- airport signage
Airfield lighting is a tough environment in which shocks, vibrations and great changes in temperature (frost or intense heat from the sun and surrounding tarmac) are the norm, so very robust installations are required. Currently the main provision for AGL is still in the form of incandescent lamps using a filament. These suffer from a number of weaknesses, in particular a relatively short average life as filaments are burnt out after 1 000 to 2 000 hours. However, some recent models have a longer lifespan, German lighting manufacturer Osram, for instance, offers new long-life versions of its tungsten halogen lamps, claiming these average 3 000 hours with "more than 6 000 hours possible".
Incandescent lights weaknesses
According to aviation expert, Paolo Mazzaracchio, former head of the Italian Civil Aviation Authority's Bureau for Airport Technologies and erstwhile Secretary of TC 97, another weakness of incandescent lights is their "poor spectral emission, this means that when operated at very low intensity they emit a very reddish colour and, on the contrary, a very bright white, almost blueish light, at maximum intensity". This has consequences for filtered lights too, where variable results can be observed. "For instance, a white lamp used with a green filter will give an amber colour at low intensity and a green-blue light at high intensity, continuously changing," Mazzaracchio told e-tech. This can prove confusing in specific weather conditions or on certain stretches of runways or taxiways.
The inherent weaknesses of incandescent lights may disappear from AGL in coming years as LED lighting solutions are introduced. LEDs have been gradually appearing in industrial and home lighting environments and, more recently in the automotive industry, for car headlights.
LED airfield lighting products first appeared in autumn 2002 in the form of elevated taxiway edge lighting installations. They were followed by versions for in-pavement taxiway centreline and obstruction lighting and are gradually being followed by fittings for runways and approach lighting. A growing number of lighting equipment manufacturers are developing LED products for AGL and they are being adopted by increasing numbers of airports.
Lifespan, cost and visual benefits
LED lights present significant advantages over their incandescent counterparts, notably in terms of their average lifespan, operating costs and light spectrum characteristics.
The average life of LED lights is many times that of incandescent lamps. They are rated by manufacturers at between 30 000 and 60 000 hours and even "far over 100 000 hours in normal operating conditions", according to several airport lighting product manufacturers. Their power consumption is also much lower: for the same light intensity, an LED light requires 10-15 % of the power used by a conventional incandescent light. LEDs can achieve power savings of between 60 and 85 % (including power line losses), depending on their operation, which means also that their CO2 emissions are reduced proportionally. Both longer life and lower power consumption have highly significant cost implications, particularly the former.
"Today an LED light may cost 40 % more than a conventional light," Mazzaracchio said, "but the advantages – maintenance in particular – are such that the choice is obvious: if you have to replace lights after 50 000-80 000 hours of operation instead of 3 000 hours that makes a very big difference." This assessment is shared by several airport authorities in Sweden, Qatar, Australia and elsewhere, which gave "reduced maintenance costs and power consumption" as their main reasons for ordering LED lighting. AGL lighting manufacturers estimate that LEDs can reduce airfield lighting maintenance costs by as much as 70 %.
The overall initial cost of installations using LEDs is also lower. The low power requirement means that smaller cables, smaller CCRs (Constant Current Regulators) and smaller electrical systems are needed, reducing the cost of infrastructure.
The reliability of LED lights is a further significant advantage. Data inferred from more than one billion operating hours accumulated so far by LED lights from a single manufacturer indicates a MTBF (mean time between failures) of more than 250 000 hours.
"Another major benefit of LEDs for airfield lighting, in contrast to the poor spectral emission of existing incandescent lights," Mazzaracchio explains, "is their chromaticity, which means that they have the same colour at each intensity level of light." LEDs ensure that the correct colour is obtained directly, meaning that the coloured filters that have conventionally been used are no longer required. This, in turn, means no energy losses and no colour shifts when viewed at various angles or under fluctuating temperatures or currents.
"In poor weather conditions, LEDs are slightly better too," he adds. "If you place two lights, one LED and a conventional light, next to each other and operate them at the same intensity, the LED will appear much brighter, crisper. So when you have fog conditions – not very thick fog, obviously – LEDs are better."
LED technology is now also being introduced in AGL for signs which are currently lit using incandescent or fluorescent lights. Neither of these is ideal: incandescent lights must be kept at maximum intensity, while fluorescent lights are very sensitive to low temperatures and cannot operate under -25 °C. On the other hand, LED lights are good, bright and less sensitive.
Outstanding difficulties being resolved
Technical limitations meant that, until very recently, LED use was restricted to taxiways and apron lights, which require medium-intensity lights. However, runways and approaches demand high-intensity lights. High-intensity lights with high-intensity LED sources presented the industry with some problems, notably in dispersing the heat produced at each junction within each single LED lamp.
High temperatures affect adversely both the light intensity and service life of LEDs. Until recently, temperature was the main limitation to LED use for high intensity applications. The higher the temperature, the shorter the lifetime of the LED and the lower its light intensity. The technical challenge has largely been solved by the use of thermal management solutions. These have allowed AGL manufacturers to develop high-intensity LED touch-down zone and runway centreline insets with minimal external diameters and installation heights. They present fewer contact surfaces for cleaning equipment, snowploughs and aircraft, resulting in a longer service life.
Straightforward and gradual transition
An interesting feature of LED lights is their backward compatibility with current equipment. They can be installed into existing fittings when these need changing, so that incandescent lights can be replaced progressively with LEDs, without having to overhaul the installations completely. However, the maximum benefit of LED AGL solutions is gained through new installations or when an entire AGL installation is being renovated. This allows for customized solutions that achieve maximum power savings and service life.
However, from an optical perspective, conventional lights cannot be mixed with LEDs on the same segment. "You can't just replace an incandescent light with an LED light because of the different behaviours, because the intensity, the colour are quite different," Mazzaracchio explained. "You have to be careful, as incandescent lights, especially old ones, will appear much weaker than corresponding LED lights. If you have a very long row of incandescent lights followed by another long row of LED lights, they have to be controlled separately in order to adjust their light intensity properly and achieve the same perceived light intensity."
Special care is also required in adjusting the intensity of LED lights if there are many of them, otherwise one may end up with what was once called "blue sea effect" on aprons. The intensity has to be adjusted with the rest of the lighting system.
Tomorrow's solution today
Given all their significant advantages over incandescent lights – lower running costs from reduced power consumption, longer service life and reduced maintenance, backward compatibility – LED products that are already, or will soon become, available for all AGL systems are bound to replace incandescent lights fairly rapidly, given also the latter's relatively short service life.
"In my opinion LED lighting is the most significant change we've had in the last 50 years," Mazzaracchio says. He concludes: "I don't think that they are the future, but they are what we must introduce now!"