The Federal Aviation Administration requires all obstacles that present hazards to aircraft navigation, notably television and radio antenna towers, to be clearly marked at all times. Flashing red beacons are generally recommended at 150 feet and are mandatory at 200 feet and above, depending upon the structure’s proximity to an airport. In addition, towers situated along highways often must be marked, regardless of height.
Tower owners typically fulfill these marking requirements by using red lights at night – both flashing beacons and steady-state obstruction lights – and white strobe lights during the day. Although the FAA permits tower operators to use white strobe lights 24 hours a day, studies show they can strain community relations and adversely affect migratory birds.
An alternative approach is to paint towers FAA-specified orange or white, but this requires costly repainting every five to six years. To date many tower owners have used incandescent lamps to meet the FAA red lighting requirements. Rated at 2000 to 8000 hours, these lamps can last anywhere from six to 18 months. If they fail, they must in some cases be replaced immediately. As a result, many tower operators replace the lights annually as part of routine maintenance. Although incandescent lamps cost only $20, the cost to have someone climb a 2000-foot tower to replace one could exceed $4000.
Another option is light-emitting diodes (LEDs), which last at least five years. They have only recently become available for high-intensity, outdoor applications such as tower lighting. LEDs were developed in the late 1960s, primarily for use in electronic systems as status indicators. They demonstrated advantages over incandescent lights, notably longer service life and reduced power consumption. These factors captured the imagination of design engineers seeking more efficient lighting solutions for a wide range of outdoor applications, including tower lighting.
Conventional incandescent light bulbs and LEDs operate on entirely different technologies. Because incandescence involves heating something to produce light, it is by definition a self-destructive process. Luminescence, by contrast, is a process that emits light by deriving energy from an essentially non-thermal semiconductor reaction, giving LEDs virtually infinite life.
Incandescent bulbs are composed of a thin resistive filament suspended between two electrodes encased in a glass sphere. This filament burns away as electric current passes through it to generate light. Most of the energy is dissipated as heat, which is why incandescent lamps are so energy-inefficient. As the filament continues to burn, it becomes even thinner and more brittle, depositing a layer of tungsten inside the bulb, which further decreases light output. Moreover, the low resistance of the filament when it is cold permits an in-rush current that can be 10 times the rated operating current. Because of these factors, the service life of incandescent bulbs rarely exceeds several thousand hours.
By contrast, LEDs are solid-state devices encapsulated in an optical-grade epoxy. They consist of a semiconductor die mounted on a conducting lead with a bond wire connecting the top of the die to another conducting lead. Current flows through them in a forward direction, emitting light as the electrons move between the two semiconductor materials. This process converts more of the energy supplied to the device into light with less dissipated as heat. Due to their semiconductor construction, LEDs are also less likely to fail prematurely from shock and vibration.
The past 25 years have seen dramatic improvements in LED technology. Today’s LEDs are over 10 times more efficient than earlier generations. In 1985, the first bright red and infrared LEDs became available. These sunlight-viewable devices were composed of gallium-arsenide with an aluminum outer layer. They were used in applications with relatively short service lives, i.e., less than 10,000 hours.
When used in applications where the expected service life was much longer, it was found the external aluminum layer would oxidize, resulting in significant light loss after 10,000 to 20,000 hours of operation. In 1993, a new type of LED chip was introduced that protects the aluminum layer from oxidation with a layer of indium.
This material became the standard for traffic signals, until the development in 1998 of super high-flux LEDs by Lumileds, a joint venture of Agilent Technologies and Philips Lighting. These LEDs provide more than 10 times the flux of conventional high-intensity LEDs, permitting the development of LED-based flashing red beacons and steady-state red side tower lights.
Designed to meet the requirements of the FAA, Transport Canada and the International Civil Aviation Organization (ICAO), the new LED-based flashing red beacon can replace conventional 300 mm incandescent units using existing wiring, controllers and monitors.
In addition, it lasts at least 10 times as long as an incandescent light and, at 115 W compared with 1240 W, consumes 90 percent less energy. On-off transition is less abrupt than strobe lights, and potentially disturbing ground-lighting effect is minimized. Like the red flashing beacon, LED-based L-810 tower side lights in both single and dual versions easily replace incandescent lights and last up to 10 times as long, extending replacement intervals and reducing maintenance costs. And at 12.5 W, they use only a fraction of the energy of a standard 116-watt incandescent bulb.
LED-based tower lighting offers significant maintenance and energy savings, while eliminating the need for costly conversions to alternative high-intensity lighting solutions. Although warranted for five years, LED-based lighting has a projected service life of seven to 10 years, so maintenance costs become fixed rather than variable. As a result, the initial investment can usually be recovered in less than three years and, in some cases, in less than one year.
Douglas Woehler is business development manager, Aviation Lighting, for Dialight.