LED Technology in AGL Systems

LED (Light Emitting Diode) technology has revolutionised airfield ground lighting. Unlike halogen or incandescent lamps that produce light by heating a filament, LEDs are solid‑state devices that emit light when current passes through a semiconductor junction. The result is higher efficacy (lumens per watt), dramatically longer life (50,000+ hours vs. 1,000–2,000 for halogen), and the ability to produce precise colours without filters.

In AGL systems, LEDs are now the default choice for new installations and are increasingly being retrofitted into existing fixtures. However, their electrical characteristics (low forward voltage, need for constant current drive, and sensitivity to reverse polarity) require careful integration with the traditional series‑circuit architecture and CCRs. The transition involves not just swapping lamps, but often replacing the entire light engine and sometimes the isolating transformer or its secondary circuit.

A pilot judging a runway's edges or a stop bar at night is trusting that every fixture in the circuit is still producing the intensity it was commissioned at — a halogen lamp's filament degrades gradually and largely invisibly between scheduled photometric checks, so a fixture can be running under-intensity for months before anyone notices. LED technology's purpose in AGL is not simply efficiency: its stable, near-constant light output over a very long service life closes that gap, keeping the fixture closer to its rated intensity for far longer, while also reducing how often technicians need to be out on live movement areas to replace lamps — itself a source of airside safety exposure. Realising that benefit is also why LEDs cannot simply be dropped into existing fixtures: because the series circuit and CCR are built to regulate constant current rather than constant voltage, the LED driver and light engine have to be engineered to work within that architecture, which is why an LED retrofit is frequently a light-engine and transformer change rather than a lamp swap.

LEDs have been adopted across the full range of AGL fixtures, with varying levels of maturity:

• Runway Edge and Centreline: LED versions of elevated and inset fixtures are now standard. They offer better intensity control and longer intervals between maintenance.
• Taxiway Edge and Centreline: Blue and green LED edge/centreline lights are widely used, with improved colour stability over lamp life.
• Approach Lighting: LED approach light bars and crossbars are common, with some systems using pulsed LED for sequenced flashing.
• PAPI / VASI: LED PAPI units are replacing halogen, providing sharper colour transition and lower energy consumption.
• Stop Bars and Guard Lights: Red and yellow LED stop bars and guard lights are standard, with faster switching for dynamic control.
• Airfield Signage: LED‑lit signs are more uniform and easier to maintain than fluorescent or lamp‑lit versions.
• Obstruction Lighting: LED obstruction lights are the de facto choice for all intensity classes, often with solar options.

Retrofit kits are available for many legacy fixtures, allowing airports to upgrade without replacing the entire housing, though compatibility with the existing series circuit and transformer must be verified.

LED light engines are fitted inside the existing fixture housing in the same position the halogen lamp and reflector previously occupied, but because LEDs run on regulated constant current rather than the higher voltage a halogen lamp tolerates, an isolating transformer swap or secondary-side adjustment is usually part of the same installation rather than a separate step. Retrofit installation is normally done fixture by fixture on a live circuit, with each unit taken out of service, the halogen lamp assembly and reflector removed, the new LED module and driver fitted and wired to the transformer secondary, and the fixture returned to service and intensity-checked before moving to the next; this staged approach keeps the runway or taxiway circuit largely operational throughout the changeover rather than requiring the whole circuit to go dark at once. Compatibility between the new LED load and the existing transformer's regulation characteristics is verified at each fixture during installation, since an LED module's low forward voltage and constant-current requirement do not always suit a transformer that was originally sized and wound for a halogen load.

In an LED AGL fixture, the LED module is driven by a constant‑current driver circuit that accepts the AC secondary voltage from the isolating transformer and converts it to a regulated DC current suitable for the LEDs. The driver may also incorporate dimming or pulse‑width modulation (PWM) to achieve the required intensity steps, which are typically controlled by the primary current level (the same 2.8A / 3.4A / 4.1A / 5.2A / 6.6A CCR steps used for halogen fixtures).

Key functional aspects include:

• Colour Control: LEDs produce light in specific wavelengths; red, green, blue, and white LEDs are available without filters, leading to more accurate and stable colours over life.
• Dimming and Intensity Steps: LED drivers can be designed to respond to the primary current level by reducing LED current or using PWM to emulate lower intensities. This ensures compatibility with existing CCR step settings.
• Instant Restrike: LEDs reach full brightness immediately, unlike halogen lamps that require a warm‑up time. This is advantageous for stop bars and other rapidly switched applications.
• Thermal Management: LEDs generate heat at the junction, which must be conducted away via heat sinks or active cooling. Proper thermal design is critical to maintain life and colour stability.
• Optical Design: LEDs are small point sources, allowing advanced optical systems (lenses, reflectors) to shape the beam precisely to meet photometric requirements without the bulk of halogen reflectors.

Inside an LED AGL fixture, the driver circuit is itself a small closed-loop controller: it accepts the AC secondary voltage from the isolating transformer, rectifies and regulates it, and continuously senses the current actually flowing through the LED array, adjusting its output to hold that current at the level the LEDs require regardless of supply variation or thermal drift. The primary-side intensity step set by the CCR is read by the driver as an input signal, which the driver translates into the corresponding LED current or PWM duty cycle, so the fixture's visible brightness tracks the same step commands used for legacy halogen fixtures without any separate control channel. A thermal management loop runs alongside the current loop: temperature sensed at the LED junction or heat sink feeds back into the driver, which derates current if the junction runs hot, protecting lumen output and colour stability at the cost of a small, self-limiting intensity reduction rather than an outright failure.

An AGL LED light engine is built around one or more semiconductor LED die mounted on a metal-core printed circuit board (MCPCB), chosen for its ability to conduct heat away from the junction efficiently, since LED lifetime and light output both degrade rapidly if junction temperature is allowed to rise excessively. The MCPCB is thermally bonded, typically with a thermal interface pad or paste, to a heat sink integrated into the fixture housing, which may be finned aluminium or simply the die-cast housing itself acting as a heat spreader. A driver circuit, converting the transformer secondary's AC or DC output into the regulated current the LED array requires, is housed in a separate potted or conformal-coated module within the fixture, protected from moisture and vibration to the same ingress protection standard as the rest of the luminaire. Optical elements, whether a moulded lens, reflector, or diffuser, are positioned directly over the LED array to shape the beam pattern, and the entire light engine assembly is gasket-sealed within the fixture housing so the LED array and its electronics remain protected while still allowing heat generated at the die to conduct outward to the ambient air.

An airport lighting LED fixture uses an LED module and driver in place of a lamp. It gives a long service life, colour without filters and lower power use. On a series circuit, the LED load must be compatible with the isolating transformer and CCR, so check the power factor, the rated current and the transformer rating before replacing lamps. See the LED module and LED driver pages.