What Is Airfield Ground Lighting (AGL)?

Airfield Ground Lighting is the network of specialised lighting fixtures, signs, and supporting electrical equipment installed across an airport's movement area — runways, taxiways, aprons, and approach zones. Its job is to give pilots continuous, reliable visual guidance for every phase of ground and near-ground aircraft movement.

AGL systems are designed according to applicable aviation standards and requirements, including ICAO, FAA, DGCA, and relevant national standards, so that the guidance a pilot sees is consistent and recognisable at certified airports worldwide (with a small number of documented differences between ICAO and FAA practice).

In simple terms: AGL is how an airport talks to a pilot using light, in a language defined by international aviation standards.

The main purposes of AGL are to:

• Provide visual guidance to pilots during aircraft movement
• Identify runway and taxiway boundaries
• Support safe aircraft take-off and landing
• Assist aircraft during low-visibility operations
• Provide approach guidance before landing
• Indicate runway thresholds, runway ends, and stop bars
• Improve airport operational safety during darkness and adverse weather
• Support efficient aircraft movement on the ground

An airport AGL system consists of several types of lighting equipment and supporting electrical systems. Each component has a specific function and contributes to the safe operation of aircraft.

Runway lighting provides pilots with visual guidance during take-off and landing. Common runway lights include edge lights, centreline lights, threshold lights, runway end lights, touchdown zone lights, and rapid exit taxiway indicator lights.

• Runway edge lights define the lateral limits of the runway (white, changing to yellow over the caution zone at the far end of the runway)
• Centreline lights help pilots maintain correct alignment, particularly in low visibility
• Threshold lights (green) mark the beginning of the runway available for landing
• Runway end lights (red) mark the end of the runway

Taxiway lighting guides aircraft between the runway, apron, terminal, hangar, and other airport operating areas. It may include taxiway edge lights, centreline lights, guidance signs, stop-bar lights, runway guard lights, clearance-bar lights, and intermediate holding position lights.

Taxiway centreline lights help pilots follow the correct route, while edge lights indicate the boundaries of the taxiway and help prevent aircraft from leaving the designated movement area.

Stop-bar lights are used at runway holding positions and indicate where aircraft must stop before entering or crossing a runway: a lit red bar means stop, and the bar is switched off (by ATC or the control system) to clear the aircraft to proceed.

The Approach Lighting System (ALS) provides visual guidance to pilots during the final approach phase before landing. Installed before the runway threshold, it helps pilots identify runway alignment, determine the approach direction, estimate the aircraft's position relative to the runway, transition from instrument to visual flight, and maintain the correct approach path. The exact type used depends on runway classification, approach category, and airport operational requirements.

Visual glide path systems help pilots maintain the correct vertical approach angle. The most common are PAPI (Precision Approach Path Indicator), VASI (Visual Approach Slope Indicator), and APAPI (Abbreviated PAPI). PAPI is the ICAO-recommended standard and the most widely installed type today, while VASI is an older system still found at some airfields. PAPI normally consists of four light units beside the runway (APAPI uses two), set for the approach angle of the runway (commonly about 3°), showing a combination of white and red lights that tells the pilot whether the aircraft is on the correct glide path.

PAPI is a critical visual aid because it supports safe, stable approaches without the pilot needing to interpret anything beyond a simple colour pattern.

Other lighting systems improve safety and visibility across the wider airport, beyond the runway and taxiway itself. Some of these (such as apron floodlighting, road and service-area lighting) are not AGL in the strict sense and are usually supplied from separate circuits:

Airfield signs provide information such as runway identification, taxiway direction, location, holding positions, and mandatory instructions.

Most traditional AGL systems use a series electrical circuit. Power flows from a Constant Current Regulator (CCR), which supplies controlled current to the circuit and maintains the required current level (commonly up to 6.6 A at maximum brightness) even when the number of operating lights or the electrical load changes, within its rated load range. The CCR also provides different intensity steps (typically 3 or 5) so the lighting output can be adjusted for visibility and operational requirements.

Isolation transformers are installed between the primary series circuit and individual light fittings. Their main functions are to electrically isolate the fitting from the primary circuit, transfer power to it, improve system safety, allow individual fittings to operate within the series circuit, and support maintenance and fault isolation. They also keep the series loop complete if a lamp fails, so one failed lamp does not take the whole circuit down.

The primary cable carries series-circuit power, at potentially lethal voltage, between the CCR and the isolation transformers, while the secondary cable connects the transformer to the individual light fitting. L-823 connectors and other approved cable accessories provide reliable electrical connections throughout.

A series circuit means every fitting shares the same current — which is exactly why isolation transformers and correct cable practice matter so much for reliability.

Modern AGL systems may be controlled and monitored through local control panels, remote control panels, an Airfield Lighting Control and Monitoring System (ALCMS), PLC systems, SCADA systems, or wider airport operational control systems.

These systems allow operators to control lighting intensity, monitor equipment status, identify faults, and record operational conditions — turning a field of individual fittings into one supervised system.

AGL is essential because aircraft must be able to operate safely in different weather and visibility conditions. The system helps pilots identify the runway during approach, maintain runway alignment, recognise taxiway routes, locate runway holding positions, understand runway and taxiway boundaries, maintain the correct approach angle, and operate safely during darkness and low visibility.

Without a reliable AGL system, airport operations may be restricted or suspended, particularly at night and in poor weather.

AGL systems can experience several types of failures and performance issues, including:

Regular inspection, testing, preventive maintenance, and photometric verification are what keep this list of possible failures from becoming actual ones.

AGL maintenance generally follows four disciplines, each with a distinct purpose:

Photometric testing confirms that light fittings are producing the required intensity, colour, beam spread, and directional performance — it's the final check that closes the loop between design intent and what's actually installed on the airfield.

• AGL is a complete engineered system, not just a collection of lights — power, control, monitoring, and photometry all work together
• Runway, taxiway, approach, and glide-path lighting each answer a different question a pilot needs answered
• Most AGL systems run on a series circuit built around a CCR, isolation transformers, and cabling
• A reliable system depends on correct design, quality equipment, and disciplined inspection, preventive, corrective, and testing regimes
• AGL systems are designed and maintained to ICAO, FAA, DGCA, and applicable national standards