AGL STUDY NOTES

INTRODUCTION TO AIRFIELD GROUND LIGHTING (AGL)

Airfield Ground Lighting, commonly called AGL, is the network of lighting and visual guidance systems used on an aerodrome to help pilots identify, approach, land on, taxi along and operate around the airfield. It becomes especially important during darkness, restricted visibility and instrument operations.

It is not simply a collection of lights. An AGL installation combines lighting equipment + electrical circuits + control systems + installation + testing + maintenance + operational requirements.

Why is AGL required?

Without appropriate visual guidance, runway and taxiway geometry is harder to identify, particularly at night or in reduced visibility. AGL helps pilots identify:

• Runway edges
• Runway threshold
• Runway end
• Runway centreline
• Touchdown zone
• Taxiway routes
• Holding positions
• Runway entrances
• Approach alignment
• Vertical approach guidance

Main AGL areas

Simple AGL concept diagram

APPROACH AREA
   ↓  Approach lighting  →  PAPI / APAPI
   ↓
┌─────────────────────────────┐
│ RUNWAY                      │
│  Edge lights  Centreline    │
│  TDZ lighting               │
└─────────────────────────────┘
   ↓
TAXIWAY  (edge / centreline / stop bar)
   ↓
APRON  →  AIRCRAFT STAND

AGL as a complete system

A common beginner mistake is studying every light in isolation. Understand the whole chain:

If one part fails, the visible symptom may appear somewhere else. That is why AGL people need both component knowledge and system knowledge. FAA specifications include dedicated requirements for constant-current regulators, L-823 connectors, isolating transformers and airport lighting cable, showing how these form distinct parts of the system.

RUNWAY LIGHTING SYSTEMS

Runway lighting provides visual guidance during take-off and landing. Study each light by connecting Location → Colour → Function → Operational meaning.

1. Runway edge lights

Define the lateral limits of the runway. FAA guidance classifies them by intensity: HIRL (High), MIRL (Medium), LIRL (Low). They are normally white, with yellow replacing white on the last 2,000 ft or half the runway length, whichever is less, on instrument runways. ICAO Annex 14 uses yellow on the last 600 m or one-third of the runway length, whichever is less; check the applicable AIP.

2. Threshold lights

Identify the beginning of the landing portion of the runway. In FAA configurations they are green when viewed from the approach side.

3. Runway end lights

Identify the end of the usable runway. FAA bidirectional fixtures show red when viewed from the runway and green when viewed from the approach side. Red = runway end; Green = threshold.

4. Runway centreline lights

Provide accurate alignment, especially on precision approach runways and in reduced visibility. FAA AIM describes 50 ft spacing. The FAA colour sequence seen from the landing threshold is White → Alternating White/Red → Red:

5. Touchdown zone lights (TDZL)

Identify the touchdown zone on applicable precision approach runways. FAA describes rows of transverse light bars, symmetrically placed about the centreline, steady-burning white.

6. REIL

Runway End Identifier Lights: two synchronised flashing lights placed laterally near the threshold for rapid, positive identification of the approach end of a runway.

7. RETIL

Rapid Exit Taxiway Indicator Lights give distance-to-go information to the nearest rapid exit taxiway and improve situational awareness during landing and rollout. ICAO-based provisions describe fixed, unidirectional yellow lights in a 3-2-1 pattern. Check the aerodrome AIP for the runways that have RETILs. REIL identifies the runway end; RETIL indicates distance toward a rapid exit.

Quick revision table

TAXIWAY LIGHTING SYSTEMS

Taxiways guide aircraft between runways, aprons, terminals, maintenance areas and other locations. Colours and configurations communicate location and movement guidance.

FAA guidance confirms blue edge, green centreline and alternating green/yellow lead-on and lead-off lighting in applicable configurations, and describes runway guard lights as flashing yellow.

1–2. Edge and centreline

Edge lights outline the taxiway (Blue). Centreline lights guide along the route (Green).

3. Lead-off lights

Guide aircraft exiting a runway onto a taxiway. FAA describes alternating green and yellow, beginning with green, from the runway centreline toward the holding position/critical-area boundary. Leaving runway → Lead-Off.

4. Lead-on lights

Guide aircraft entering the runway from a taxiway, using the same alternating green/yellow pattern as lead-off lights, in bidirectional fixtures where provided. In ICAO practice, lead-on lights are green and work with the stop bar: lit when the stop bar is off. Going toward runway → Lead-On.

5–6. Stop bars and runway guard lights

Stop bars are red and mark a stopping position. Runway guard lights are flashing yellow, in elevated or in-pavement arrangements, and identify a runway holding position. Don't confuse Stop Bar → Red with Runway Guard → Flashing Yellow. A lit stop bar must not be crossed. ATC switches it off when it clears the aircraft or vehicle to proceed; contingency procedures apply only if it cannot be switched off.

7. Runway entrance lights (REL)

Part of the FAA Runway Status Lights system: red in-pavement fixtures along the taxiway centreline near a holding position. When activated they indicate runway traffic or an aircraft on final approach within the activation area. Runway Guard = flashing yellow; REL = red. They are different systems.

APPROACH LIGHTING SYSTEMS & PRECISION APPROACH AIDS

Approach lighting helps pilots transition from instrument conditions to visual references for landing. It is installed in the approach area beyond the threshold. FAA describes ALS as providing information on runway alignment, height perception, roll guidance and horizontal references.

• Runway alignment
• Longitudinal perspective
• Approach path reference
• Threshold identification
• Lateral reference
• Instrument-to-visual transition

Main FAA approach lighting terms

ICAO / India: Annex 14 describes simple approach lighting (centreline about 420 m with a crossbar at 300 m) and precision approach lighting CAT I (centreline 900 m, crossbar at 300 m) and CAT II/III (900 m, extra crossbar at 150 m and red side rows over the inner 270 m). FAA also uses MALS, MALSF, SALS and ODALS. Always confirm against the applicable AIP.

PAPI: Precision Approach Path Indicator

Visual approach slope guidance. FAA describes two or four lights in a single row, normally to the left of the runway. Common four-light interpretation:

More white = high. More red = low. 2 white + 2 red = on path.

VASI: Visual Approach Slope Indicator

Common two-bar interpretation: White/White = above path; Red/White = on path; Red/Red = below path. Three-bar VASI can give two visual glide paths.

AIRFIELD ELECTRICAL SYSTEMS FOR AGL

The fixture is only one part of an AGL system. Behind every light is an electrical system delivering controlled power. The series circuit is one of the most important concepts for electrical and airport engineering students.

POWER SUPPLY → CCR → PRIMARY CABLE → PRIMARY CONNECTOR
   → ISOLATING TRANSFORMER → SECONDARY CONNECTOR → AGL LIGHT

1. Constant Current Regulator (CCR)

Supplies controlled current to the series circuit, provides intensity levels in applicable systems and interfaces with airport lighting control. FAA lists AC 150/5345-10H for CCRs and regulator monitors. Why a CCR? Because series-circuit lighting is designed to operate with controlled constant current. Typical FAA values (check the specification): 6.6 A maximum; 3-step 4.8 / 5.5 / 6.6 A; 5-step 2.8 / 3.4 / 4.1 / 5.2 / 6.6 A.

2. Series circuit

CCR out ─► [TX 1] ─► [TX 2] ─► [TX 3] ─► … ─► [TX N] ─┐
  ▲           │         │         │              │    │
  │        Light 1   Light 2   Light 3        Light N │
  │                                                   │
  └─────  return to CCR: the loop is continuous  ─────┘

The circuit is one continuous loop: the same current leaves the CCR, passes through every isolating transformer primary and returns to the CCR.

3. Isolating transformer

Provides electrical isolation between the primary series circuit and the fixture, transfers power to the secondary side, and lets individual fixtures connect to the series circuit. FAA AC 150/5345-47C covers series-to-series isolation transformers.

4–5. Primary cable and connector

The primary cable is part of the high-voltage series circuit; FAA AC 150/5345-7F specifies L-824 underground cable. Primary connectors join cable lengths and connect the cable to the transformer primary; in FAA systems they are normally L-823. A poor connection can look like a circuit or fixture fault.

6–7. L-823 and secondary connection

L-823 connectors are used for underground power cables, isolation transformer leads and fixture leads (FAA AC 150/5345-26E), covering both primary and secondary connectors. The transformer secondary supplies the fixture through the secondary connection. L-823 = airport lighting cable connector.

AGL fault-finding logic

Don't immediately replace the light. Ask, in order:

1. Is the CCR operating correctly?
2. Is the series circuit energised?
3. Is the primary circuit continuous?
4. Are the connectors satisfactory?
5. Is the isolating transformer healthy?
6. Is the secondary connection satisfactory?
7. Is the light fixture working?

Revision table

ADVANCED & STANDARDS STUDY NOTES

Once the five core notes are clear, go deeper into equipment, circuits, visual aids, testing, photometry and the ICAO and FAA standards behind them. Each note has its own page with exam questions.

Equipment & Electrical Circuits

Visual Aids, Signs & Markings

Testing, Photometry & Maintenance

Operations & Standards

QUICK REVISION SHEET

STUDY METHOD FOR AGL