Touchdown Zone Lighting (TDZL): Aviation Glossary and In-Depth Technical Reference

Touchdown Zone Lights (TDZL) are a cornerstone of modern runway lighting infrastructure, providing essential visual guidance for pilots during the most critical phase of flight: landing. Their design, layout, and technical specifications are precisely regulated to ensure uniformity and operational safety at airports around the world.

What Are Touchdown Zone Lights (TDZL)?

Touchdown Zone Lights are arrays of high-intensity, steady-burning white lights installed symmetrically in the touchdown zone of runways equipped for precision instrument approaches. They begin 100 feet (30 meters) beyond the runway threshold and extend up to 3,000 feet (900 meters) or to the runway midpoint, whichever is less. TDZLs consist of two parallel rows of lights placed on either side of the runway centerline.

TDZLs are mandated or recommended by aviation authorities such as the FAA and ICAO for runways supporting Category II and III Instrument Landing System (ILS) operations, enabling safe landings in low visibility and at night.

TDZL Layout Example

Image: Schematic showing typical TDZL arrangement with parallel rows starting 100 feet from the threshold and extending up to 3,000 feet.

Purpose and Function of TDZL

Enhancing Landing Accuracy and Safety

TDZLs provide unmistakable visual cues for pilots to identify the optimum landing area, especially when painted runway markings are obscured by darkness, precipitation, fog, or other visibility-reducing factors. They directly support operational safety by:

• Clearly delineating the touchdown zone, reducing the risk of long landings and runway excursions.
• Supporting pilots during the transition from instrument to visual flight, particularly on CAT II/III approaches where Runway Visual Range (RVR) may be as low as 75 meters.
• Reinforcing proper landing technique by marking the “aiming point” and optimal touchdown window.

Integration with Other Lighting Systems

TDZLs are designed to operate seamlessly alongside Runway Centerline Lights (RCL), Runway Edge Lights (REL), and Approach Lighting Systems (ALS), creating a continuous and logical sequence of visual information for pilots. This integration is vital during low visibility operations, where the first external cue at minimums is often the TDZLs.

Regulatory Compliance

TDZLs are required for CAT II/III runways by both the FAA (see AC 150/5340-30) and ICAO (Annex 14). Their presence is often a precondition for low visibility operations and must be maintained to stringent standards for intensity, placement, and redundancy.

Technical Specifications of TDZL

Layout and Placement

• Start: 100 feet (30 meters) beyond runway threshold
• End: Up to 3,000 feet (900 meters) or runway midpoint
• Arrangement: Two parallel rows, each 30 feet (9 meters) from the centerline
• Bar Configuration: Each bar consists of three or more light units, placed at 100-foot intervals along the runway axis

Light Characteristics

• Color: Steady-burning white
• Intensity: 10,000–20,000 candela, adjustable to match other in-pavement runway lights
• Beam Control: Optimized for visibility without glare, compliant with ICAO and FAA photometric standards

Construction and Reliability

• Recessed, load-bearing housings designed to withstand heavy aircraft
• High-strength borosilicate glass lenses for durability and chemical resistance
• Sealed fixtures to prevent ingress of water and debris
• Dedicated electrical circuits with backup power and looped fault-tolerant wiring at major airports

Control and Redundancy

• Integrated with Runway Lighting Control Systems for manual, ATC, automated, or pilot-controlled intensity
• Redundant power supplies (generators or UPS) to maintain operation during grid outages

TDZL Technical Configuration Table

Operational Usage: Pilot Procedures and Visual Cues

TDZLs play a vital role in pilot situational awareness and landing technique, especially in challenging conditions.

Visual Identification

• TDZLs appear as bright, parallel white lines extending from just beyond the runway threshold.
• In low visibility or at night, they are often the first positive runway reference a pilot acquires.

Landing Technique

• Pilots are trained to land within the illuminated TDZL area, maximizing runway available for stopping and reducing overrun risks.
• Standard operating procedures require a go-around if touchdown is not made by the end of the TDZL segment.

Integration with Other Visual Aids

• TDZLs work with approach lights, centerline, and edge lights to provide a layered sequence of cues:
  • ALS for alignment and descent
  • Threshold and REILs for runway identification
  • TDZLs for touchdown guidance
  • Centerline/edge for rollout and stopping

Example Pilot Briefing Callout

“Expect runway 16L, equipped with TDZLs and centerline lighting. Minimums for CAT II. Touchdown zone starts 100 feet after threshold, ends at 3,000 feet. If not down by end of lights, initiate go-around.”

TDZL Operational Use Table

Regulatory Standards: FAA, ICAO, and Global Harmonization

FAA Standards

• Required for CAT II/III runways per AC 150/5340-30 and AIM Section 2-1-5
• Specifies installation, intensity, maintenance, and outage reporting (via NOTAM)
• Outages may restrict approaches or increase minimums

ICAO Annex 14

• International baseline for TDZL design and operation
• Requires white lights, two rows, 60–900 meters (200–3,000 ft) from threshold
• Mandates photometric testing and compliance for certification

Maintenance & Outage Reporting

• Regular inspection, cleaning, and photometric checks are required
• Any unserviceability is reported via NOTAM, and corrective action must be prompt

TDZL vs. Other Runway Lighting Systems

Design, Maintenance, and Limitations

Fixture Durability

• Recessed, high-strength construction resists aircraft loads, snow, chemicals, and weather

Electrical Reliability

• Redundant circuits, backup power at major airports
• Fault detection and looped wiring

Limitations

• Not installed on all runways due to cost/complexity
• Effectiveness may be reduced by snow, rubber, or water accumulation
• Precise alignment and integration required to avoid visual confusion

Outage Protocols

• Outages reported by NOTAM
• Operations may be restricted if TDZLs are inoperative
• Regular maintenance includes photometric testing and cleaning

TDZL in Low Visibility Operations (LVO)

Role in CAT II/III Approaches

• TDZLs are often the first visible runway cue in LVO, enabling safe landings at RVRs as low as 75 meters
• Required for LVO certification of airports and airlines

Operational Redundancy

• Redundant power and control systems mandated
• Included in operational readiness checks before LVO approaches

Mnemonics and Memorization Aids for Pilots

• Extent mnemonic: “100 to 3,000, keep it in the zone.”
• Action mnemonic: “If the lights run out, go around.”

Key Takeaways

• TDZLs are essential for safe, accurate landings on precision runways, especially in low visibility.
• Strictly regulated by ICAO and FAA, they are required for CAT II/III instrument approaches.
• Proper use and maintenance of TDZLs are central to airport safety, pilot training, and operational reliability.

For more details, regulatory documents, or consulting on TDZL upgrades, contact our team or schedule a live demo .