Displaced Threshold

Definition

A displaced threshold is an operational feature of a runway where the touchdown point for landing aircraft is moved a specified distance inward from the physical end of the paved surface. This adjustment is made to enhance safety, accommodate environmental or infrastructure constraints, and optimize runway use. The area before the displaced threshold is clearly marked and is not used for the landing touchdown in that direction, though it remains available for takeoff, taxi, and rollout from the opposite direction.

Purpose and Reasons for Use

Displaced thresholds serve several important purposes:

• Obstacle Clearance: Ensures aircraft approach over obstacles (trees, buildings, terrain) at a safe height, as required by ICAO Annex 14 and FAA regulations.
• Pavement Protection: Prevents landing on runway sections that are not structurally suitable for touchdown forces, which may be older or temporarily repaired pavement.
• Noise Abatement: Shifts landing further from noise-sensitive areas, reducing disturbance to nearby communities.
• Safety Area Compliance: Ensures adequate Runway Safety Area (RSA) or Runway End Safety Area (RESA) in accordance with regulatory standards.
• Construction or Maintenance: Allows runway use while work is performed near the original threshold.
• Proximity to Other Airfield Features: Avoids interference with intersecting taxiways, runways, or protected areas.

Each application is based on evaluating safety, operational efficiency, and regulatory compliance.

Operational Details

• Landing: Aircraft must not touch down before the displaced threshold. Doing so may be unsafe or a regulatory violation.
• Takeoff: The entire runway, including the displaced section, is typically available for takeoff rolls in both directions.
• Landing Roll-Out (Opposite Direction): Aircraft landing from the opposite direction may use the displaced area for roll-out, maximizing stopping distance.
• Taxiing: The displaced area may be used for taxiing, depending on airport layout and procedures.

Declared Distances

• LDA (Landing Distance Available): Begins at the displaced threshold, not the physical end.
• TORA (Takeoff Run Available): Typically includes the full paved length.
• TODA (Takeoff Distance Available): Also generally includes the displaced area.
• ASDA (Accelerate-Stop Distance Available): Includes the full paved length unless otherwise specified.

Example:
A 10,000 ft runway with a 2,000 ft displaced threshold has:

• LDA: 8,000 ft (for landing in direction of displacement)
• TORA, TODA, ASDA: 10,000 ft (full length)

Types of Displaced Thresholds

• Permanent: Result from enduring obstacles, long-term noise abatement, or unrecoverable pavement limitations. Depicted on all official charts.
• Temporary: Due to short-term construction, maintenance, or temporary obstacles. Communicated by NOTAMs and updated charts.

Both types must be well-marked and documented for flight crews.

Markings, Signs, and Identification

Displaced thresholds are distinguished by:

• White arrows along the runway centerline pointing toward the threshold.
• A wide solid white threshold bar (ten feet wide) perpendicular to the centerline.
• Arrowheads across the width just before the threshold bar.
• Runway numbers start only after the threshold.
• Green threshold lights at the displaced threshold (approach side).
• Red runway end lights at the true end of the pavement (departure side).
• White runway edge lights along both the displaced and landing portions.

Markings and lighting comply with ICAO Annex 14 and FAA AC 150/5340-1.

Lighting for Displaced Thresholds

• Threshold Lighting: Green lights at the displaced threshold for approach.
• Runway End Lighting: Red lights at the physical end, visible to departing or opposite-direction aircraft.
• Edge Lighting: White, possibly amber near runway end.
• Approach Lighting Systems: May extend beyond the displaced threshold for additional guidance.

Temporary displacements might use portable lights and extra signage.

Declared Distances and Calculation

Declared distances change as follows:

• LDA is shortened by the displacement; starts at the displaced threshold.
• TORA, TODA, ASDA usually remain the full runway length unless otherwise noted.

Pilots must use these published values for performance calculations and compliance.

Operational Implications and Practical Considerations

For Pilots

• Preflight Planning: Always check current airport diagrams, charts, and NOTAMs.
• Landing: Never land before the displaced threshold bar.
• Takeoff: Use the full paved runway unless local restrictions apply.
• Situational Awareness: Recognize arrow markings, threshold bars, and green lights.
• Performance Calculations: Use the correct LDA for safe stopping.

For Airport Operators

• Publish Accurate Data: LDA, TORA, TODA, and ASDA must be current and precise.
• Mark and Light Thresholds: Strictly maintain markings and lighting to standards.
• Communicate Changes: Issue NOTAMs and update charts for any threshold changes.

For Surveyors and Planners

• Measure Precisely: Ensure accurate depiction of physical ends and displaced thresholds in all documentation.
• Ongoing Assessment: Regularly evaluate the need for threshold displacement due to obstacles, safety area compliance, or environmental changes.

Examples and Use Cases

• Farnborough Airport (EGLF): Both runways have a 2,099 ft displaced threshold due to noise abatement and obstacle clearance, reducing LDA to 5,906 ft.
• San Jose International (KSJC), Runway 30L: 2,537 ft displaced threshold, LDA reduced to 7,614 ft out of 11,000 ft.

Summary Table: Displaced Threshold Area Usage

Related Terms

• Runway Threshold: The beginning of the portion of the runway available for landing.
• Runway Safety Area (RSA): Cleared area to reduce risk to aircraft from undershoots or overruns.
• Runway Stopway: Area beyond runway for aborted takeoff roll.

Further Reading

• ICAO Annex 14 — Aerodromes
• FAA AC 150/5300-13B — Airport Design
• FAA AC 150/5340-1 — Standards for Airport Markings

Displaced thresholds are critical for safe, efficient, and compliant airport operations. Understanding their markings, operational implications, and regulatory background is essential for pilots, airport operators, planners, and anyone involved in aviation safety.