Displaced Thresholds: Purpose, Markings, and Aviation Safety
In the complex environment of airport operations, runway markings provide critical instructions to pilots to ensure safe takeoffs and landings. One such specialized marking is the displaced threshold (DTHR), a runway threshold located at a point other than the physical beginning or end of the runway pavement.
While a runway may physically start at one point, the displaced threshold indicates that the landing area begins further down the strip. This creates a distinct section of pavement that serves different purposes depending on whether an aircraft is arriving, departing, or taxiing.
Key Facts
- A displaced threshold moves the landing point further down the runway.
- The area behind the threshold can be used for takeoffs in either direction.
- Landing aircraft may use the displaced area for rollout after landing from the opposite direction.
- Common reasons for displacement include obstacle clearance, noise mitigation, and pavement wear.
- Landing aircraft are strictly required to touch down beyond the displaced threshold bar.
Why are Displaced Thresholds Used?
Airports implement displaced thresholds for several operational and safety reasons. The primary goal is often to ensure that arriving aircraft have sufficient clearance over obstructions—physical objects located near the runway approach path—without sacrificing the total runway length available for departing aircraft.
Beyond physical obstacles, displaced thresholds are used for:
- Noise Mitigation: By shifting the landing point, airports can reduce the noise impact on surrounding communities during the final approach.
- Pavement Preservation: If the beginning section of a runway can no longer sustain the high-impact force of landing aircraft, a displaced threshold is introduced to protect the pavement.
Because departing aircraft and those rolling out from the opposite direction do not impact the pavement with the same force as a landing aircraft, they are permitted to use the section behind the displaced threshold, even if the displacement was caused by lowered pavement resistance.
Identifying a Displaced Threshold
Pilots identify the displaced section of a runway through three specific visual markings:
- Center Line Arrows: White arrows located along the center line of the runway.
- Threshold Arrowheads: White arrowheads spanning the width of the runway just before the threshold bar.
- The Threshold Bar: A white bar, 3.0 meters (10 feet) wide, that stretches across the width of the runway to mark the official start of the landing area.
Real-World Examples
The impact of a displaced threshold on available runway length can be significant, as seen in these global examples:
Indira Gandhi International Airport (Delhi, India)
On Runway 29L, a displaced threshold is positioned 1,460 meters (4,790 feet) from the physical start of the pavement. This is necessary to ensure aircraft avoid the Shiva statue located near the runway. Consequently, 1,500 meters (4,900 feet) of the runway cannot be used for landing in this direction, reducing the available landing length to 2,970 meters (9,740 feet).
John F. Kennedy International Airport (New York, USA)
Runway 22R utilizes a displaced threshold specifically for noise abatement. This operational decision shortens the landing distance available to 2,376 meters (7,795 feet), despite the physical runway length being 3,682 meters (12,079 feet).
| Airport | Runway | Primary Reason | Landing Distance Available |
|---|---|---|---|
| Indira Gandhi Intl | 29L | Obstacle Clearance (Shiva Statue) | 2,970 m (9,740 ft) |
| John F. Kennedy Intl | 22R | Noise Abatement | 2,376 m (7,795 ft) |
Frequently Asked Questions
Can a plane take off from the displaced section of a runway?
Yes. The portion of the runway behind a displaced threshold may be used for takeoff in either direction.
Can a plane land on the displaced section?
No. Aircraft are expected to land beyond the displaced threshold bar.
What happens if a plane lands from the opposite direction?
Aircraft landing from the opposite direction may use the area behind the displaced threshold for their landing rollout.
How wide is the white threshold bar?
The white threshold bar is 3.0 meters (10 feet) wide.
Why would pavement resistance cause a displaced threshold?
If the beginning of the runway can no longer support the continuous, heavy impact of landing aircraft, the threshold is displaced to move the impact point to a stronger section of pavement.