Jazmin Li
A displaced threshold is a runway threshold located at a point other than the physical beginning or end of a runway. Displaced thresholds are used to give arriving aircraft clearance over an obstruction, while still allowing departing aircraft to use the maximum amount of runway available. They may also be introduced as a noise mitigation measure or if a beginning section of the runway is no longer able to sustain the continuous impact from landing aircraft. Displaced thresholds can be temporary, for example, due to construction works, or permanent.
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What You'll Learn
Obstacle avoidance
Displaced thresholds are often used to ensure that aircraft maintain clearance over obstacles on the approach slope. This could include permanent obstacles such as trees, buildings, power lines, or roads, as well as temporary obstacles like passing vehicles. By using a displaced threshold, aircraft are required to aim for a point further down the runway, which gives them more space to clear these obstacles. This is particularly important for landing aircraft, as they require a longer distance to safely touch down.
In some cases, displaced thresholds can also be used for noise abatement, ensuring that aircraft don't fly too low over residential areas, which can reduce noise pollution for nearby communities. Additionally, they can be used if a beginning section of the runway is no longer able to sustain the continuous impact of landing aircraft.
It's important to note that a displaced threshold is different from a stopway. A displaced threshold can bear the weight of an aircraft and is made from the same surface as the main runway. In contrast, a stopway is usually made of a different material or softer ground and is intended as a last resort to stop a runaway plane from veering off the end of the runway into obstacles such as trees or buildings.
To ensure safety, runway markings and lights are used to indicate a displaced threshold. For example, red lights may be used to indicate the displaced threshold, while the lights beyond it may be white or yellow. Additionally, chevron markings may be used to indicate that the area before the threshold is paved but not suitable for aircraft movement.
While displaced thresholds can enhance safety and noise abatement, they also introduce some risks. For example, if pilots are unaware of a displaced threshold or misjudge their landing, they may not have enough runway available, potentially resulting in a runway excursion or other incidents. Therefore, proper pilot awareness and adherence to procedures are crucial when operating on runways with displaced thresholds.
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Runway length optimisation
The portion of the runway behind a displaced threshold can be utilised for takeoffs in either direction and landings from the opposite direction. This flexibility ensures that departing aircraft can take advantage of the maximum available runway length, enhancing takeoff performance. Additionally, after landing at one end, aircraft can use the area behind the displaced threshold for rollout, further optimising the runway's efficiency.
Displaced thresholds are often implemented to provide arriving aircraft with clearance over obstacles, such as buildings, roads, or permanent structures. By displacing the threshold, aircraft can maintain a safe distance from these obstacles while still utilising the runway. This is particularly important for takeoff, as it typically requires more distance than landing.
In some cases, displaced thresholds are introduced as a noise mitigation measure. By displacing the threshold, aircraft on approach can maintain a higher altitude, reducing the impact of noise on nearby communities. This strategy not only improves the quality of life for residents but also addresses concerns related to low-flying aircraft over residential areas.
It is important to distinguish between a displaced threshold and a stopway. A displaced threshold is structurally capable of bearing the weight of an aircraft and is constructed from the same material as the main runway. In contrast, a stopway is typically made of a different material or softer ground and serves as an emergency measure to halt a runaway aircraft.
The optimisation of runway length through displaced thresholds offers several benefits, including enhanced safety, improved aircraft performance, and reduced noise pollution. However, it is crucial for pilots to be aware of these thresholds and adhere to proper landing and takeoff procedures to ensure successful operations. Markings and lighting systems are employed to clearly indicate displaced thresholds, aiding pilots in their decision-making processes.
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Noise mitigation
Displaced thresholds are used to increase the clearance between arriving aircraft and obstacles, such as buildings, trees, and houses. By using these thresholds, planes are higher up when they aim for landing, which gives them more space from obstacles. This also helps to keep the glide slope on target. Displaced thresholds are also cheaper to build and maintain than touchdown zones.
In addition, displaced thresholds can be used for "noise abatement" over residential areas. By increasing the height of the aircraft at a point of arrival, compared to landing on a threshold at the beginning of the runway, noise is reduced for those living nearby.
The Federal Aviation Administration (FAA) has established a Residential Noise Mitigation Program to address aircraft noise. This program is based on an Annual MSP Noise Contour Analysis and is more expansive than any other U.S. airport's sound mitigation program. The calculation for DNL considers the time of day an aircraft operates and applies a 10-decibel penalty for aircraft arriving or departing between 10 p.m. and 7 a.m. The current federally-established threshold of significance is 65 dB DNL.
The Metropolitan Airports Commission (MAC) also offers a residential sound mitigation program for qualifying homes located within aircraft noise exposure areas of 60 dB DNL associated with flight activity at Minneapolis-St. Paul International Airport (MSP). Eligibility for this program is based on an Annual MSP Noise Contour Analysis and is determined by an updated aircraft noise exposure contour area map associated with the airport.
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Runway maintenance
There are several reasons why airports use displaced thresholds, and one of the primary reasons is to ensure the safety of arriving aircraft by providing clearance over an obstruction. For example, at Runway 29L in Indira Gandhi International Airport, Delhi, India, there is a displaced threshold of 1,460 m (4,790 ft) to ensure aircraft can clear a Shiva statue located near the runway. Similarly, at John F. Kennedy International Airport, Runway 22R has a displaced threshold to mitigate noise for nearby communities.
Displaced thresholds can also be introduced if the beginning section of the runway can no longer withstand the continuous impact of landing aircraft. In such cases, the displaced threshold allows departing aircraft to still utilise the maximum amount of runway available for take-off, while landing aircraft are expected to land beyond the displaced threshold. This helps to reduce the impact force on the runway and prolong its lifespan.
The use of displaced thresholds can be either temporary or permanent. Temporary displacements may occur due to construction works or runway repairs, while permanent displacements may be necessary when a runway is shortened or when obstructions cannot be removed. In both cases, proper markings and lighting are crucial to indicate the displaced threshold to pilots. A displaced threshold is marked by a 10-foot-wide white threshold bar and white arrows along the centreline. The runway edge lights in the approach direction are red, while they remain white or yellow in the opposite direction.
It is important to note that displaced thresholds can pose challenges for pilots if they are not clearly marked or if pilots are unfamiliar with the runway layout. There have been several incidents where aircraft have commenced take-off from a displaced threshold without realising, which can lead to reduced take-off distance and potential safety hazards. Therefore, proper signage, lighting, and pilot awareness are critical aspects of runway maintenance when utilising displaced thresholds.
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Safety
Displaced thresholds are often used to give arriving aircraft clearance over an obstruction, while still allowing departing aircraft to use the maximum length of the runway. For example, at Runway 29L at Indira Gandhi International Airport in Delhi, India, there is a Shiva statue located near the runway. The displaced threshold ensures aircraft don't come too close to it. Similarly, at John F. Kennedy International Airport, Runway 22R has a displaced threshold to ensure aircraft don't fly too low over residential buildings during final approach. In some cases, the obstruction might be temporary, such as passing trucks or cars on a road near the runway.
Displaced thresholds can also be introduced as a noise mitigation measure, reducing the impact of aircraft noise on nearby communities. This was the case at Runway 22R at John F. Kennedy International Airport. Additionally, a displaced threshold may be necessary if the beginning section of a runway is no longer able to sustain the continuous impact of landing aircraft.
It is important to note that displaced thresholds are different from stopways. A displaced threshold can bear the weight of an aircraft and is made from the same surface as the main runway. It can be used for taxiing and commencing takeoff runs. However, pilots should not land on it. A stopway, on the other hand, is usually made of a different material or softer ground and is intended to stop a runaway plane as a last resort.
The use of displaced thresholds has been associated with some incidents and accidents. For example, in 2018, a Boeing 787-9 crew at Gatwick Airport inadvertently commenced takeoff from the displaced threshold of the departure runway instead of the full length required for the rated thrust used. Similar incidents have occurred at other airports, leading to risk reduction actions by airport operators. Proper markings, lighting, and pilot awareness are crucial to ensuring the safe use of displaced thresholds.
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Frequently asked questions
A displaced threshold is a runway threshold located at a point other than the physical beginning or end of the runway.
Airports use displaced thresholds for noise mitigation or to give arriving aircraft clearance over an obstruction. They are also used if a beginning section of the runway is no longer able to sustain the continuous impact from landing aircraft.
Displaced thresholds reduce noise by increasing the distance between aircraft on approach and the communities overflown.
Aircraft are expected to land beyond the displaced threshold. However, in some locations, such as Canada, landing on a displaced threshold may be permitted.
