What is THSHD on Boeing 737? (Threshold)

The threshold, also known as the THSHD, is a crucial component of the Boeing 737 aircraft. It plays a vital role in aircraft operations, particularly during takeoff and landing. In this article, we will dive deeper into what the threshold is on the Boeing 737, its significance, and how it affects the aircraft’s performance.

Understanding the Threshold

The threshold on the Boeing 737 refers to the beginning of the runway where the aircraft initiates its takeoff or landing roll. It is marked by the Threshold Markings, which typically consist of white stripes that extend across the width of the runway. These markings help pilots identify the exact position where the aircraft is expected to touchdown during landing. The threshold also marks the starting point for measuring the runway length for takeoff performance calculations.

During landing, the goal is to land the aircraft as close to the threshold as possible without touching down before it. The threshold provides the necessary reference point to help pilots accurately judge their landing flare and touchdown point. Landing before the threshold may compromise safety, as there might not be sufficient runway remaining for the aircraft to come to a complete stop.

On the other hand, during takeoff, the threshold serves as the starting point for determining the required takeoff distance. Takeoff performance calculations take into account various factors such as the aircraft’s weight, temperature, wind speed, and runway conditions. These calculations enable pilots to determine the minimum runway length required to safely take off, including any factor for obstacle clearance.

The threshold also plays a role in the calculation of the aircraft’s V1, VR, and V2 speeds. V1 represents the maximum speed at which the pilot can abort the takeoff and safely stop the aircraft using the remaining runway length. VR is the speed at which the pilot initiates rotation, causing the aircraft’s nose to pitch up for liftoff. Finally, V2 is the takeoff safety speed, which is the minimum speed required for the aircraft to safely climb and clear any potential obstacles.

It’s important to note that the threshold markings may vary based on the type of runway and airport. For example, some runways may have precision approach lighting systems or other visual aids to assist pilots in their landing approach. These additional features enhance safety and provide pilots with important visual cues as they approach the threshold.

Threshold Considerations

Several factors come into play when considering the threshold on the Boeing 737. Let’s take a closer look at them:

Runway Length

The runway length is a crucial consideration for the threshold. A longer runway allows the pilot more room to safely land or take off and provides additional margin for error. The Boeing 737 has different threshold requirements depending on the specific variant, such as the 737-700, 737-800, or 737 MAX. Each variant has its own takeoff and landing performance characteristic, including specific threshold speeds and distances.

For example, the Boeing 737-800 typically requires a runway length of around 6,660 feet for takeoff at its maximum takeoff weight. This includes the distance needed for the aircraft to accelerate to V1, rotate at VR, and climb safely at V2. Similarly, during landing, the threshold plays a crucial role in calculating the landing distance required for the aircraft to come to a complete stop.

Pilots must adhere to the prescribed threshold speeds and distances to ensure the aircraft’s safe performance during takeoff and landing. Any deviation from these guidelines can have serious consequences for the aircraft’s ability to remain within the designated runway length.

Runway Conditions and Wind Effects

The condition of the runway and the effects of wind also factor into threshold considerations. Wet or contaminated runways, for example, can significantly affect the aircraft’s braking performance and tire traction, potentially requiring a longer landing distance. Similarly, crosswinds can influence the aircraft’s ability to maintain runway alignment during takeoff and landing, impacting the threshold’s importance.

Wind can affect the aircraft’s airspeed and groundspeed, potentially altering the threshold speeds and distances required for takeoff and landing. Tailwinds increase groundspeed and reduce the aircraft’s required runway length, while headwinds have the opposite effect. Pilots must consider these factors and adjust their calculations accordingly to ensure safe operations.

Obstacle Clearance

Obstacle clearance is another critical consideration for the threshold. An obstacle, such as a building, mountain, or tower, near the end of the runway can impact the aircraft’s ability to safely take off or land. Pilots must factor in these obstacles when calculating the required takeoff and landing distances.

Additional margin for obstacle clearance may be necessary, especially in airports with challenging terrain or significant obstructions in close proximity to the runway. The threshold, in conjunction with the aircraft’s performance characteristics, helps pilots determine whether the required obstacle clearance can be achieved within the available runway length.

It is worth noting that each airport has its own specific requirements regarding obstacle clearance and threshold operations. These requirements are established to ensure safe aircraft operations and prevent potential collisions or accidents while taking off or landing.

The Importance of Accurate Threshold Calculations

Accurate threshold calculations are crucial for the safe operation of the Boeing 737 aircraft. The specific threshold speeds and distances are determined based on extensive testing, simulations, and analysis conducted by Boeing’s engineering teams. Any variation or deviation from these calculations can have serious consequences, jeopardizing the aircraft’s ability to safely operate within the designated runway length.

Pilots rely on these calculations to ensure the safe takeoff and landing of the aircraft, considering factors such as weight, temperature, wind speed, runway conditions, and obstacle clearance. Adhering to the prescribed threshold speeds and distances helps pilots maintain control over the aircraft and reduces the risk of accidents or incidents associated with inadequate performance margins.

It is worth mentioning that modern aircraft systems, such as the Boeing 737’s Flight Management System (FMS), provide pilots with accurate real-time data to aid in threshold calculations. These systems consider a wide range of variables and provide instant feedback to help pilots make informed decisions during takeoff and landing.

To ensure the highest level of safety, pilots undergo extensive training on threshold operations and performance calculations. This training enables them to accurately determine the required threshold speeds and distances based on the specific operational conditions and aircraft characteristics.

To further enhance safety, airports and regulatory bodies continuously review and update threshold operations guidelines and regulations. These measures aim to improve standardization and promote uniform practices across the industry, ensuring consistent and accurate threshold calculations for Boeing 737 aircraft.


The threshold on the Boeing 737 is a critical component that affects aircraft performance during takeoff and landing. It provides pilots with important reference points for touch down, takeoff distance calculations, and speed determination. Threshold considerations encompass various factors such as runway length, runway conditions, wind effects, and obstacle clearance.

Accurate threshold calculations are essential for ensuring the safe operation of the Boeing 737. Pilots rely on these calculations to maintain control over the aircraft and adhere to performance margins that are vital for safety. The combination of well-defined threshold guidelines, advanced aircraft systems, and pilot training ultimately contribute to the optimal operation of the Boeing 737 and the overall safety of air travel.

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