What is TCH in Aviation? (Threshold Crossing Height)

In aviation, the threshold crossing height (TCH) refers to the vertical distance between the aircraft’s landing gear and the runway threshold at the point where the aircraft passes over it during landing. It is an important concept that pilots use to ensure a safe and smooth touchdown. The TCH is usually measured in feet and is a crucial parameter for pilots to consider during their approach and landing phases.

Understanding Threshold Crossing Height

During the landing phase, the aircraft descends gradually to the runway. The TCH determines the height at which the aircraft’s landing gear passes over the runway threshold. It is measured from the highest part of the aircraft, typically the main landing gear or the fuselage, to the runway surface at the threshold. The TCH is one of the many factors that pilots must account for to ensure a safe landing.

The TCH is influenced by several factors, including the aircraft’s approach speed, configuration, and aircraft type. Each aircraft has its own specific threshold crossing height, which can vary depending on its design characteristics. For example, larger aircraft with lower-wing-mounted engines might have a higher TCH compared to smaller aircraft with higher-wing-mounted engines.

Additionally, the length of the aircraft’s landing gear, the touchdown point on the runway, and the slope of the runway also affect the threshold crossing height. It is important for pilots to consider these factors when planning their approach and landing to ensure a smooth touchdown and prevent any potential contact between the aircraft and the runway surface.

The TCH is typically provided in the aircraft’s performance charts or manuals, which pilots consult to determine the appropriate approach and landing parameters for their specific aircraft. By knowing the TCH, pilots can adjust their approach angle and touchdown point accordingly to meet the safety requirements and achieve a smooth touchdown.

The Importance of Threshold Crossing Height

The threshold crossing height plays a critical role in ensuring the safety of an aircraft’s landing. By properly calculating and maintaining the correct TCH, pilots can avoid the risk of dragging or striking the landing gear or any other part of the aircraft on the runway surface during touchdown. This is particularly crucial for larger aircraft that have a higher TCH and are more susceptible to runway contact.

Maintaining the appropriate TCH also helps to minimize the level of impact forces experienced by the landing gear during touchdown. When the aircraft’s landing gear reaches the TCH, the vertical speed of descent should ideally be minimized to minimize the stress on the landing gear components. This helps to prolong the lifespan of the landing gear and reduce maintenance costs.

Moreover, the TCH affects the overall landing performance of the aircraft. Pilots aim to achieve a smooth touchdown, allowing the aircraft to absorb the impact forces gradually and smoothly. By maintaining the correct TCH, pilots can ensure that the aircraft lands at the desired touchdown point without any hard landings or bouncing, enhancing passenger comfort and safety.

Calculating and Adjusting the Threshold Crossing Height

To calculate the threshold crossing height, pilots take into account the aircraft’s approach speed, glide path angle, and the visual perspective from the cockpit during approach. These factors help the pilots estimate the height at which the aircraft will cross the runway threshold.

During the approach phase, pilots typically establish a desired glide path angle to guide the aircraft towards the runway threshold. The glide path angle is usually defined by Instrument Landing System (ILS), which helps pilots maintain the correct descent profile. By adjusting the aircraft’s approach speed and descent rate, pilots can control the TCH to meet the specific requirements for their approach and landing.

Additionally, pilots can also make adjustments to the TCH based on external factors such as runway conditions, wind, and obstacles in the approach path. If the runway is wet or contaminated, pilots may choose to increase the TCH to account for the potential reduced braking performance during landing. Similarly, if there are obstacles near the runway threshold, pilots may increase the TCH to provide additional clearance.

It is important for pilots to consistently monitor and adjust the TCH during the landing phase. As the aircraft approaches the runway threshold, pilots can visually confirm the TCH by referencing the outside visual cues, such as the runway markings and lights. Any adjustments to the aircraft’s approach speed, descent rate, or glide path angle can be made to ensure that the aircraft crosses the threshold at the desired height and position.

Overall, the threshold crossing height is a crucial element of aviation that pilots must understand and consider during landing. By controlling the TCH, pilots can achieve safe and smooth landings, minimizing the risk of runway contact, reducing landing gear stress, and enhancing passenger comfort. It is a key parameter that contributes to the overall safety and efficiency of aviation operations.

For More: What is GPP in Aviation? (General Practices And Procedures)