What is HAT in Aviation? (Height Above Touchdown)

In aviation, Height Above Touchdown (HAT) refers to the vertical distance between the runway and a given point during the approach and landing phase of an aircraft. It is an essential term used by pilots and air traffic controllers to determine the aircraft’s position and altitude in relation to the runway surface.

Understanding Height Above Touchdown

The concept of Height Above Touchdown is crucial for pilots as they execute a safe landing. When an aircraft approaches the runway, the pilot needs to be aware of their position and altitude to ensure a smooth touchdown. HAT provides this information by measuring the vertical distance from a specified point on the runway to the aircraft, typically the landing gear.

HAT is measured in feet and is often used in conjunction with the Decision Altitude (DA) or Decision Height (DH) to establish a stabilized descent path during the landing phase. The Decision Altitude (DA) or Decision Height (DH) is the designated altitude at which the pilot must make a decision to continue the approach or initiate a missed approach procedure.

Let’s consider an example to understand the concept of Height Above Touchdown better. Suppose an aircraft is on an instrument approach to a runway with a Height Above Threshold (HAT) of 50 feet. This means that the aircraft must be at least 50 feet above the runway threshold when it crosses over the specified point, usually defined by a specific navigation aid or instrument approach procedure.

The Significance of Height Above Touchdown

Height Above Touchdown plays a vital role in maintaining the safety and precision of landings in aviation. By knowing the HAT, pilots can appropriately adjust their descent rate and flight path, ensuring a smooth touchdown and avoiding any potential obstacles or obstacles on the runway.

Precision Approach Path Indicators (PAPIs) or Visual Approach Slope Indicators (VASIs) are often installed alongside runways to assist pilots in maintaining the correct approach slope and HAT. These visual aids use a series of lights to indicate the aircraft’s vertical position in relation to the desired approach path.

Furthermore, the HAT value is used in calculations for landing performance, obstacle clearance, and approach lighting systems. It helps determine the required descent angles, touchdown zones, and runway lighting requirements, ensuring optimal conditions for safe landings.

Factors Affecting Height Above Touchdown

Several factors can influence the Height Above Touchdown during an aircraft’s approach and landing. These factors must be considered to ensure a safe and accurate landing:

1. Aircraft Performance: The performance characteristics of the aircraft, including its approach speed, descent rate, and glide slope intercept altitude, play a significant role in determining the HAT. Different aircraft types may have different recommended HAT values for safe landings.

2. Atmospheric Conditions: Weather conditions, such as wind speed and direction, can affect the aircraft’s descent rate and groundspeed. These factors can have an impact on the HAT and require pilots to make adjustments to maintain the desired approach path.

3. Runway Elevation: The elevation of the runway above sea level can affect the HAT value. Higher elevation runways may require a greater HAT to ensure obstacle clearance and a safe touchdown.

4. Obstacle Clearance: The presence of obstacles, such as buildings, trees, or terrain near the runway, can impact the HAT. Adequate clearance must be maintained to ensure the aircraft’s safe descent and landing.

By considering these factors, pilots can make informed decisions and maintain the appropriate Height Above Touchdown for a safe landing.


Height Above Touchdown (HAT) is a critical concept in aviation, providing pilots with essential information during the approach and landing phase. By understanding the HAT value, pilots can ensure a safe and accurate descent, maintaining the appropriate altitude above the runway surface. Factors such as aircraft performance, atmospheric conditions, runway elevation, and obstacle clearance must be considered to achieve a smooth and secure landing. Height Above Touchdown contributes to the precision and safety of aviation operations, ultimately ensuring the well-being of passengers and crew.

For more information on aviation terminology and procedures, visit FAA.

For More: What is BVR in Aviation? (Beyond Visual Range)