Flight Redemptions

What is OAS in Aviation? (Obstacle Assessment Surface)

Updated: March 06, 2024

Understanding Obstacle Assessment Surface (OAS) in Aviation

When it comes to aviation, safety is of utmost importance. Pilots and aviation authorities take several measures to ensure the safety of flights and minimize the risk of accidents. One such measure is the establishment of Obstacle Assessment Surfaces (OAS). In this article, we will delve into the concept of OAS, its significance in aviation, and how it contributes to flight safety.

The Importance of Obstacle Assessment Surface

Obstacle Assessment Surfaces (OAS) play a crucial role in the planning and construction of airports and heliports. These surfaces are designed to create a clear and safe airspace around the airport, minimizing the risk of collisions with obstacles such as buildings, trees, or terrain. By defining specific criteria for the height and location of obstacles, OAS ensure that pilots have a clear and unobstructed flight path.

One of the key objectives of OAS is to establish safe approach and departure paths for aircraft. These surfaces are typically defined in relation to the runways at an airport, taking into account factors such as the aircraft's performance capabilities, approach angles, and runway lengths. By adhering to the defined OAS, pilots can safely navigate the airspace and avoid any potential obstacles that may pose a threat to the aircraft or its passengers.

It is important to note that OAS are not limited to just the immediate vicinity of the airport. They extend beyond the runway ends, ensuring that aircraft have a safe climb gradient and obstacle clearance during takeoff. Similarly, OAS also consider the approach path and provide adequate obstacle clearance during landing.

The Three Components of Obstacle Assessment Surface

Obstacle Assessment Surfaces consist of three main components, each serving a specific purpose in ensuring flight safety:

1. Approach Surface

The Approach Surface is designed to provide a safe and obstacle-free path for aircraft during the landing phase. It extends beyond the runway threshold and is inclined upwards to provide adequate obstacle clearance. The height and angle of the approach surface vary depending on the type of aircraft and the runway length. It is crucial for pilots to adhere to the defined approach surface to ensure a safe landing and prevent any potential collisions with obstacles.

For example, in the case of a Category I Instrument Landing System (ILS) approach, the approach surface is typically defined by a glide slope angle of 3 degrees. This means that for every 300 feet of horizontal distance, the aircraft will descend by 100 feet vertically. By following this glide slope and staying within the defined approach surface, pilots can ensure a smooth and obstacle-free landing.

2. Transitional Surface

The Transitional Surface connects the Approach Surface and the Takeoff Surface, providing a smooth transition for aircraft during the initial climb phase. It ensures that there are no sudden changes in obstacle clearance and helps maintain a safe flight path. The Transitional Surface is particularly important for airports located in hilly or mountainous areas, where terrain elevation changes rapidly.

By adhering to the defined Transitional Surface, pilots can safely navigate the airspace during the critical phase of climb after takeoff. It provides a gradual increase in obstacle clearance, allowing the aircraft to gain altitude without any risk of collision with nearby obstacles.

3. Takeoff Surface

The Takeoff Surface is designed to ensure a safe climb gradient for aircraft during takeoff. It provides a clear and obstacle-free path for the aircraft to gain altitude after leaving the runway. The height and angle of the Takeoff Surface are determined based on the aircraft's performance capabilities and takeoff speeds.

Adhering to the defined Takeoff Surface is crucial for pilots to ensure a safe climb and avoid any potential obstacles that may pose a risk during the initial phase of flight. It allows the aircraft to gain the necessary altitude and establish a safe flight path without any hindrances.

The Role of Obstacle Assessment Surface in Aviation Planning

Obstacle Assessment Surfaces play a vital role in the planning and design of airports and heliports. Prior to the construction of an aviation facility, a comprehensive assessment of the surrounding area is conducted to identify any potential obstacles that may pose a risk to aircraft operations.

This assessment takes into account factors such as the height and location of existing buildings, trees, or other structures that could obstruct the flight path. By defining the OAS, aviation authorities can ensure that the proposed airport or heliport meets the necessary safety standards and provides pilots with adequate obstacle clearance.

Furthermore, OAS also play a crucial role in determining the suitability of a site for aviation operations. If an area cannot meet the required OAS criteria, it may not be suitable for the establishment of an airport or heliport. This ensures that aviation facilities are only constructed in areas where flight safety can be guaranteed.

Overall, Obstacle Assessment Surfaces are an essential component of aviation planning and design. They contribute significantly to flight safety by providing clear and obstacle-free flight paths for aircraft during takeoff, landing, and climb phases. By adhering to the defined OAS, pilots can minimize the risk of collisions with obstacles and ensure the safety of their passengers and crew.

For more information on Obstacle Assessment Surfaces and their significance in aviation, you can refer to the FAA Advisory Circular 150/5300-13C.

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