What is OIS in Aviation? (Obstacle Identification Surface)

Understanding Obstacle Identification Surface (OIS) in Aviation

When it comes to aviation safety, identifying and avoiding obstacles is of paramount importance. One of the crucial elements in this process is the Obstacle Identification Surface (OIS). The OIS is an imaginary surface established to ensure the safety of aircraft during various phases of flight, including takeoff, approach, and landing.

The OIS is defined by the Federal Aviation Administration (FAA) as the “volume of airspace in the vicinity of an airport or heliport that is established to define where obstructions may penetrate the surface and to provide obstruction-free airspace for aircraft operations.” In simpler terms, the OIS sets the boundaries within which structures, such as buildings, antennas, or trees, should not exceed in order to maintain the safety of aircraft operations.

The Importance of Obstacle Identification Surface

The establishment of the OIS is crucial in ensuring proper clearance and safety for aircraft during critical flight operations. Here are a few key reasons why the OIS is essential in aviation:

1. Obstacle Clearance: The primary purpose of the OIS is to provide sufficient clearance between aircraft and surrounding obstacles. By defining the airspace that is free from obstructions, the OIS enables pilots to have a clear and safe flight path during takeoff, approach, and landing. These clearances help minimize the risk of collisions and ensure the safety of passengers and crew.

2. Visual Guidance: The OIS also serves as a visual reference for pilots, aiding in their situational awareness. By delineating the boundaries within which structures should not exceed, the OIS allows pilots to easily identify potential obstructions and make timely adjustments to their flight path. This visual guidance is especially crucial during low visibility conditions or when operating in unfamiliar airports.

3. Regulatory Compliance: The OIS is established based on national and international regulations governing aviation safety. Compliance with these regulations is mandatory for airport operators and developers. By adhering to the guidelines set by the OIS, airport authorities ensure that new constructions or modifications near airports do not compromise the safety of aircraft operations.

4. Preventing Accidents: Accurate identification and assessment of potential obstacles near airports are crucial in preventing accidents. The OIS helps in identifying structures that may pose a risk to aircraft operations, such as tall buildings, power lines, or communication towers. By establishing clear boundaries for these structures, the OIS enables authorities to take appropriate measures to mitigate the risks and prevent accidents.

Establishing the Obstacle Identification Surface

Now that we understand the importance of the Obstacle Identification Surface, let’s delve into how it is established.

1. Site Survey: Before the construction or modification of an airport or heliport, a detailed site survey is conducted to identify potential obstructions in the vicinity. This survey takes into account existing structures, terrain features, and natural obstacles that may affect aircraft operations.

2. Airport Master Plan: The information gathered from the site survey, along with other relevant factors, is incorporated into the Airport Master Plan. This comprehensive document outlines the long-term development objectives for the airport, including the establishment of the OIS.

3. Design Criteria: Based on the regulations and guidelines provided by aviation authorities, specific design criteria are established for the OIS. These criteria include the maximum height limits for structures within the OIS, the slopes of approach and departure surfaces, and the dimensions of the OIS volumes.

4. Obstacle Limitation Surfaces: The OIS is further divided into three specific surfaces, known as the approach surface, departure surface, and transitional surface. Each surface has its own set of criteria and dimensions, ensuring adequate obstacle clearance for aircraft during various flight phases.

5. Monitoring and Compliance: Once the OIS has been established, regular monitoring and compliance checks are conducted by airport authorities. Any new constructions or modifications that may affect the OIS are thoroughly assessed to ensure compliance with the established criteria.


The Obstacle Identification Surface (OIS) plays a crucial role in aviation safety by defining the boundaries within which structures should not exceed to maintain the safety of aircraft operations. It provides obstacle clearance, visual guidance, regulatory compliance, and helps prevent accidents. The establishment of the OIS involves site surveys, the incorporation of information into the Airport Master Plan, the establishment of design criteria, and the division of the OIS into specific surfaces. Regular monitoring and compliance checks are essential to ensure ongoing safety. By adhering to the guidelines set by the OIS, airports can promote safe aircraft operations and minimize the risk of accidents caused by obstacles.

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