Obstacle Assessment

A high-rise building, a series of wind turbines or electricity poles may affect the safety of aircraft operations. International regulations from ICAO as described in Annex 14 specify an airspace around aerodromes to be maintained free from obstacles. This airspace is defined by surfaces that depend on the airport and its runways. Some of these surfaces should not be penetrated by any objects, while others may be penetrated under certain conditions. E.g., penetration may be allowed when the visibility of the object is increased by navigation lights or painting.

According to ICAO regulations such protrusions are only allowed when it is shown by means of an aeronautical study that the safety or regularity of flight operations are not significantly affected. In such cases a flight technical assessment is carried out and very often objective facts can be given in support of the building plans.

In general four main issues may play a role:
Collision risk

This issue is mainly dealt with by International Civil Aviation Organisation Annex 14, Aerodromes, and sometimes some local regulations. The safety issue is in general that the obstacle protrudes one of the defined Obstacle Limitation Surfaces (OLS). The OLS define the maximum height that an obstacle near an airport is allowed to have. Usually the so-called Inner Horizontal Surface (IHS) or Conical Surface are of most interest. The IHS is a circular surface a height of 45m with a radius ranging from 2 to 4 km, depending on the runway reference length. The Conical Surface slopes outward from the IHS with a slope of 5%, up to a height of 80 to 145m, also depending on the runway reference length. In case one of these surfaces is protruded an aeronautical study is required to demonstrate that the obstacle does not affect the safety and regularity of the operations to/from the airport.

Aircraft operational impact

Due to the erection of obstacles flight procedures may need to be changed, or aircraft performance requirements may be affected (for instance one-engine out performance). The impact of obstacles on procedure design is mainly covered by ICAO Doc 8168 (the so-called PANS-OPS).

Disturbances of radar, navigation aids and communication aids

Objects may also pose a danger to air traffic because of electromagnetic interference: disturbing communication, navigation or radar surveillance systems. All these services are safety-critical and should not be disturbed too much by obstacles.

The disturbance of radar, navigation aids and communications systems are mainly addressed by ICAO Annex 10 (Volumes 1-5). Within Europe further guidance material is provided by ICAO EUR Doc 15 on managing building restricted areas. Furthermore in the Netherlands a requirement exists that any wind turbine within 15 km from the instrumented range of a primary radar needs to be assessed for potential disturbances.

NLR also has the expertise to assess the potentially disturbing effects of an object.

Wind hindrance

Due to the large dimension of today's wind turbines the wake and turbulence may become hazardous for aircraft passing downwind behind the wind turbine. However, no specific international regulations do exist that protect aircraft from entering the potential hazardous wake or induced turbulence behind a large wind turbine.

Problems and solutions for wind turbine siting in the vicinity of airports, by Peter J. van der Geest

Read the paper: Obstacle Lighting of Onshore Wind Turbines
Balancing aviation safety and environmental impact by H.T.H. (Jeroen) van der Zee

If you have concerns about an existing or planned object in the vicinity of an airport, we can help you to:

  • Indicate relevant obstacle limitation surfaces
  • Execute an obstacle assessment
  • Provide safety arguments when applicable
  • Provide recommendations for visual marking
  • Report the results to appropriate authorities

When deemed necessary, we also advise on procedure design because of needed changes to approach or departure procedures in order to create the required obstacle clearance.

Our experience includes:
Aeronautical study for planned high-rises, 2010

The Fonds d’Urbanisation et d’Amenagement du Plateau de Kirchberg is planning to develop a number of high-rises at the Kirchberg Plateau, near Luxembourg airport. The location and height of two of these high-rises are such that they protrude through the Obstacle Limitation Surfaces that are intended to safeguard a defined volume of airspace around the airport. According to ICAO regulations such protrusions are only allowed when it is shown by means of an aeronautical study that the safety or regularity of operations of aircraft are not significantly affected. NLR-ATSI has conducted the required aeronautical study.

Safe positioning of power pylons, 2010

The Dutch power company TenneT plans to build a new 380 kV high-voltage transmission line. A section of the envisaged line of pylons runs alongside Schiphol’s Polder runway, following the route of an existing 150kV transmission line. NLR-ATSI analysed the potential risks to flight safety. read more...

Obstacle limitations in the defined zones at Schiphol terrain
Obstacle assessment of wind turbines Teuge, 2009

Obstacle assessment of planned wind turbines near Teuge airport and identification of risk mitigating measures. read more...

Obstacle assessment of wind turbines curacao, 2009

Obstacle assessment of wind turbine park near Hato airport. Both obstacle clearance issues were investigated as well as interference of the wind turbines with the surveillance equipment on the airport. read more...

Obstacle analysis high-rise zone Novartis North near Euro-airport Basel-Mulhouse
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