Tilting at the Windmills

Wind turbines are one of the most popular renewable energy technologies. However when their intended location is close to an aerodrome, a conflict of interests may occur.

Such a situation arose when an energy projects developer (Evelop Ontwikkeling B.V.) in the Netherlands planned to erect 5 large wind turbines near the Ecofactorij industrial estate near Teuge airport (jurisdiction of the City of Apeldoorn).

A review of applicable regulations regarding obstacle limitations revealed that building wind turbines at this location is allowed. ICAO notes however, that when high obstacles are located outside the Obstacle Limitation Surfaces, they might cause significant operational restrictions.

In addition, due to the growing number of large wind turbines planned near airports, CAA-NL is considering the need for tighter national regulations regarding obstacle limitations. And last but not least, the management of Teuge airport, the Dutch branch of AOPA (Aircraft Owners & Pilots Association), and CAA-NL expressed their concerns regarding flight safety.

Considering these circumstances Evelop and local authorities asked NLR-ATSI to perform an analysis of safety risks that the wind turbines might cause to air traffic and to investigate whether or not these risks could be acceptable.

Teuge airport (EHTE) is uncontrolled (up to 1500 ft) and operates two runways; a grass runway (03/31; 700 m) and a concrete runway (09/27; 1199 m). Only VFR (Visual Flight Rules) traffic is permitted (aircraft up to a max. wingspan of 24 m). Average annual usage is approximately 65000 movements (2009).

The Ecofactorij wind turbine park is located approximately 4700 m South of the ARP (Aerodrome Reference Point) and about 1250 m West of reporting point SIERRA. The 3 MW wind turbines (Vestas V90) reach to about 500 ft AAL (Above Aerodrome Level).

For approach and landing on runway 09/27 mostly 'Circuit Area South' is in use. Traffic is then required to pass over compulsory reporting point SIERRA (700 ft) to subsequently fly North along a small canal to enter the Circuit Area South. Circuit Altitude is 700 ft AAL. Refer to the approach plate.

The risk analysis focused on 3 areas:

  • Which safety risks will wind turbines at this location introduce?
  • What is the magnitude of these risks, how are they substantiated and are they acceptable?
  • What measures are needed to reduce any potentially unacceptable risk to an acceptable level?

In a preceding assessment (2008) NLR-ATSI already concluded that potential wind disturbances (wake) caused by the planned Ecofactorij wind turbines are of such magnitude that they will not introduce a significant safety risk. Therefore, this risk element remained outside the scope of the new safety assessment.

All parties involved participated in identifying the risks. Experts of Teuge airport, nearby Lelystad airport and NLR-ATSI used a probability - severity matrix to classify all possible risks.. No unacceptable risks were identified; most risks were considered acceptable. For traffic approaching the compulsory reporting point SIERRA or traffic on track from SIERRA to the circuit, two risks were rated between acceptable and unacceptable:

Following the ALARP (As Low As Reasonably Practicable)2 principle, mitigating measures were identified that could help reduce the risks:

Using the northerly circuit
Relocation of reporting point SIERRA
Dedicating the area around the wind turbines as 'area to be avoided'
Visually marking the turbines
Also the installation of an innovative Obstacle Collision Avoidance System (OCAS) is considered, although this system is currently not yet certified in the Netherlands.

The overall conclusion of NLR-ATSI is that the placement of five wind turbines near Teuge airport is acceptably safe if reasonable mitigating measures are taken to minimize the risks identified.


Ultimately the local authorities in cooperation with the project developer decide which of the measures are reasonably practicable and if the remaining risks are acceptable. The NLR-ATSI safety assessment provides all information needed to make a well-founded decision.

  • The risk of collision with the wind turbines due to evading conflicting traffic
  • The risk of limited maneuvering space

Following the ALARP (As Low As Reasonably Practicable) [1] principle, mitigating measures were identified that could help reduce the risks:

  • Using the northerly circuit
  • Relocation of reporting point SIERRA
  • Dedicating the area around the wind turbines as 'area to be avoided'
  • Visually marking the turbines

Also the installation of an innovative Obstacle Collision Avoidance System (OCAS) is considered, although this system is currently not yet certified in the Netherlands.

The overall conclusion of NLR-ATSI is that the placement of five wind turbines near Teuge airport is acceptably safe if reasonable mitigating measures are taken to minimize the risks identified.

Ultimately the local authorities in cooperation with the project developer decide which of the measures are reasonably practicable and if the remaining risks are acceptable. The NLR-ATSI safety assessment provides all information needed to make a well-founded decision.



[1] The idea of ALARP is to strive to risk reduction to a level that is as low as possible without requiring excessive investments, i.e., as long as the investments made are not grossly disproportionate to the benefit gained. If it is demonstrated that all measures that are ‘reasonably practicable’ have been taken, the remaining risk can be accepted.



Touch&Go

From 1998 until 2008, the amount of electricity generating wind turbines in the Netherlands grew by 750 to 1941 wind turbines in total. Of these 1941 wind turbines in 2008, 310 were higher than 71 meters. In 1998 all wind turbines were lower than 71 meters. (Source: Statistics Netherlands).