The Winds of Change
Wind Turbines and Their Effects on the National Airspace System
By Michael Rauchle, FAA Flight Procedures and Airspace Group
You do not have to look very far to realize that there is a global “push” for greener energy. The concept of using wind power to bring well water to the surface, operate sawmills, and generate electrical power has been utilized since at least the mid-1800s. Currently, there are more than 72,000 wind turbines throughout the U.S., with the majority concentrated in the nation’s midsection.
Wind turbine technology has experienced significant advances including composite base fixture construction and increased efficiency of the rotating airfoils to generate more power. While wind energy innovation improves our ability to harness renewable energy, it also introduces additional challenges for the aviation industry. This article educates pilots about wind turbines and wind turbine farms and how the FAA is ensuring the safety of the National Airspace System (NAS). We’ll discuss how wind turbines and wind turbine farms are marked, lighted, and charted, including how NAS users can play an active role in the evaluation process, as well as provide some recommendations to help pilots safely avoid encounters with wind turbines.
A Closer Look at Wind Turbine Data
The FAA maintains a Daily Digital Obstacle File (DDOF) of all known obstructions in the NAS. The FAA’s Flight Technologies and Procedures Division, with assistance from the Safety Analysis and Promotion Division, recently completed a data review and analysis of the DDOF regarding wind turbines.
The data review and analysis revealed some interesting trends. Over the past twenty years, both the population and height of wind turbines have grown significantly. Older wind turbines have an average height of 344 feet above ground level (AGL) or less. More recently, wind turbines are being built around the country with heights exceeding 499 feet AGL, with the tallest turbines exceeding 800 feet AGL. When first introduced, wind turbines affected only uncontrolled (Class G) airspace. However, recent trends indicate that new and re-powered wind turbines are encroaching on controlled (Class E) airspace. Increasing heights of the wind turbines directly correlate to the potential for impacting more aircraft flying in the NAS. You might now be wondering how the FAA evaluates existing and proposed wind turbines for their potential NAS impact. Let’s take a look.
How Wind Turbines are Reviewed and Evaluated
In accordance with Title 49 of the United States Code, section 44718, and 14 CFR part 77, Safe, Efficient Use and Preservation of the Navigable Airspace, the FAA is responsible for conducting aeronautical studies to ensure the safety of air navigation. To fulfill this obligation, the FAA’s Flight Standards Service, along with many other offices within the FAA, review and comment on aeronautical studies such as wind turbine farms and other types of potential obstructions that may affect navigable airspace. FAA Joint Order (JO) 7400.2P, Procedures for Handling Airspace Matters, prescribes the policy and general process for FAA evaluations of proposed wind turbine farms.
Wind turbines are typically lit with medium intensity, flashing red lights, placed as high as possible on the turbine nacelle.
Flight Standards is responsible for evaluating proposed wind turbines and wind turbine farms for potential “VFR Effect.” As part of that process, Flight Standards identifies potential VFR routes, flyways, and other concentrations of VFR traffic within two statute miles of the proposed location(s). The Obstruction Evaluation Group within the FAA’s Air Traffic Organization determines the actual traffic volume using traffic data sources. When the aeronautical study is completed, the FAA issues a determination letter to the person or entity submitting the construction proposal. The letter states whether the proposed structure constitutes a hazard to air navigation (or not) and includes a list of provisions such as how the structure(s) should be marked and lighted. Submitters file requests for aeronautical studies through the Obstruction Evaluation/Airports Airspace Analysis (OE/AAA) public portal at bit.ly/OEAAA.
The FAA circulates some aeronautical studies to give the public an opportunity to review and comment on such proposals. The OE/AAA public portal allows a user to register, subscribe to notifications, view, search, and comment on aeronautical studies that are available. The FAA encourages anyone with an interest in participating in this process to register for an account with the OE/AAA public portal and configure your email preferences to be notified of circularized cases (account required). Here’s a link to view circularized cases bit.ly/OEAAAcases.
Wind Turbine Marking and Lighting
The FAA publishes recommendations for marking and lighting of obstructions to improve daytime and nighttime visual conspicuity. Wind turbines are normally painted white or light gray to improve daytime visibility. The color palette was determined by an internal FAA study that strikes a balance between aviation safety and being “community friendly.” In addition, snow-prone areas are evaluated for additional markings to improve the visual conspicuity of the wind turbines against a snow-covered landscape.
Wind turbines are typically lit with medium intensity, flashing red lights, placed as high as possible on the turbine nacelle. The lights should be synchronized to flash together; however, not all wind turbine units within a wind turbine farm (a group of three or more wind turbines in the same location) need to be lighted, depending on their location and height. Sometimes, only the perimeter of the wind turbine farm and/or an arrangement of interior wind turbines are lit.
Some wind turbine farms utilize Aircraft Detection Lighting Systems (ADLS), which are proximity sensor-based systems designed to detect aircraft as they approach the obstruction. This system automatically activates the appropriate obstruction lights until they are no longer needed based on the position of the transiting aircraft. For more information on how obstructions like wind turbines are marked and lighted, refer to Advisory Circular 70–7460–1, Obstruction Marking and Lighting. Lastly, it is worth mentioning that anyone can report potential observed outages of obstruction lighting of wind turbines or other lighted obstructions via the public Obstruction Light Outage Reporting tool at bit.ly/3Z80kQv (account required), or by calling (877) 487–6867 or (800) 478–3576 if in Alaska.
Pilots should also be aware that wind turbines in motion could result in limitations of air traffic services in the vicinity of the wind turbine farms. Pilots utilizing VFR flight following should be aware that ATC may have difficulty observing non-transponder and/or non-ADS-B Out equipped VFR aircraft when flying in the vicinity of wind turbine farms (see Aeronautical Information Manual 4–5–1 paragraph B(1)(g)). In extreme circumstances, this can extend for more than one nautical mile horizontally around the nearest turbine, and at all altitudes above the wind turbine farm.
Many pilots are unaware of the several unique hazards that wind turbines and wind turbine farms pose.
Wind turbines may also adversely affect doppler weather radar returns due to the reflectivity and motion of the rotating blades. Pilots should be aware of this when evaluating weather radar data near wind farms to avoid mistaking the motion of the wind turbine blades for actual weather. For more details on this, go to weather.gov/mkx/windfarm.
Wind Turbine Safety Data and Potential VFR Safety Concerns
According to NTSB accident data, there have been at least six wind turbine related accidents from 2003 to 2023 with eight fatalities. A cursory review and analysis of these accidents indicate that insufficient preflight planning and/or aeronautical decision-making may be contributing factors. This would indicate that many pilots are unaware of the several unique hazards that wind turbines and wind turbine farms pose.
For starters, the rotating blades are unlighted obstructions that can extend several hundred feet above the lighted nacelle/blade hub. Note that the boxed elevation figure on VFR aeronautical charts associated with wind turbine farms accounts for the mean sea level height of the wind turbine blades when passing the 12 o’clock position.
In addition, a 2014 University of Kansas and Kansas Department of Transportation joint study, Wind Farm Turbulence Impacts on General Aviation Airports in Kansas, has identified that wind turbines can generate low altitude turbulence up to several miles downwind of the turbine(s). The amount of turbulence depends on the prevailing wind speed. Therefore, pilots should exercise caution and avoid low-altitude flight downwind of wind turbine farms. Pilots should also be vigilant when flying in the VFR traffic pattern near wind turbine farms that are located within a few miles of an airport. Pilots should report (via a PIREP) turbulence encounters and the suspected source to ATC or the nearest Flight Service Station.
In accordance with 14 CFR section 91.119, pilots must maintain appropriate separation (laterally and vertically) from wind turbines. Attempting low-altitude flight between wind turbines could be hazardous because meteorological towers (METs), some less than 199 feet AGL, may be embedded within the wind turbine farm. METs are very difficult to acquire visually and are not always marked, lighted, or charted. Finally, pilots are reminded of the basic VFR weather minimums contained in 14 CFR section 91.155 and that some wind turbines may significantly exceed 499 feet AGL and extend into Class E airspace (700 feet AGL floor and greater).
VFR Charting and Flight Planning Tips
As a result of industry feedback as well as internal review, the FAA recognized the need to enhance how the agency depicts wind turbines and wind turbine farms on visual charts. In early August 2023, the FAA’s Aeronautical Information Services Visual Charting Team began implementing charting enhancements to sectional aeronautical, VFR terminal area, and helicopter route charts to depict the location of wind turbine farms more clearly. This change will also be published in the FAA’s Aeronautical Chart User’s Guide.
VFR pilots, as part of their comprehensive preflight planning, should carefully evaluate their intended route of flight, ensuring that minimum visibility and cloud clearances can be maintained as well as maintaining safe distances from obstructions. Pilots should carefully study their charts, noting all obstructions along the proposed route of flight and elevation figures to select their VFR cruising altitudes. The boxed elevation figures, as well as other charted elevations and the charted (quadrangle) maximum elevation figure (MEF) should be compared and taken into consideration.
Also, VFR pilots should carefully review NOTAMs (notice to air missions) for temporary or permanent obstructions along the planned route of flight during their preflight preparations. Emphasis should be given to obstructions in the vicinity of the approach and departure ends of the runway complex, or any other areas where flight below 500 feet AGL is planned or likely to occur.
Lastly, always leave yourself an “out” and consider multiple contingency plans (including returning or diverting), which may be a safer course of action. VFR night flying along a route that includes wind turbines and wind turbine farms introduces additional challenges. As we mentioned earlier, not all the wind turbines within a farm may be lighted — it might just be those on the outer perimeter. Pilots should also be aware that wind turbine farms utilizing ADLS may cause a temporary “startle effect” due to unexpected activation of the lights as their aircraft approaches.
Watching Which Way the Wind Blows
Wind-powered energy will continue to proliferate for the foreseeable future and now you should be familiar with many aspects regarding wind turbine impacts to the NAS. As the wind energy industry continues to evolve, new challenges may emerge for the aviation industry. The FAA and industry-stakeholder partnerships will continue to look for solutions to ensure that both wind energy and aviation can safely co-exist.
If you have questions or comments regarding this article, please direct them to the FAA’s Flight Technologies and Procedures Division at 9-AWA-AVS-AFS420@faa.gov with the subject line: Wind Turbines.
Learn More
- FAA AC 70–7460–1, Obstruction Marking and Lighting
- FAA’s Obstruction Light Outage Reporting Tool
- Obstruction Evaluation /Airports Airspace Analysis Portal
- FAA Aeronautical Chart User’s Guide
- FAA Daily Digital Obstacle File
- 14 CFR part 77, Safe, Efficient Use and Preservation of the Navigable Airspace
- FAA JO 7400.2P, Procedures for Handling Airspace Matters
Michael Rauchle is an aviation safety inspector with the FAA’s Flight Procedures and Airspace Group.
