Airspace Issues in Wind Turbine Siting
Wind turbine projects need to clear many hurdles before they can proceed to construction. One of the most important milestones in any wind project is securing a determination from the Federal Aviation Administration (FAA) that the project does not adversely affect air traffic or radar systems. This can be a complicated and uncertain process, and many projects have run into unexpected delays. The primer below explains how anticipate and avoid some of these conflicts, and how the FAA review process works. Several case examples are presented.
The FAA’s Role and Procedures
The FAA has oversight of any object that could have an impact on the navigable airspace or communications/navigation technology of aviation (commercial or military) or Department of Defense (DOD) operations. The FAA requires that a Notice of Proposed Construction (Form 7460-1) be filed for any object that would extend more than 200 feet above ground level (or less in certain circumstances, for example if the object is closer than 20,000 feet to a public-use airport with a runway more than 3,200 feet long). As wind turbine heights have increased during the past couple of decades, this filing requirement has applied to increasing numbers of projects.
For any filed project, the FAA undertakes an initial aeronautical study within the relevant FAA region, and issues either a Determination of No Hazard to Air Navigation (DNH) — the “green light” for the project — or a Notice of Presumed Hazard (NPH). If an NPH is issued, the FAA will then initiate an in-depth technical analysis (commonly called an extended study), which will explain the cause of the NPH and evaluate impacts on air operations. If after the extended study, which may include a public comment period, there remains an operational impact, the FAA will try to negotiate an acceptable height for a project that has received a DNH. If no agreement can be reached, FAA will issue a Determination of Hazard (DOH). A DOH can be appealed to FAA Washington Headquarters. If the appeal does not secure a DNH, the proponent’s main recourse is to bring the issue before a Federal Court.
The FAA analysis considers several types of airspace impacts: (1) imaginary surface penetration, (2) operational impacts, and (3) electromagnetic interference. A primer is helpful:
- Imaginary Surfaces: Both aircraft flight routes and the airspace near airports require great swaths of defined space dedicated to the takeoff, travel, and landing of aircraft. We might imagine these spaces as giant rooms and tunnels of airspace with very specific boundaries. The walls, ceilings, and floors of those rooms and tunnels are defined as imaginary surfaces. If an imaginary surface is penetrated by a constructed object (e.g., a turbine), the FAA then does an extended study to determine whether the turbine poses an operational problem for the relevant airport or for a specific visual flight route between airports. If the penetration does not pose an operational impact it may be determined not to be a hazard.
- Operational Impacts: The thousands of flights that leave and arrive each day at large airports, such as Boston’s Logan International Airport (Logan), are possible because of very complex and exacting protocols known as visual flight rules (VFR) and instrument flight rules (IFR). Operational impacts are those that affect VFR and IFR operations. Examples of operational impacts include increasing the minimum flight altitude in a specific area (either for “enroute” air traffic or for circling at an airport), diverting air traffic away from an obstacle, increasing the minimum climb gradient (steepness) for airport departure, or increasing the minimum descent altitude at the obstacle location for airport arrivals. For obvious reasons, the air transport industry is generally resistant to operational changes that would increase workload in the cockpit during critical takeoff and landing periods or impose non-standard flight restrictions.
- Electromagnetic Interference (EMI): Experience has shown that wind turbines can degrade performance of air traffic control (ATC) or air defense radar. The phenomenon can include sudden or intermittent appearance of radar contacts at the location of the wind turbine because of blade motion or rotation of the turbine to face the wind. For ATC radar the interference is generally limited to wind turbines that are within the radar line of sight. Studies indicate that this problem may be minimal for turbines more than 5 nautical miles from the radar.
A September 27, 2006 Department of Defense (DOD) report titled The Effect of Windmill Farms on Military Readiness identifies similar conflicts with air defense radar. These conflicts can extend for tens of miles from the radar facility due to atmospheric refraction).
And finally, interference with microwave transmissions is another form of EMI that is of concern because public safety radio systems typically use microwave-based technologies.
Resolving Adverse Impacts. As part of a Notice of Presumed Hazard or Determination of Hazard, the FAA may suggest modifications that could render the project acceptable – for example, a change in the height of a proposed turbine or the re-siting of a turbine at a greater distance from an airport. Such modifications can make a project possible while accommodating regulatory needs and preserving the quality of airspace or navigational operations at the nearby airport. For wind project proponents, these modifications often represent a tradeoff of efficiency for safety. In rare cases the FAA might agree to a procedural change that would sidestep the problem.
For conflicts with radar systems, there are technologies, such as geosensor mapping and “masking” that may prevent air traffic control (ATC) radar from picking up wind turbines as targets. However, masking also renders the radar ineffective in the subject area. DOD’s recent report concludes that the only way to prevent signal degradation of air defense radar is to keep wind turbines out of the radars’ lines of sight.
Identifying Potential Conflicts Early. Wind project developers may find it beneficial to consider how the FAA evaluates potential airspace obstruction as they screen potential sites for wind projects. MTC has found it useful to obtain an “Airspace Obstruction Report and Opinion Letter” from a qualified consultant at an early stage in a project. This relatively low-cost effort can help project proponents identify potential airspace conflicts early and avoid wasting time and effort on a dead-end project. It is important to note that the FAA’s evaluation of potential conflicts with air traffic is significantly more predictable than its evaluation of radar issues. At present, the only reliable approach to identifying radar issues is to have the FAA conduct a “propagation study” as part of its formal review of a Notice of Proposed Construction.
Case Examples: Learning by Doing
MTC’s understanding of airspace siting issues has evolved through experience with some early wind projects in Massachusetts. The following case examples provide some history and context for current conversations about securing approval for wind turbine sites, especially in the Boston Harbor area.
Hull and the IBEW: Early Successes
- The Hull Municipal Light Plant installed Massachusetts’ first large scale wind turbine, a 164-foot turbine located at Windmill Point, in 2001. Five years later, it installed a 340 foot turbine on the site of a town landfill. Happily, both received DNH status from the FAA and are operational. Logan Airport air traffic control radar.
- In 2005, Local 103 of the International Brotherhood of Electrical Workers received a DNH for a 150-foot wind turbine at their Dorchester office site. Today the turbine spins away. It is unclear whether the FAA will ultimately conclude that this turbine has some effect on Logan Airport air traffic control radar.
Boston Harbor: Airspace Concerns
Wind projects in the vicinity of Logan Airport raise special concerns, since Logan is one of the nation’s busiest and most compact airports. MTC and project proponents filed Notices of Proposed Construction near the beginning of the development process for several Boston Harbor area projects so that any airspace or radar problems would be known early in the development process. For the Deer Island, Long Island, and Lynn projects (see below), the FAA had initial concerns about the heights of turbines.
- In late 2005 the Massachusetts Water Resource Authority (MWRA) filed a Notice of Proposed Construction for 394-foot turbines at five possible locations at the wastewater treatment plant on Deer Island in Boston. When the FAA concluded that turbines at the proposed heights were a potential hazard to aviation, the MWRA and MTC revised the project to use 190-foot turbines. In March 2008, FAA issued Determinations of No Hazard for two turbine sites. They will act on the remaining proposed sites after evaluating the impact of turbine operations on Logan airport radar.
- The City of Boston, with support from MTC’s Community Wind Collaborative, proposed a series of up to four 394-foot turbines on Long Island in Boston Harbor, a bit more than three nautical miles from Logan airport. Notices of proposed construction were filed on October 15, 2005. On February 10, 2006 the FAA issued initial NPH designations for all four locations based on its finding that “the structure[s] as described exceed[s] obstruction standards and/or would have an adverse physical or electromagnetic interference effect upon navigable airspace or air navigation facilities.” (The FAA identified reduced heights at which the turbines may have been acceptable as 190, 233, 256, and 247 feet.) At that point, MTC hired an airspace specialist to determine whether there were options for overcoming the FAA’s concerns. That specialist proposed a “step-down fix” by which minimum aircraft heights over Long Island would be increased. The FAA conducted an extended study of this proposal including a public comment process. In September of 2006, the FAA issued a DOH for the Long Island sites, saying that the “cumulative impact of the proposed structure[s] . . . is considered to be significant.” It cited, among its reasons: the turbines would be in the only remaining quadrant around Logan that is not currently impacted by obstructions; building a step down fix would increase cockpit workload during final approach; and that, without complex geosensor mapping for each turbine, interference with air traffic control radar was likely. MTC and the City of Boston elected not to appeal those FAA determinations.
- The City of Lynn Community Wind Collaborative wind turbine project is slated for a regional wastewater treatment plant site. The initial FAA filing for the 397-foot structure resulted in a NPH, in part because of turbine height. Subsequent evaluation by an airspace specialist under contract to MTC indicated that the FAA would be unlikely to accept a modification of minimum aircraft altitudes for the Lynn location. Informal contact with the FAA indicated that there might also be conflicts with Logan Airport air traffic control radar. MTC and the City then accepted a lower wind turbine height (254 feet) and asked that the FAA investigate the radar issue in greater depth. After further FAA study, the FAA issued a DNH in January 2007.
Outside the Boston Area: Barnstable and Paxton
- MTC is collaborating with Cape Cod Community College on a wind project to be built on the College’s campus in West Barnstable. Initially, the project involved a 397-foot turbine on the east side of the campus. Early in the project development process, MTC and the college discussed the project with authorities at the local airport, and received informal assurances that the project should not raise airspace concerns. However, a subsequent filing with the FAA resulted in, first, a NPH, and then, a DOH. In response to the FAA’s evaluation, the College modified the proposal to a 253-foot turbine on the west side of the campus. As of February 2007 the College and MTC are waiting for final FAA approval of this smaller turbine in the alternate location.
- In mid-2005 MTC filed a Notice of Proposed Construction for two wind turbine locations on Asnebumsket Hill in Paxton, Massachusetts. There are currently several telecommunications and radio transmitter towers at the site, one of which is 366 feet above ground level. In December 2005 the FAA issued NPH’s for the two locations indicating that any object greater than 200 feet above ground level would result in a DOH. Subsequent evaluation by MTC airspace consultants led us to the conclusion that there was little change of mitigating FAA concerns. (This evaluation also concluded that the minimum height for one turbine location should have been 125 feet above ground level.) An important aspect of the Paxton situation is that the proposed wind turbine locations were closer to Worcester airport than the existing radio/telecommunications towers. Thus, wind turbines at the proposed heights would have constituted an additional deterioration of navigable airspace which was unacceptable to the FAA.
Guidelines and “Best Practice” Recommendations
It is not possible to predict perfectly the outcome of an FAA determination on a given project. However, from the above project experiences (and others), MTC has generated the following recommendations for the planning and development of wind turbine projects.
- Review airspace considerations early in the project development process; this increases efficiency, saving time, money, and stress.
- Be aware of the following general guidelines: a turbine under 200 feet in height is rarely problematic for navigable airspace, and a turbine site more than five nautical miles from an airport runs the least risk of a NPH or DOH on radar issues.
- Conduct, as early as possible, due diligence with local authorities, proponents, and stakeholders. This avoids, among other challenges, unanticipated opposition at the public comment stage of an FAA study.
- Enlist the services of professional aviation consultants, and bring them into the process early to advise on the technicalities of FAA regulations, which can be daunting. (A report commissioned by MTC states that FAA criteria “are highly complex, especially as they relate to operational impacts on air traffic. There are many instances where[in] FAA operational criteria . . . are modified on a case-by-case basis . . . . only an experienced airspace analyst should attempt to apply the Operational Impact criteria.”). The consultant’s analysis is relatively inexpensive (approximately $500) and results will be available quickly, whereas the FAA review process takes considerable time.
With this information in hand, you will be better prepared to tackle the exhilarating (and sometimes cumbersome) task of bringing a wind turbine project to life. At MTC we are constantly striving to provide accurate and current information regarding your wind turbine siting and permitting needs — please do not hesitate to contact us!
Resources/Links
This summary of FAA considerations in the siting of wind power projects draws on numerous resources and technical reports, available below:
Massport airspace maps for Logan Airport: The Massachusetts Port Authority has developmed a series of maps illustrating various imaginary surfaces associated with Logan International Airport in Boston.
Memo - Daytime Lighting Requirements for Wind Turbines
Airspace Obstruction and Electromagnetic Interference Considerations for Wind Power Projects, prepared by Aviation Systems, Inc. for MTC, January 2007.
Filing with the FAA
- FAA Obstruction Evaluation / Airport Airspace Analysis website.
- Notice of Propose Construction (form 7460-1). This form must be submitted for any structure that exceeds 200 feet above ground level, no matter where it is located.
- Advisory Circular – Proposed Construction or Alteration of Objects that May Affect the Navigable Airspace
- Part 77 Airspace Obstruction Analysis – This webpage provides contact information, contains links to form 7460-1 and provides a concise description of how the FAA application & notification process works.
- All determined and proposed air obstruction cases filled with the FAA can be found here. You can also file your 7460-1 form electronically from this webpage. However, you do need to register as a new user.
Table 1 – Summary of Wind Projects in Massachusetts and FAA Determinations
Project/Location |
Proposed Structure Height (feet)1 |
Distance from Airport Center |
Status |
Eastern Massachusetts |
|||
Boston (IBEW) |
150 |
4.2 |
|
Boston |
394 |
3.47 |
|
|
1st Filing – 394 |
2.19 |
|
Boston (MWRA Nut Island) |
397 |
Approx. 5.5 Logan |
|
Cape Cod Community College |
1st Filing – 397 |
2.75 |
1st - DOH |
Falmouth |
394 |
3.8 |
|
Gloucester (Varian) |
480 |
12.0 |
DNH |
Hull - 1 |
241 |
5.5 |
DNH not available |
Hull - 2 |
340 |
9.05 |
|
Lynn |
1st Filing - 397 |
5.7 |
|
Orleans |
340 |
4.9 |
|
Central Massachusetts |
|||
Paxton |
397 |
2.24 |
|
Western Massachusetts |
|||
Florida and Monroe |
340 |
6.3 |
|
Hancock |
338 |
8.6 |
|
Hancock |
394 |
7.0 |
|
1 – This includes the height of the turbine only (blade tip at highest point) and not the elevation of the site which FAA analysts include in their study.
2 – The City of Boston considered 4 different locations, all of which received a DOH.
3 – MWRA considered 5 locations at the wastewater treatment plant on Deer Island, all of which received a DPH.