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23 3. Restricts the clear view of runways, helipads, taxiways, or traffic patterns from the airport control tower cab; 4. Derogates airport capacity and/or efficiency; 5. Affects future proposed IFR and VFR operations; or 6. Affects the usable length of an existing or future runway. The FAA also considers whether or not the proposal will have an effect on a significant volume of aeronautical activity on an airport, which is a case-by-case determination. Signifi- cant volume effects vary for different activities (e.g., effects on departures and arrivals may be a daily impact, whereas instrument procedures and minimum altitudes may be utilized weekly). The FAA will make a substantial adverse effect determination if the structure causes electromagnetic inter- ference on facilities and aircraft or if there is a combination of adverse effects listed previously and an impact on significant volume (FAA 2008a). FIGURE 17 Gorgonio Wind Farm, California (courtesy: NASA Procedures for Handling Airspace Matters provides more Earth Observatory website). specific guidance on whether or not a significant adverse effect will occur. For example, it states that structures that necessi- tate an alteration to a Minimum En Route Altitude cause an different from other large structures and, therefore, additional adverse effect. However, flight procedures and air traffic per- consideration beyond normal minimum separation distances sonnel may consider conducting more detailed analysis to and obstacle avoidance is not necessary (CAA 2010). determine if the structure will result in a substantial adverse effect depending on the location of the structure relative to Numerical simulations have shown that natural turbu- flight traffic and extent of use. The loss of altitude for a cardi- lence in the atmosphere will destabilize the wind turbine nal direction is generally considered to result in a substantial creating vortices at a distance of 26 rotor-radii (250750 ft) adverse effect except when the aeronautical study determines (Troldborg et al. 2007). Aircraft flying at the same eleva- that the Minimum En Route Altitude is not normally flown by tion as the wind turbine rotor (200450 ft above ground) at aircraft nor used for air traffic control purposes. a distance where turbulence is projected to occur is deter- mined to be operating in an unsafe location. Turbulence Another example is provided in Terminal Instrument Pro- downwind of a wind turbine could be a consideration for cedures (TERPS) guidance (TERPs 2010). A structure that assessing the suitability of very light sport aviation such as penetrates the 40:1 departure slope for IFR departures is con- parachuting, hang gliding, paragliding, and microlight oper- sidered to be an obstruction to air navigation. If the obstacle ations (CAA 2010). penetrates the departure slope by more than 35 ft, it is pre- sumed to be a hazard and a Notice of Presumed Hazard is MITIGATION OPTIONS issued. Further analysis by flight procedures and air traffic is then necessary to determine if the structure poses a substan- The following mitigation options have been considered for tial adverse effect. minimizing impacts of wind farms on aviation: Guidance from the Civilian Aviation Authority (CAA) Allow appropriate siting to avoid physical penetration of the United Kingdom states that proposed structures large and communication systems impacts. enough to cause a potential impact to radar (including wind Provide developers with the opportunity to fund gap- turbines) should notify the CAA for an impact assessment fill radars or contribute to the cost of replacing long- if the structure is within 15 miles of a radar facility. How- range radar, thereby providing a dual benefit of allowing ever, impacts are not likely for structures beyond 6.2 miles renewable energy development and upgrading aging (or 10 km) (CAA 2010). radar systems. Re-route air traffic around the wind energy facilities to ROTOR BLADE TURBULENCE avoid potential shadow effects and disruptions associ- ated with wind farm radar clutter as part of operational Rotor-induced turbulence can occur downwind of a WTG mitigation. Negative effects of an increased noise foot- where the wind flow is disrupted after passing through the print and CO2 emissions from longer flight tracks need rotor producing a chaotic and turbulent airflow (see Figure 17). to be considered. Analysis of the extent of disruption downwind of the wind tur- Turn off radar that is receiving false returns for the wind bine suggests that the amount of turbulence is not significantly farm area and use supplemental radar that is available in