Experimental evaluation of wind turbine wake turbulence impacts on a general aviation aircraft

roll hazards may exist up to 4.5 km (2.8 mi) downwind of a wind turbine, they based their calculations on a 64.4 km h−1 (40 mi h−1) wind speed (which is unrealistically high for the operation of light aircraft) and did not account for the re- duction in turbine thrust that occurs at higher wind speeds using pitch-regulated control (Wang et al., 2015). More re- cent studies by Tomaszewski et al. (2018) and Varriale et al. (2018) used wake velocity fields generated from compu- tational fluid dynamics (CFD) models to study potential im- pacts on aircraft flight. Tomaszewski et al. (2018) assessed the roll hazard induced by flight through the wake via a static analysis of roll moment generation acting on the air- craft in hundreds of simulated wake passes. Results showed that 99.99 % of roll moments encountered within 10 rotor di- ameters (RD) of the turbine were classified as low-hazard, with 0.01 % classified as medium-hazard and no cases in the high-hazard category. Varriale et al. (2018) simulated aircraft flight through the wake using a six-degrees-of-freedom flight dynamic model of a Cessna 172 airplane. This study showed that the loads experienced by the aircraft in the wake passes were reasonably small and did not approach the structural limits of the aircraft. Wang et al. (2015, 2017) present piloted simulation studies in which several pilots flew a flight sim- ulation of a light aircraft through wake velocity fields gen- erated from a numerical model. These results showed that yaw disturbances due to wake encounters were less than 10°, and pilots rated the overall disturbances due to turbulence as minor. Several studies have also considered the potential effects of added wake turbulence on helicopters. Bakker et al. (2018) report the preliminary results of a variety of sim- ulation studies, showing that under typical wind conditions, disturbances to helicopters in forward flight are likely to be minimal when crossing turbine wakes, although the paper suggests that potential hazards may exist in hover or low- speed flight when operating very close to turbines at high wind speeds. Finally, de Jong et al. (2020) analyzed hun- dreds of hours of accelerometer data from helicopters servic- ing offshore wind farms, comparing the vertical and lateral accelerations experienced inside and outside turbine wakes. The authors found no apparent difference in acceleration data between segments of flights inside and outside the wakes at offshore wind farms, indicating that added turbulence inside the wake did not noticeably impact vehicle control or stabil- ity. While the majority of the above studies conclude that added turbulence in wind turbine wakes is of sufficiently low magnitude to avoid posing a risk to light aircraft, there is a lack of experimental data, particularly from fixed-wing gen- eral aviation aircraft, with which to confirm these findings. For instance, in Varriale et al. (2018) the authors express the need for “a real-world flight or simulation test involv- ing a pilot”, while de Jong et al. (2020) state that “a test flight with special equipment could help in the analysis” of turbulence effects on aircraft at land-based wind farms. The current paper fills this gap in experimental data by present- ing the results of a dedicated flight experiment in which a general aviation aircraft was flown in close proximity to op- erating wind turbines at a land-based wind energy facility. The test flights were performed with a Cessna 206 aircraft near a Vestas V136 turbine at the Whitla Wind facility in southern Alberta, Canada. Thirteen wake passes were per- formed at distances ranging from 2125 m (15.6 rotor diam- eters, or RD) to 223 m (1.6 RD) from the turbine base. Air- craft state data, wind turbine operational data, meteorologi- cal mast data, and video footage were gathered during each wake pass. Analysis of the resulting data showed that the flight disturbances encountered by the aircraft while in the turbine wake were uniformly small and well below the level that would lead to the risk of loss of control or structural dam- age. Beyond 6 RD, load factor and orientation disturbances inside the wake could not be distinguished from those ex- perienced outside the wake, while at closer distances, small perturbations were evident in the flight data but were only slightly higher than those caused by atmospheric turbulence outside the wake. Collectively, analysis of flight data, visual inspection of in-aircraft video recordings, and pilot feedback indicated that there was no safety risk to the aircraft when flying through turbine wake, even at close distances. Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1851 Figure 1. Whitla Wind facility turbine locations and area of flight operations. Turbine T1 (highlighted) is the northwesternmost turbine located at Whitla. Figure 2. The Cessna 206 used in flight tests (photo courtesy of Raina Naomi of Edmonton Skydive). Table 1. Whitla Wind facility V136 turbine characteristics. Parameter V136 turbine Rotor diameter 136 m Hub height 105 m Tip height 174 m Rated power 3.6 MW Cut-in speed 3 m s−1 Cut-out speed 27.5 m s−1 Rated speed 9.5 m s−1 2 Flight test methodology This section provides detailed information regarding the Whitla Wind facility where the flight tests were conducted, the test aircraft, the data gathered during the experimen- tal flight, and the weather conditions that prevailed during each wake pass. The test trials were performed on 16 Octo- ber 2023 during the hours of 13:30–15:30 mountain daylight time (MDT). 2.1 Description of Whitla Wind facility The Whitla Wind facility (“Whitla”) is located approxi- mately 16 km south of Bow Island, Alberta, Canada. The 353 MW facility is comprised of 98 Vestas V136 turbines. The characteristics of the V136 turbines installed at Whitla are provided in Table 1. The turbine locations at Whitla are shown in Fig. 1. At the time selected for the test flights, the winds were generally blowing from the southeast. As a result, the turbine wakes were propagating northwest of the project area. The area se- lected for flight operations was downwind of the farthest tur- bine at the northwest corner of the project, turbine T1. The location of this turbine and the selected area of flight opera- tions are shown in Fig. 1. https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1852 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Table 2. Characteristics of the Cessna 206 aircraft. Parameter Cessna 206 Wing span 11.0 m Wing area 16.2 m2 Length 8.61 m Height 2.85 m Stall speed (landing config.) 101.9 km h−1 (55 kn) Maximum gross weight 3806 lb (1726 kg) Installed power plant Continental IO-520, 213 kW (285 hp) Table 3. Data collected for each wake encounter. Data collected Source Aircraft latitude, longitude (GPS) Sensor Logger Aircraft pitch and roll angle Sensor Logger Load factor (acceleration) Sensor Logger Aircraft altitude (GPS) Sensor Logger Aircraft bearing Sensor Logger Aircraft groundspeed (GPS) Sensor Logger Wind speed at hub height Turbine T1 SCADA Wind direction at hub height Turbine T1 SCADA Turbine T1 power output Turbine T1 SCADA Wind speed at 35 m altitude Meteorological mast Aircraft cabin video Cabin-mounted GoPro camera 2.2 Description of flight test aircraft and pilot qualifications The aircraft used for the test flights was a Cessna 206 Station- air owned and operated by Edmonton Skydive. The aircraft used was a Cessna P206 (P stands for passenger model), reg- istration number C-GFBE, S/N P206-044. A picture of the aircraft is shown in Fig. 2. This aircraft was selected as it is similar to other high-winged Cessna aircraft models such as the Cessna 172, Cessna 180, and Cessna 182 commonly flown out of small airports and airstrips in agricultural ar- eas. The Cessna 206 has a wing area that is nearly identi- cal to those of the Cessna 172 and Cessna 182, but it can be operated at a higher maximum gross weight of 1726 kg (compared to 998 kg for the Cessna 172 and 1338 kg for the Cessna 182). In order to maintain similarity to these smaller aircraft, the Cessna 206 used for the flight tests had all the interior components of the cabin removed and was operated at a gross weight of approximately 2300 lb during the wake passes. Note that gross weight and wing area are two of the most important drivers of the response of an airplane to tur- bulence according to US Air Force Handbook 15-10 (US Air Force, 2019), and thus this operational similarity of the flight test aircraft to other aircraft commonly flown near wind farms ensures the relevance of the results. Other characteris- tics of the flight test aircraft are provided in Table 2. The pilot who flew the wake passes holds a commercial pi- lot’s license with a multi-engine rating and a group 1 IFR (in- strument flight rules) rating. He is an experienced skydiving pilot and, at the time of the flight trials, regularly flew sky- diving missions at Edmonton Skydive. At the time of the test flights, the pilot had 520 h of total flight time, with approxi- mately 247 flight hours as pilot in command of a Cessna 206 acquired during the 3 months prior. A passenger (who was not a licensed pilot) was also present in the aircraft to assist with flight planning and data recording. The pilot and pas- senger knew beforehand that the purpose of the flights was to assess the level of turbulence experienced by the aircraft in a wind turbine wake and were asked to fly a set of test points at different distances from the turbine. 2.3 Description of data collection The Cessna 206 does not have an onboard flight data record- ing system. To record flight data during the wake encounters, an iPhone 13 Pro was temporarily mounted in the cockpit of the aircraft and configured to record data during the wake encounters using an application called Sensor Logger. The iPhone 13 Pro is equipped with a range of sensors includ- ing a three-axis accelerometer, three-axis gyroscope, three- axis magnetometer, and GPS receiver. The iPhone operating system is equipped with attitude estimation algorithms that fuse data from the accelerometers, gyroscope, and magne- tometer to determine the device’s roll, pitch, and yaw an- gles via the Core Motion software framework (Apple, 2024). The Sensor Logger application provides a means to easily record raw sensor data as well as orientation estimates from the phone’s operating system. Numerous recent publications have evaluated the accuracy of the acceleration and orienta- tion data from the iPhone and found it to be similar in accu- racy to commercial low- or medium-grade inertial measure- ment units (e.g., Grouios et al., 2023; Mourcou et al., 2015). For each flight, the Sensor Logger app was used to record position, speed, bearing, and altitude with respect to mean sea level (MSL) derived from GPS; roll, pitch, and yaw ori- entation; and three-axis accelerometer outputs. The GPS data are derived directly from the GPS receiver in the iPhone 13 Pro. Mean sea level altitude is converted to altitude above ground level (AGL) by subtracting the ground height at tur- bine T1 (note that the terrain in the vicinity of T1 is generally flat, so the local ground elevation at any aircraft position is assumed to be the same as the ground elevation at the loca- tion of T1). The roll, pitch, and yaw orientations are the es- timates provided by the phone’s operating system, while the acceleration data are taken directly from the phone’s three- axis accelerometer. In this work, the primary acceleration component of interest is the acceleration in the vertical direc- tion with respect to the aircraft, i.e., the aircraft load factor. The load factor is the ratio of the acceleration experienced in the aircraft body-fixed vertical direction to the gravita- tional acceleration. This value is significant in that the air- craft structural limitations are defined in terms of the limit load factor for a specific make and model of aircraft. Thus, with the phone mounted flat on the aircraft dashboard, only the vertical-axis component of the acceleration is of interest. Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1853 Figure 3. Example raw and filtered accelerometer data (a); amplitude spectrum of raw data and areas of band-stop filtering (b). A summary of the collected flight data and meteorological data is provided in Table 3. An accelerometer that is rigidly mounted to the aircraft will measure loads across a wide frequency range. Gener- ally, signal components at frequencies of less than 10 Hz are due to turbulence and control inputs, while those at higher frequencies result from engine vibrations or excitation of air- craft structural modes (Mansfield and Aggarwal, 2022). En- gine vibrations occur at the propeller rotation frequency (ap- proximately 2500–2700 rpm for this flight) and its harmon- ics. Figure 3 shows raw vertical-axis accelerometer data for one of the wake passes along with the single-sided ampli- tude spectrum obtained from the Fourier transform of the data. Note the presence of a peak at the propeller rotation frequency around 42 Hz and at a subharmonic around 21 Hz. A smaller peak around 37 Hz was also evident in some of the data and could be due to vibration of the instrument panel on which the sensor was mounted or other aircraft structural vibrations. Note that the peaks at harmonics of the propeller rotation frequency at 21 Hz and 42 Hz nearly ex- actly match similar accelerometer data from a piston-engine aircraft shown in Ilic et al. (2017). Because the purpose of this study is to isolate the loads caused by turbulence, a frequency-domain low-pass filter was applied to the ac- celerometer data to remove frequency components higher than 20 Hz, as shown in Fig. 3b, resulting in the filtered ac- celerometer data shown in the orange trace in Fig. 3a. These filtered accelerometer data reflect the loads experienced due to turbulence and can be compared to the limit load factor to assess any potential for structural damage. Hub height wind direction, wind speed, and power pro- duction data from turbine T1’s supervisory control and data acquisition (SCADA) system were collected for the time pe- riod of the flight trials. These SCADA data were measured at hub height and are used to assess the wind conditions and wake propagation direction during each wake encounter. In addition, the wind data from the turbine can be combined with the groundspeed measurements of the aircraft from GPS to obtain an approximate airspeed of the aircraft. This air- speed approximation is performed by resolving the aircraft groundspeed and wind speed into north and east components, subtracting the wind from the groundspeed components, and computing the resulting vector magnitude. Note that this air- speed is an approximation and does not represent a precise airspeed measurement. To complement the hub height wind data from T1’s SCADA system, lower-altitude wind condi- tions during the flight trials were recorded by a meteorolog- ical mast located within the Whitla project boundaries at an altitude of 35 m. These additional data provide a measure- ment of wind speed fairly close to the ground surface during the flights. There were no sensors that directly recorded the control inputs to the aircraft. Control input information may be par- ticularly useful in evaluating the pilot’s response and any in- crease in workload observed when flying through the turbine wakes. In order to assess pilot behavior during the flights, a GoPro HERO10 Black camera was mounted in the cabin to record video footage for each trial. This video footage is useful in assessing pilot response and workload as well as providing a qualitative assessment of the experience of fly- ing through the turbine wake. The video files are available as supplemental data accompanying this paper. 2.4 Description of wake passes A total of 13 wake passes were performed. Three aircraft speed and flap configurations were used: airspeed of 90 knots (kn; 1 kn = 0.514 m s−1) with 0 flaps, airspeed of 80 kn with 10° flaps, and airspeed of 75 kn with 10° flaps. Wing flaps are retractable extensions on the trailing edge of the wing that are typically extended (i.e., deflected downward by a cer- tain angle) during approach and landing to allow for higher https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1854 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts lift production at lower flight speeds. The above flight con- ditions are representative of departure, approach, or landing operations, which are the flight segments in which a general aviation aircraft is most likely to be flying at low altitude in the vicinity of a wind turbine near an airport. The pilot was asked to fly two passes with closest distances of approach to the turbine of 15, 10, 5, and 3 RD, with one pass at each distance flown with the 90 kn configuration and one flown with the 80 kn configuration. In addition, the pilot was asked to repeat the 5 RD pass at 80 kn four times to pro- vide more data for this specific configuration. The pilot was also granted latitude to fly additional passes closer to the tur- bine as desired. The resulting passes, listed in Table 4, were flown at different lateral distances from the turbine with the closest point of approach ranging from 1.6 to 15.6 RD. The flights were flown at altitudes of 45–122 m above the ground, with most flights flown at an altitude near the hub height of the turbine (105 m). Figure 4 shows the GPS ground tracks of the 13 wake passes. Table 4 shows the time, flight config- uration, closest point of approach to the turbine, wind speed, wind direction, and turbine power output for each wake pass. Note that the closest distance of approach to the turbine re- quested for each trial sometimes differed slightly from the closest distance actually flown (as listed in Table 4) due to small errors in flight path tracking. For instance, wake pass 1 was intended to approach the turbine within 15 RD but ap- proached within 13.5 RD instead, while wake passes 7–10 were intended to approach within 5 RD of the turbine but varied between 4.5 and 5.5 RD. The wind speeds at the time of the test flight shown in Table 4 were slightly lower than the annual average wind speed at Whitla of about 8 m s−1. However, wind conditions were generally favorable for the creation of added turbu- lence in the wind turbine wake. It is well-known that turbine- added turbulence is a function of the turbine thrust coefficient (Quarton and Ainslee, 1990; Crespo and Hernandez, 1996). Modern turbine control systems operate under high-thrust conditions when there are low wind speeds so as to avoid tower resonance, with thrust coefficients typically exceeding 0.96 at wind speeds below 7–8 m s−1 (Martinez-Tossas et al., 2022). The wind speeds recorded by turbine T1’s SCADA system during the flight trials varied from 4.7–7.5 m s−1 and thus were conducive to the generation of added turbulence in the wake due to operation at a relatively high thrust coeffi- cient. Furthermore, the wind speeds recorded by the meteoro- logical tower at 35 m altitude in Table 4 were in the range of 4.9–6.6 m s−1, which would be favorable for recreational fly- ing. One factor that may have reduced the level of turbulence encountered in the wake during the test flights is daytime- heating-induced vertical mixing, which is known to dilute the wake (Lee and Lundquist, 2017). Although nighttime con- ditions have the potential for higher levels of wake turbu- lence due to reduced vertical mixing, general aviation aircraft flight through a turbine wake at night would be extremely in- frequent. The two most common scenarios where a general aviation aircraft may be expected to fly below tip height in the wake region of a turbine would be during aerial appli- cator (crop-dusting) missions or on takeoff or landing from an airport. Aerial application is done only during the day- time. Furthermore, in North America, airports located near turbines tend to be private or small public airstrips which have minimal nighttime operations. As a result, the flight test is representative of a situation in which a general aviation aircraft would encounter a turbine wake and embodies an al- most worst-case scenario in which the turbine is producing relatively high levels of added turbulence in the wake in day- time conditions. 3 Flight test results and discussion The flight test data consist of sensor data recorded from the iPhone mounted to the aircraft dashboard as well as videos captured during each wake pass. This section presents time histories of the relevant aircraft states, a description of the video data, and a qualitative description from the pilot re- garding the level of turbulence encountered and the flight control response required. 3.1 Sensor data This section presents the sensor data recorded during each wake pass. Figures 5 and 6 show the GPS tracks for the wake passes along with the approximate location of the wake. In these figures, the GPS tracks are shown with a black line, the turbine location is shown with a red dot, and the approximate wake boundaries are shown with dashed blue lines. The wake boundaries are approximated as a region that is two rotor di- ameters wide (272 m), directly downwind of the turbine. The wake boundaries shown for each wake pass are computed using the wind direction recorded during that wake pass by the turbine SCADA system. It is acknowledged that the wake does not always propagate directly downwind of the turbine due to effects such as wake meandering, and thus the wake boundaries shown are only approximate. Figures 7–12 show the time histories of distance to the tur- bine, groundspeed and estimated airspeed, roll and pitch an- gles, bearing angle, altitude above ground level (AGL), and load factor for wake passes 2, 6, 7, 9, 12, and 13. Flight data from these wake passes are shown to illustrate general trends; data from the remaining wake passes are shown in the Sup- plement. In the flight data plots, the vertical dashed black lines indicate the approximate times when the aircraft enters and exits the wake. These times are determined from the GPS timestamps recorded when the aircraft crosses the approxi- mate wake boundaries shown in Figs. 5 and 6. Although a target airspeed of either 90, 80, or 75 kn was selected for each wake pass, it is difficult to maintain a target airspeed precisely when flying the aircraft due to normal disturbances and imprecision when controlling the aircraft. Thus, in the Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1855 Figure 4. GPS ground tracks of each wake pass. Table 4. Description of wake passes. Pass Local time Configuration Closest turbine T1 hub height/35 m T1 hub height T1 output no. (16 Oct 2023) distance height wind speed wind direction power (MW) 1 13:47 MDT 90 kn, 0 flaps 1843 m (13.5 RD) 4.7 m s−1/5.7 m s−1 151° 0.46 2 13:53 MDT 90 kn, 0 flaps 1428 m (10.5 RD) 4.6 m s−1/5.7 m s−1 161° 0.33 3 13:58 MDT 90 kn, 0 flaps 768 m (5.6 RD) 5.1 m s−1/5.4 m s−1 147° 0.50 4 14:02 MDT 90 kn, 0 flaps 397 m (2.9 RD) 5.3 m s−1/4.2 m s−1 146° 0.54 5 14:57 MDT 80 kn, 10° flaps 2125 m (15.6 RD) 5.4 m s−1/5.7 m s−1 146° 0.73 6 15:02 MDT 80 kn, 10° flaps 1480 m (10.9 RD) 6.1 m s−1/5.7 m s−1 145° 0.87 7 15:10 MDT 80 kn, 10° flaps 676 m (5.0 RD) 6.8 m s−1/5.7 m s−1 141° 1.0 8 15:14 MDT 80 kn, 10° flaps 752 m (5.5 RD) 6.8 m s−1/5.7 m s−1 142° 1.0 9 15:17 MDT 80 kn, 10° flaps 721 m (5.3 RD) 6.8 m s−1/5.1 m s−1 140° 1.2 10 15:20 MDT 80 kn, 10° flaps 607 m (4.5 RD) 7.2 m s−1/5.7 m s−1 137° 1.4 11 15:24 MDT 80 kn, 10° flaps 521 m (3.8 RD) 7.5 m s−1/4.9 m s−1 149° 1.2 12 15:30 MDT 75 kn, 10° flaps 223 m (1.6 RD) 7.4 m s−1/6.6 m s−1 149° 1.1 13 15:33 MDT 75 kn, 10° flaps 283 m (2.1 RD) 6.6 m s−1/5.7 m s−1 148° 1.0 results below, the airspeed and groundspeed deviate slightly from the selected speeds, both inside and outside the wake. Several trends are evident in Figs. 5–12 and the figures in the Supplement. The GPS tracks show that the pilot flew nearly perpendicularly to the wake in each pass, and Figs. 7– 12 show that the aircraft was present in the wake region of T1, as estimated by the above procedure, for between 5–10 s during each pass. In most wake passes, the levels of pitch and roll disturbances, load factor magnitudes, and speed and bearing deviations are not noticeably higher inside the wake compared to outside. In some passes close to the turbine, such as wake pass 7 (Fig. 9), a small roll angle deviation of approximately 7.5° and an increase in the magnitude of load factor perturbations are noted inside the wake region. The severity of these disturbances will be compared to relevant benchmarks in a subsequent section. https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1856 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 5. GPS tracks for wake passes 1–6. 3.2 Pilot feedback regarding experience flying through wake Prior to discussing the pilot’s comments regarding his ex- perience of flying through the wake, it is helpful to present general turbulence categories used by the aviation commu- nity for reference. The United States Federal Aviation Ad- ministration (FAA) Aeronautical Information Manual (AIM) (FAA, 2021a) describes four categories of turbulence: light, moderate, severe, and extreme. The definitions of these tur- bulence categories are provided below (as direct quotes from FAA, 2021a): – Light turbulence. Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). – Moderate turbulence. Turbulence that is similar to “light turbulence” but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indi- cated airspeed. – Severe turbulence. Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1857 Figure 6. GPS tracks for wake passes 7–13. https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1858 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 7. Sensor data from wake pass 2 (closest distance of approach is 10.5 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. large variations in indicated airspeed. Aircraft may be momentarily out of control. – Extreme turbulence. Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage. Note that light or moderate turbulence generally does not pose a risk to aircraft safety from either a loss-of-control or a structural-load standpoint. The pilot was debriefed about his experiences during the wake passes. The pilot was asked to provide an overall sum- mary of his experiences in writing and to answer more spe- cific questions related to workload, the magnitude of flight disturbances, and the presence of any safety risk (each of which is discussed below). A summary of the pilot’s expe- riences and feedback is provided here. First, the pilot noticed that during the flight down to the wind farm, the air was generally smooth, with no atmospheric turbulence notice- able. This indicates that a stable atmosphere prevailed dur- ing the flight experiment. It is well-known that stable atmo- spheric conditions, i.e., low ambient turbulence intensity, are conducive to the longer propagation of turbine-added wake turbulence (Tomaszewski et al., 2018; FAA, 2021a). Within 3–4 km of the wind farm, when flying below the turbine tip heights, the pilot noticed “very light turbulence”. This would be expected downwind of a wind farm given the combined wake effects of 98 turbines at the Whitla Wind facility. The pilot’s impression was that the turbulence experienced was negligible. The videos of each flight trial provide an indica- tion of the very light turbulence experienced (the aircraft is operating downwind of the wind farm within 3–4 km and be- low tip height during the entire portion of all the videos). The pilot noted that in the wake passes flown beyond four rotor diameters (all wake passes except 4, 11, 12, and 13), the flight was “just a little bumpy” and “didn’t have any real disturbance to attitude”. Those encounters were described as “definitely good overall”. In trials performed closer than 4 RD, the pilot noted that some attitude disturbances did oc- cur in the form of a slight rolling motion, “probably 10–15°”, on either entry to or exit from the wake. This observation by the pilot matches the magnitudes of 10–11° recorded in the flight data for wake passes 12 and 13 shown in Figs. 11–12. The pilot described the turbulence encountered during trials 12 and 13, flown around 200 m downwind of the turbine, as Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1859 Figure 8. Sensor data from wake pass 6 (closest distance of approach is 10.9 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. “moderate”. It is worth noting that, unlike flight in moder- ate atmospheric turbulence, flight through the turbine wake at distances of one to four rotor diameters can only happen over a 5–10 s time span for typical general aviation aircraft since the pilot must be flying perpendicularly to the wake, as in the flight trials performed here. This is because flying to- ward the turbine at this distance for a sustained period would cause a collision, and flying away from the turbine at this distance would quickly lead to spacing beyond four rotor di- ameters. Thus, the aircraft would only experience this level of turbulence for several seconds. Several questions were posed to the pilot following the flight experiment to assess workload and the perceived level of risk. First, the pilot was asked to comment on the level of control action required to maintain straight and level flight when flying through the wake of turbine T1, compared to flight outside the wake. The pilot’s response to this question was as follows: Overall I was trying to carefully set up the aircraft before entering the wake zone so that the data of that turbine’s turbulence would be as good as pos- sible. Once in the wake zone I was prepared to re- act however necessary. However, I did not end up flying differently inside the wake as compared to outside. I remained using small inputs to try and hold altitude and airspeed, except for the very last pass which I did, which was within a few hundred feet of the turbine. In the last pass, I used moder- ate control input to correct for the brief roll distur- bance that occurred. Second, the pilot was asked whether the turbulence en- countered in the wake of turbine T1 ever posed a risk to the aircraft, led to an increase in pilot workload, or placed the air- craft in (or close to) an “upset” condition. Note that the FAA Airplane Flying Handbook (FAA, 2021b) defines an upset condition, which can be a precursor to loss of control, as a roll angle exceeding 45° and pitch angle less than −10° or greater than 25°. The pilot’s response to this question was as follows: The turbulence inside the wake did not pose a safety risk to the aircraft. My workload was al- ready high as I was trying to hold altitude and air- speed as closely as possible to test requirements, https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1860 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 9. Sensor data from wake pass 7 (closest distance of approach is 5.0 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. while flying through air that was not smooth to be- gin with. Entering the wake zone behind the tur- bine did not increase my workload further. If I was flying with a lower workload, I do not think my workload would have increased. At no time during the requested test runs did the aircraft come close to entering an unusual attitude. Only on the last ex- tra pass [wake pass 13] did the aircraft experience a wing drop that could have resulted in an unusual attitude without normal corrective action. This nor- mal corrective action is similar to what would be used by a pilot in moderate atmospheric turbulence to correct for disturbances. The pilot was also asked whether the turbulence experi- enced inside the wake of turbine T1 was similar to atmo- spheric turbulence he experienced outside the wake on this flight and on other flights not in proximity to wind farms. He replied, “I would say that it was the same.” Finally, the pilot was asked whether he would have any concerns about flying in wake turbulence from a turbine again, to which he replied that he would not. 3.3 Video recordings during wake passes Each wake pass was recorded by a GoPro HERO10 Black video camera mounted rigidly at the back of the cabin, fac- ing forward. Video files are available for viewing and down- load at https://doi.org/10.5281/zenodo.11085457 (Rogers, 2024a). Each video begins prior to entry into the wake of tur- bine T1 and ends after the aircraft exits the wake. The videos were all filmed while the aircraft was below the turbine tip heights. Examination of the videos reveals the light or very light turbulence experienced when outside the wake of T1 at the beginning and end of each video segment, corroborating the description provided by the pilot. The turbulence expe- rienced in the middle of each video segment as the aircraft passes through T1’s wake can be compared with this back- ground level of turbulence to assess the additional level of disturbances to the aircraft motion caused by flight through T1’s wake. Review of the video evidence shows that there was no no- ticeable change in the magnitude of the aircraft perturbations as the aircraft flew through the wake in passes 1, 2, 3, 5, 6, 8, and 9 (flown at 13.5, 10.5, 5.6, 15.6, 10.9, 5.5, and 5.3 RD, Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1861 respectively). Several of these wake passes (1, 2, 5, and 6) were flown beyond 10 rotor diameters from the turbine, while others (passes 3, 8, and 9) were flown closer than 6 rotor di- ameters. In videos from wake passes 4, 7, 10, 11, 12, and 13 (flown at 2.9, 5.0, 4.5, 3.8, 1.6, and 2.1 RD, respectively), it is possible to notice some slight perturbations to the air- craft as it passes through the wake. In wake passes 4, 7, 10, and 11, the perturbations are only very slightly noticeable. In passes 12 and 13, which were performed closer than two ro- tor diameters from the turbine, brief roll perturbations as well as small vertical disturbances are experienced. These distur- bances were short-lived and, in the video, seemed to be easily corrected by the pilot. The video in these trials generally cor- roborates the pilot’s descriptions of the turbulence magnitude and control actions required. The video evidence did not show increased pilot workload or control action when flying through the wakes. Pilot con- trol action and workload appeared to remain the same during the flight trials, regardless of whether flying inside or outside the wake. This video evidence again corroborates the pilot feedback provided after the flight trials. 4 Discussion The primary goal of this study is to measure the level of tur- bulence experienced by a general aviation aircraft in the wake of a utility-scale wind turbine and to compare the distur- bances experienced to relevant thresholds to assess whether a safety hazard existed. When flying through turbulence, air- craft may encounter risks from both a loss-of-control stand- point and a structural-load standpoint. Thus, both of these factors must be considered when assessing the risk to the air- craft as it flies through the wake. For the purposes of this study, the risk of aircraft loss of control is measured by com- paring the roll and pitch disturbances experienced by the aircraft as it traverses the wake to the definition of an up- set as defined by the FAA Airplane Flying Handbook (FAA, 2021b). An upset condition may be a precursor to loss of con- trol per the FAA (2021b) and therefore serves as a suitable benchmark. Aircraft loads experienced in the wake are com- pared to the aircraft limit load factor, which is defined by the FAA (2021b) as “the limit load [that] can be sustained with- out compromising the integrity of the aircraft structure”. For normal-category aircraft (such as the Cessna 206 and most other general aviation aircraft), the positive limit load factor is +3.8 g and the negative limit load factor is −1.52 g (the positive limit is reduced to +3.5 g with flaps extended). The flight data shown in Figs. 7–13 and the Supplement, as well as the video evidence and pilot feedback, support the conclusion that the aircraft did not experience a safety haz- ard when flying through the wake, even in close proximity to the turbine. First, consider the load factors experienced in the wake. For wake passes beyond six rotor diameters, there was no noticeable difference between the load factors experi- enced inside the wake and outside the wake. For flights flown closer than six rotor diameters, there was a very slight in- crease in the load factor perturbations on the order of 0.1– 0.2 g beyond the loads observed in steady-state flight outside the wake of T1. Table 5 shows the maximum and minimum load factors experienced within 5 s of entering and exiting the wake regions (using the 2 RD wake width criterion used in Figs. 5–12). This 5 s buffer guards against uncertainty in predicting where the wake was at the time the aircraft passed through it. The maximum load factor recorded was 1.30 g (wake pass 10), and the minimum was 0.57 g (wake pass 11). These load factor disturbances did not approach the load fac- tor limits of a normal-category aircraft. To explore this further, consider the load factor plots from the flight trials flown beyond 6 RD from turbine T1 in Fig. 13 and the load factor plots from the trials flown closer than 6 RD in Fig. 14. Examining Fig. 13 first, which covers wake passes 1, 2, 5, and 6, it is evident that the load factor pertur- bations inside the dashed lines are of similar magnitude to those experienced throughout the remainder of the trial, out- side the wake. Next, examining Fig. 14, there is still no large difference between the load factor perturbations experienced inside the wake region and outside the wake region. How- ever, in some (but not all) of the wake passes in Fig. 14, the maximum or minimum load factors experienced during the trials occurred in or near the predicted wake region (either inside or near the dashed-line region). This occurs in wake passes 4, 7, 9, 11, 12, and 13 but not in wake passes 3, 8, and 10. For instance, in wake pass 9, the load factor reaches its lowest point of about 0.71 g inside the wake region. How- ever, this is only 0.1 g lower than the minimum load factor that occurs outside the wake region in level flight and is still extremely far from the limit load factor of −1.52 g. Likewise, in wake pass 11, the load factor reaches a minimum value of 0.57 g near the end of the wake region. This load factor mag- nitude is only about 0.1 g less than the 0.68 g disturbance experienced earlier in the flight outside the wake of T1. Another way to view these data is in terms of the max- imum and minimum load factors experienced during each trial as a function of the closest distance of approach of each wake pass. These data are shown in Figs. 15 and 16. Al- though there is some scatter in the data, both figures show noticeable trends where larger load factor disturbances (both maximum and minimum) are experienced as the aircraft flies closer to the turbine, with maximum load factors reaching 1.25–1.30 g and minimum load factors reaching 0.5–0.75 g within 4 RD of the turbine. In summary, at distances greater than six rotor diameters, there was no noticeable effect of the wake in the recorded load factor data. At distances closer than six rotor diame- ters, a slight increase in vertical acceleration disturbances was noted in some of the wake passes, but they were of such a small magnitude that they did not pose a safety risk to the aircraft. Although the trends showed that higher load dis- turbances were experienced as the aircraft got closer to the https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1862 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 10. Sensor data from wake pass 9 (closest distance of approach is 5.3 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. Table 5. Maximum state disturbances in or near the turbine wake. Wake Minimum aircraft– Maximum roll angle Maximum pitch angle Max/min load pass turbine distance magnitude (°) magnitude (°) factor (g) 1 1843 m (13.5 RD) 4.2 2.6 1.24/0.76 2 1428 m (10.5 RD) 5.5 2.5 1.15/0.82 3 768 m (5.6 RD) 5.8 3.1 1.21/0.81 4 397 m (2.9 RD) 4.7 4.9 1.27/0.67 5 2125 m (15.6 RD) 10.3 4.7 1.17/0.81 6 1480 m (10.9 RD) 3.0 3.1 1.18/0.85 7 676 m (5.0 RD) 7.5 3.0 1.28/0.70 8 752 m (5.5 RD) 5.3 4.2 1.17/0.81 9 721 m (5.3 RD) 6.4 5.3 1.22/0.71 10 607 m (4.5 RD) 5.6 2.8 1.30/0.74 11 521 m (3.8 RD) 6.2 3.9 1.22/0.57 12 223 m (1.6 RD) 11.4 5.4 1.26/0.71 13 282 m (2.1 RD) 11.5 3.9 1.25/0.67 Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1863 Figure 11. Sensor data from wake pass 12 (closest distance of approach is 1.6 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. turbine, the observed load factors were always well within the bounds of the limit load factor of +3.8 g (flaps up) or +3.5 g (flaps down) and −1.5 g. Furthermore, in light- ambient-turbulence conditions when the aircraft experiences similar loads due to atmospheric turbulence, loads of this magnitude may not be noticeable to the pilot or passengers. Turning to orientation disturbances, Table 5 shows the maximum roll and pitch magnitudes recorded inside the wake during each wake pass. As shown in this table, the maximum roll and pitch angles experienced were bounded by ±11.5° in roll and ±5.3° in pitch. Figures 17 and 18 show the maximum roll angle and pitch angle magnitudes versus the minimum aircraft–turbine separation distance for all wake passes. Except for the outlier in wake pass 5, there is a clear trend in the results showing that flying closer to the turbine results in larger maximum roll and pitch angle devia- tions. Regarding the higher roll angle disturbance in wake pass 5 (performed at 15.6 RD), it should be noted that the roll angle deviation was higher than other passes beyond 10 RD, but the maximum pitch angle and load factor were not. This is in contrast to, for instance, wake passes 12 and 13 where the roll, pitch (for pass 12), and load factor disturbances were all higher than wake passes performed at farther distances. Thus, it may be hypothesized that a random roll disturbance due to atmospheric turbulence happened to impact the air- craft during the time when it flew through the wake region in wake pass 5, which was separate from and unrelated to any turbine-added turbulence. Random roll disturbances of this magnitude may, for instance, be caused by updrafts due to ground heating that lead to a higher angle of attack on one side of the wing compared to the other. To assess the safety risk imposed by the roll and pitch deviations in Table 5, consider that large orientation distur- bances may present a risk to aircraft safety by inducing what is known as loss of control in flight (LOC-I). According to the European Union Aviation Safety Agency, LOC-I is “an extreme manifestation of a deviation from intended flight path” (EASA, 2024). LOC-I may result from environmen- tal factors (including severe turbulence), mechanical factors, human factors, or stall-related factors (FAA, 2021b). When considering the effects of turbulence in particular, LOC-I may follow from what is described as an upset, accord- ing to the FAA Airplane Flying Handbook (FAA, 2021b). The handbook describes an upset condition as pitch attitude greater than 25° nose up, pitch attitude less than 10° nose down, or roll attitude greater than 45° in either direction. Air- craft attitude deviations that approach or exceed these limits https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1864 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 12. Sensor data from wake pass 13 (closest distance of approach is 2.1 RD). Dashed lines indicate the time period during which the aircraft was present in the estimated wake region of turbine T1. Figure 13. Load factor vs. time for wake passes farther than 6 RD. Load factors experienced inside the wake region in the dashed lines are indistinguishable from those experienced outside the wake region. Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1865 Figure 14. Load factor vs. time for wake passes closer than 6 RD. Slight increases in load factor magnitudes when inside the wake are evident in wake passes 4, 7, 9, 11, 12, and 13 but not in wake passes 3, 8, and 10. would be cause for concern, as they pose a risk of LOC-I. However, the orientation disturbances observed in the wake passes were below these limits by a wide margin. The low magnitude of orientation disturbances suggests that, while the turbulence in the wake did affect the aircraft, the effect was small enough for it not to pose a risk from a loss-of- control perspective. It should be noted that the wake passes were performed not in a location isolated to a single turbine wake but rather in an area roughly downwind of the entire 98-turbine Whitla Wind facility. Figure 4 shows that there are multiple tur- bines in close proximity to the south and southeast of turbine T1, including a turbine only 500 m to the south and another 1500 m to the southeast. The turbine wakes would be ex- pected to intersect and influence one another to some extent given the wind direction from the southeast, producing cu- mulative wake effects in the vicinity of the wake passes. Even in the presence of cumulative wakes from multiple turbines, the pilot noticed only “very light turbulence” anytime he was within 3–4 km downwind of the wind farm at altitudes be- low the turbine tip heights (above the tip heights the air was smooth). This type of turbulence was classified as “negligi- ble” by the pilot. An interesting avenue for future work may be to model the wakes of all turbines in the northern section https://doi.org/10.5194/wes-9-1849-2024 Wind Energ. Sci., 9, 1849–1868, 2024 1866 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts Figure 15. Maximum load factor vs. minimum aircraft–turbine dis- tance. Figure 16. Minimum load factor vs. minimum aircraft–turbine dis- tance. of Whitla depicted in Fig. 4 to determine the extent of over- lapping wakes that occurred during the flight, from which more definitive conclusions could be made regarding the air- craft flight path through overlapping wake regions. Such an analysis is beyond the scope of this initial work but would be a useful future extension. 5 Conclusions In order to experimentally assess the risk of general aviation aircraft flight through wind turbine wakes, a series of instru- mented tests were performed in which a Cessna 206 aircraft flew through the wake of an operating utility-scale wind tur- bine at different downwind distances. Recorded flight data show that the maximum bank angle magnitude experienced Figure 17. Maximum roll angle magnitude vs. minimum aircraft– turbine distance. Figure 18. Maximum pitch angle magnitude vs. minimum aircraft– turbine distance. inside the wake region was 11.5° and the maximum pitch angle magnitude was 5.4°, both of which are well below the limit for an upset condition as defined in pilot training guidelines. The load factors experienced by the aircraft were between 0.57 g and 1.30 g, which are also well within the limit load factor boundaries for FAA normal-category air- craft. Sensor data, in combination with pilot feedback, sug- gest that light or very light turbulence may be experienced in the wake at distances farther than four to six rotor diameters downwind of the turbine and that light or moderate turbu- lence may be experienced at distances closer than 4–6 rotor diameters. The pilot did not report (nor did the flight data suggest) the presence of severe or extreme levels of turbu- lence in the wake region, which would have been a cause for Wind Energ. Sci., 9, 1849–1868, 2024 https://doi.org/10.5194/wes-9-1849-2024 J. D. Rogers: Experimental evaluation of wind turbine wake turbulence aviation impacts 1867 concern from a loss-of-control or structural-integrity stand- point. Overall, the results from this flight experiment showed no evidence of any safety risk to general aviation aircraft from turbine-added turbulence in the wake region. Code availability. The code used to process the flight data and the plotting routines were built in MATLAB and are available from the author upon request. Data availability. The raw data used in this pa- per are publicly available at the following DOI: https://doi.org/10.5281/zenodo.13787764 (Rogers, 2024b). They are openly available for immediate download. Video supplement. The video supplement is available at https://doi.org/10.5281/zenodo.11085457 (Rogers, 2024a). Supplement. The supplement related to this article is available online at: https://doi.org/10.5194/wes-9-1849-2024-supplement. Competing interests. The author has declared that there are no competing interests. Disclaimer. Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, pub- lished maps, institutional affiliations, or any other geographical rep- resentation in this paper. While Copernicus Publications makes ev- ery effort to include appropriate place names, the final responsibility lies with the authors. Acknowledgements. The author would like to thank James Os- ness, Santi Churphongphun, and Matthew Davis from Capital Power Generation Services and Gavin Fitch from McLennan Ross for facilitating these flight tests. In addition, the author would like to acknowledge the contributions of Ian Watson and Al Christou from Edmonton Skydive in planning and carrying out the wake pass flights. Review statement. This paper was edited by Etienne Cheynet and reviewed by two anonymous referees. 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