Achieving the highest quality wind readings is a multi-step process. First, AOC operates a dedicated Calibration Lab and engineer who maintains the sensors used for flight level measurements of parameters such as static pressure, dynamic pressure, sideslip, and angle of attack, and who ensures the instruments are regularly baselined during ground runs through a test chamber using established industry standards.
Next, these sensors are flown on dedicated calibration flights where a series of maneuvers are performed. These include racetracks flown into and with the wind at altitudes ranging incrementally from 1500 feet to 20,000 feet at 180 knots indicated airspeed (IAS), 210 knots IAS and 240 knots IAS, concentric circles at 5000 feet, and yaw maneuvers at 1500 feet and 15,000 feet. The yaw maneuver involves a sequence of 5 degree rudder displacement (until 55 millibars of differential sideslip pressure is achieved). Finally, at 20,000 feet, a sequence of 5 degree pitch up and pitch down maneuvers are performed.
Data gathered from this flight are then analyzed in the office where two minute segments of the racetrack legs are selected, along with the pitch maneuvers, to calculate the Attack Angle (AOA) slope and intercept. Data from the concentric circles and the yaw maneuvers are then analyzed to calculate the Slip Angle (SA) slope and intercept.
The post-flight analysis is a man-hour intensive effort that has recently been automated. This automation has opened the door for further research possibilities involving the use of dynamic AOA and SA coefficients that vary depending upon the regime of flight.
While wind calibration has been a critical aspect of WP-3D operations for decades, in recent years similar wind cal flights have been incorporated into AOC’s Gulfstream G-IV ops in order to achieve research quality flight level winds from that platform as well. Finally, a MET data system, with scientific wind measurements, has been added in the past two years to the NOAA DHC-6-300 Twin Otter aircraft.