Airfield Wind Advisory Systems for General Aviation
- Tuesday, 09 February 2010
These systems reduce risks associated with unknown winds during takeoffs and landings.
An Airfield Wind Advisory System (AWAS) includes a self-contained weather station, located at an airfield, that measures speed and direction of the wind, the temperature, the barometric pressure, and the humidity. This ground station digitizes these measurements and transmits the measurement data in real time, via radio, to portable units in aircraft cockpits. The portable units automatically detect the data and display the information to pilots. In 1999, a prototype of the AWAS system was demonstrated to function successfully in tests at Kennedy Space Center's Space Shuttle Landing Facility in 1999.
An AWAS ground station is designed for automatic operation with minimal maintenance, using either alternating current from a power line or power from a solar photovoltaic array with battery backup. AWAS ground stations can include solid-state transducers with no sliding parts (e.g., sonic anemometers and/or strain-gauge wind sensors) for high reliability but could also be designed to take advantage of previously installed anemometers, weather vanes, and other weather-measurement devices. AWAS ground stations can also contain Web server computers, which transmit the information to wide-area networks over intranet or Internet links upon demand. The transmitted information can include not only the data from the AWAS weather measurements but also Global Positioning System (GPS) data and/or other geophysical data from measurement devices to support scientific observations.
An AWAS airborne unit includes an antenna, receiver, microprocessor, data-storage elements, a power supply, and a back-lit liquid-crystal display device. The simplest version provides a no-clutter display that can be read by the pilot in a one-second glance, showing the following data:
- The identity of the airfield [represented by a three- or four-character label assigned by the Federal Aviation Administration (FAA) to every airfield or helipad],
- The direction of the wind in degrees measured from magnetic north,
- The steady-state and peak gust wind speeds in knots, and
- The most favorable runway for takeoff and landing.
In order to select the most favorable runway, the microprocessor in the airborne unit compares the wind vector with the runway heading (runway headings are stored in an internal database) and calculates the headwind and crosswind values. The runway indicated on the display device is the one with the highest headwind and lowest crosswind components. A switch enables the pilot to command the display of the headwind and crosswind data for the selected runway.
A standby switch shuts off the display device, but allows the remainder of the airborne unit to operate in a listening mode to conserve energy when not in range of an airfield. Advanced versions may include keypads to enable pilots to select specific airfields or runways or to enter special data or queries.
Specialized AWAS software, which may also be incorporated into wireless personal data assistants (PDAs) and hand-held GPS devices, could provide graphical depictions of airfield diagrams and winds. The Internet version of the AWAS is intended for use with wireless PDAs and cellular telephones that provide access to the Internet.
The AWAS can also be used to disseminate weather information in real time in applications other than aviation. Examples of tion in real time in applications other than aviation. Examples of potential users include the National Weather Service, the military, commercial weather services, marine and agricultural concerns, the Federal Emergency Management Agency, and emergency services (e.g., firefighters during forest fires).
The receiver in an AWAS airborne unit automatically detects the signal from an AWAS ground unit at an airfield and activates the display device when in range of the airfield. The display helps the pilot to select the optimum approach to the airfield while 8 to 10 miles (about 13 to 16 km) out, thus saving three to five minutes per landing. Under visual flight rules (VFR), approaching an uncontrolled field, the pilot is required to make contact with the UNICOM of the airfield to request wind and runway information (UNICOMs are non-government communication facilities that provide airport information at some airports). If there is no answer, as is often the case, the pilot must fly over the airfield to view the windsock while visually avoiding other traffic, estimating the wind speed, and entering the traffic pattern to land, trusting that other pilots are also making the same assumptions about the winds and the landing runway. This procedure typically involves extra maneuvering and backtracking. With the AWAS, the pilot knows the wind at the airfield and knows which runway is most favorable, and is therefore able to fly a more direct approach to the landing pattern, without guesswork or extra maneuvering. More importantly, the pilot knows the headwind and crosswind for each runway without question, and will therefore be able to plan ahead for the appropriate aircraft alignment and control inputs.