WA-

(See AIRMET.)

(See WEATHER ADVISORY.)

WAAS-

(See WIDE‐AREA AUGMENTATION SYSTEM.)

WAKE TURBULENCE- A phenomenon that occurs when an aircraft develops lift and forms a pair of ­counter‐rotating vortices.

(See AIRCRAFT CLASSES.)

(See AIRCRAFT WAKE CATEGORIES.)

(See VORTICES.)

(Refer to AIM.)

WARNING AREA-

(See SPECIAL USE AIRSPACE.)

WAYPOINT- A predetermined geographical position used for route/instrument approach definition, progress ­reports, published VFR routes, visual reporting points or points for transitioning and/or circumnavigating ­controlled and/or special use airspace, that is defined relative to a VORTAC station or in terms of ­latitude/longitude coordinates.

WEATHER ADVISORY- In aviation weather forecast practice, an expression of hazardous weather conditions ­not predicted in the Aviation Surface Forecast, Aviation Cloud Forecast, or area forecast, as they affect the ­operation of air traffic and as prepared by the NWS.

(See AIRMET.)

(See GRAPHICAL AIRMEN'S METEOROLOGICAL INFORMATION.)

(See SIGMET.)

WEATHER RADAR PRECIPITATION INTENSITY- Existing radar systems cannot detect turbulence, ­however, there is a direct correlation between turbulence intensity and precipitation intensity. Controllers must ­issue all precipitation displayed on their user display systems. When precipitation intensity is not available, ­controllers will report intensity as UNKNOWN. When precipitation intensity levels are available, they will be ­described as follows:

1. LIGHT (< 26 dBZ)
2. MODERATE (26 to 40 dBZ)
3. HEAVY (> 40 to 50 dBZ)
4. EXTREME (> 50 dBZ)

   WEATHER RECONNAISSANCE AREA (WRA)- A WRA is airspace with defined dimensions and published ­by Notice to Airmen, which is established to support weather reconnaissance/research flights. Air traffic control ­services are not provided within WRAs. Only participating weather reconnaissance/research aircraft from the ­53rd Weather Reconnaissance Squadron and National Oceanic and Atmospheric Administration Aircraft ­Operations Center are permitted to operate within a WRA. A WRA may only be established in airspace within ­U.S. Flight Information Regions outside of U.S. territorial airspace.

   WHEN ABLE-

5. In conjunction with ATC instructions, gives the pilot the latitude to delay compliance until a condition or ­event has been reconciled. Unlike “pilot discretion,” when instructions are prefaced “when able,” the pilot is ­expected to seek the first opportunity to comply.
6. In conjunction with a weather deviation clearance, requires the pilot to determine when he/she is clear of ­weather, then execute ATC instructions.
7. Once a maneuver has been initiated, the pilot is expected to continue until the specifications of the ­instructions have been met. “When able,” should not be used when expeditious compliance is required.

   WIDE‐AREA AUGMENTATION SYSTEM (WAAS)- The WAAS is a satellite navigation system consisting ­of the equipment and software which augments the GPS Standard Positioning Service (SPS). The WAAS ­provides enhanced integrity, accuracy, availability, and continuity over and above GPS SPS. The differential ­correction function provides improved accuracy required for precision approach.

   WIDE AREA MULTILATERATION (WAM)– A distributed surveillance technology which may utilize any ­combination of signals from Air Traffic Control Radar Beacon System (ATCRBS) (Modes A and C) and Mode ­S transponders, and ADS‐B transmissions. Multiple geographically dispersed ground sensors measure the ­time‐of‐arrival of the transponder messages. Aircraft position is determined by joint processing of the ­time‐difference‐of‐arrival (TDOA) measurements computed between a reference and the ground stations' ­measured time‐of‐arrival.

   WILCO- I have received your message, understand it, and will comply with it.

   WIND GRID DISPLAY- A display that presents the latest forecasted wind data overlaid on a map of the ARTCC ­area. Wind data is automatically entered and updated periodically by transmissions from the National Weather ­Service. Winds at specific altitudes, along with temperatures and air pressure can be viewed.

   WIND SHEAR- A change in wind speed and/or wind direction in a short distance resulting in a tearing or ­shearing effect. It can exist in a horizontal or vertical direction and occasionally in both.

   WIND SHEAR ESCAPE- An unplanned abortive maneuver initiated by the pilot in command (PIC) as a result ­of onboard cockpit systems. Wind shear escapes are characterized by maximum thrust climbs in the low altitude ­terminal environment until wind shear conditions are no longer detected.

   WING TIP VORTICES-

   (See VORTICES.)

   WORDS TWICE-

8. As a request: “Communication is difficult. Please say every phrase twice.”
9. As information: “Since communications are difficult, every phrase in this message will be spoken twice.”

   WS-

   (See SIGMET.)

   (See WEATHER ADVISORY.)

   WST-

   (See CONVECTIVE SIGMET.)

   (See WEATHER ADVISORY.)