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6/17/21 

Pilot/Controller Glossary 

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 VORTICES.) 
(Refer to AIM.) 

WARNING AREA

 

(See SPECIAL USE AIRSPACE.) 

WAYPOINT

 A predetermined geographical posi-

tion 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 SIGMET.) 

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 53

rd

 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

 

a. 

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. 

b. 

In conjunction with a weather deviation 

clearance, requires the pilot to determine when he/she 
is clear of weather, then execute ATC instructions. 

c. 

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 deter-
mined 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 

PCG W