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 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 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 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