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          w.e.f. - with effect from. Also w.i.e., with immediate effect.
          
 WAAS - Wide Area Augmentation System (in USA)
 
 wake turbulence - wingtip vortices generated behind a wing 
          producing lift. Behind a large heavy aircraft they can be powerful 
          enough to roll or even break up a smaller aircraft.
 
 WAT - weight-and-temperature
 
 wind shear - localised change in wind speed and/or direction 
          over a short distance, resulting in a tearing or shearing effect, 
          usually at low altitude, that can cause a sudden loss of airspeed with 
          occasionally disastrous results if encountered when taking off or 
          landing.
 
 WIP - work in progress.
 
 WP - waypoint.
 
 wt - weight
 
 Wx - weather.
 
 WX NIL - no significant weather, term used in Met reports.
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