Wash-out To destroy, or badly damage a plane. Variously applied. Sometimes applied to planes obsoleted by the air service. White roses Allied anti-aircraft artillery used high-explosive, which showed white on bursting. Germans used black powder, which showed black. Wind sock A conical strip of cloth on a staff atop the hangars to give pilots wind direction. Wipers Nickname soldiers gave to Belgian town of Ypres. Yaw off To slip off desired direction due to lack of speed or wind resistance. Zoom To pull the nose up sharply and climb at an angle too great to be long sustained.