The NTSB is encouraging pilots to review a Safety Alert regarding proper procedures for the use of deice boots.
The Safety Alert states that leading-edge deice boots should be activated as soon as icing is encountered, unless the aircraft flight manual or the pilot's operating handbook specifically directs not to activate them.
The full Alert, “Activate Leading-Edge Deice Boots As Soon As Airplanes Enter Icing Conditions” is available for download by clicking here.