An E6B is an essential tool in all your flight planning calculations.
Very useful to determine fuel consumption, speed, distance traveled, time in route, density altitude, true airspeed corrections due to non-standard atmospheric pressure and temperature calculations, wind correction angles etc.
The E6B has been used by Aviators since the early 1940's by the U.S Army Air Corps to help their navigation calculations due to no radio communications available at that time. Philip Dalton the ingeniously individual thought of this formula and lives on while being remembered as a remarkable Airman.
When tecnology fails, an E6b will have your 6 when its time to get crunching those numbers.