This piece originally ran in the July 2021 issue of EAA Sport Aviation magazine.
This Mk IIA course and speed calculator was a mechanical computer used by British and Commonwealth air forces during World War II. The computer, equipped with both a linear and a circular slide rule for drift, time, and distance calculations, was used until the E-6B — generally referred to by the British as the Dalton computer — saw widespread adoption. EAA’s example bears both Royal Air Force and Royal Canadian Air Force serial numbers, and is believed to be a wartime production model.