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He was born on the 19th of February 1916 in Königsberg, Germany. In 1935 he joined the German military as a trainee in the Luftwaffe, where he rose to the rank of Oberleutnant in 1939. At the outbreak of the war, Fiehn transferred to the Kriegsmarine where he was given the rank of Kapitänleutnant and awarded the Iron Cross 2nd Class. In 1942 he was given commission of U-191. After departing form Bergen on the 17th of March 1943, U-191 was attacked and destroyed by HMS Hesperus, and Fiehn died along with the entire crew on April 23, 1943.