Captain Knueppel was the pilot of a B-26B Invader bomber with the 730th Bomber Squadron, 452nd Bomber Wing. On April 22, 1951, while on an intruder mission striking the marshaling yards at Paup, North Korea, the aircraft was hit by anti-aircraft fire setting the left engine on fire. The crew bailed out. He was listed as Missing in Action and was presumed dead on January 31, 1954. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.