On patrol on 3 July 1943, Cundy sighted a U-boat on the surface several miles ahead. With his gunners opening fire with machine guns, he dived to attack and launched the Hedgehog anti-submarine weapon onto the submarine, , which had recently left Brest on its fourth cruise. He made a second attack despite his aircraft suffering serious damage from return fire. Cundy then dropped four depth charges which straddled the submarine. Cundy made a further attack as the submarine settled low in the water. As he circled after his third attack, it was seen that the U-boat had sunk and crewmen were in the water. Cundy nursed his aircraft back to St Eval on three engines; there was also damage to the fuel tanks and the aircraft's tail, but he made a safe landing. This was the first use of the Hedgehog anti-submarine weapon from an aircraft.