Participating in Operation Millenium, a bombing raid on Cologne on the night of 30 May 1942, Manser was captain and first pilot of Avro Manchester bomber 'D' for Dog. As he came over the target, his aircraft was caught in searchlights and although he bombed the target successfully from it was hit by flak. In an effort to escape the anti-aircraft fire he took violent evasive action, this reduced his altitude to only but he did not escape the flak until he was clear of the city. By this time the rear gunner was wounded, the front cabin full of smoke and the port engine overheating. Rather than abandon the aircraft and be captured, Manser tried to get the aircraft and crew to safety. The port engine then burst into flames, burning the wing and reducing airspeed to a dangerously low level. The crew made preparations to abandon the aircraft, by then barely controllable and with a crash inevitable. The aircraft was by now over Belgium, and Manser ordered the crew to bail out but refused the offer of a parachute for himself. He remained at the controls and sacrificed himself in order to save his crew. As the crew parachuted down they saw the bomber crash in flames into a dyke at Bree, Belgium, north east of Genk.