Operation Obviate was the October 29, 1944, RAF Bomber Command attack on the World War II German battleship Tirpitz. 37 Avro Lancasters -- 18 from No. 9 Squadron RAF, 18 from No. 617 Squadron RAF and a film unit aircraft from No. 463 Squadron RAF -- were dispatched from RAF Lossiemouth in Scotland to bomb the Tirpitz moored near the Norwegian port of Tromso. The removal of the Lancasters' mid-upper turrets and other equipment and the installation of extra fuel tanks gave each aircraft a total fuel capacity of 2,406 gallons, allowed the Lancasters to carry complete the 2,250 mile flight. After a weather reconnaissance deHavilland Mosquito reported the target area free of cloud, the Lancasters formed up at a lake near the bay in which the Tirpitz was moored and commenced the attack. However, the wind changed and a bank of cloud covered the Tirpitz 30 seconds before the first Lancaster was ready to bomb. 32 aircraft released Tallboy bombs on the estimated position of the battleship but no direct hits were scored (one near miss bent a propeller shaft). One of the No. 617 Squadron RAF Lancasters was damaged by flak and crash-landed in Sweden -- the crew was later returned to Britain. Operation Catechism subsequently sunk the Tirpitz.