'Obviate' was a British air attack on the German battleship Tirpitz at her forward base in German-occupied Norway (29 October 1944).

The attack was undertaken by Royal Air Force heavy bombers in an effort to destroy the damaged battleship after she had moved to a new anchorage near Tromsø in northern Norway, and was the successor partially successful 'Paravane' of 15 September 1944 in which Tirpitz[ had been crippled. As Allied intelligence was unaware that the battleship was no longer able to operate at sea and warships needed elsewhere were being retained in British waters to counter her, it was decided to make another attack. After a period of planning and preparations, 38 British bombers and one film aeroplane departed from bases in northern Scotland during the early hours of 29 October. The attack took place that morning, but was frustrated by clouds over the Tromsø area which made it difficult for the Allied airmen to target Tirpitz with accuracy. The battleship received no direct hits, but was damaged by a bomb that exploded near her hull. A British bomber made a crash landing in Sweden after being hit by Flak fire, and several others were damaged.

The Allies remained committed to sinking Tirpitz after the failure of 'Obviate', and the same basic plan was used for 'Catechism', the next raid on the battleship, which took place on 12 November 1944.

From a time early in 1942, Tirpitz had posed a significant threat to the Allied convoys transporting supplies through the Norwegian Sea to the USSR. Stationed in any of several fjords on the Norwegian north-western coast, the ship was capable of overwhelming the close escort forces assigned to the Arctic convoys. Tirpitz could also possibly be employed in an attempt to enter the North Atlantic to attack Allied convoys making passage to and from the UK, as her sister ship Bismarck had attempted in 'Rheinübung' during May 1941. To counter these threats, the Allies needed to keep a powerful force of warships with the British Home Fleet, and capital ships accompanied most convoys part of the way to the USSR.

Tirpitz was repeatedly attacked by British forces. RAF heavy bombers made four unsuccessful raids on the battleship between January and April 1942 while she was stationed in the Fættenfjord. From March 1943, Tirpitz was based in the Kaafjord spur of the Altafjord in the far north of Norway. During 'Source' on 22 September 1943, she was severely damaged by explosives placed beneath her hull by Royal Navy personnel who had used midget submarines to penetrate the Kaafjord. On 3 April 1944, aircraft from British aircraft carriers attacked Tirpitz in 'Tungsten' and inflicted further damage. This attack had been timed to coincide with the time in which it was believed repairs to rectify the damage caused by 'Source' were nearing completion. A series of subsequent aircraft carrier attacks, 'Mascot' on 17 July and 'Goodwood' (ii) between 22 and 29 August, were unsuccessful.

As it was now evident that further aircraft carrier raids would be fruitless, as a result of the shortcomings of the Fleet Air Arm’s aircraft and their armament, responsibility for sinking Tirpitz was transferred to Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command, and on 15 September Nos 9 and 617 Squadrons attacked the battleship in the Kaafjord in 'Paravane'. This operation employed Avro Lancaster four-engined heavy bombers armed with Tallboy bombs and 'Johnnie Walker' self-propelled mines, and was mounted from Yagodnik in the USSR. Tirpitz was struck by a single Tallboy, which caused extensive damage to her bow and rendered her unfit for combat.

A meeting involving Grossadmiral Karl Dönitz, commander of the German navy, was held in Berlin on 23 September to discuss Tirpitz. Dönitz was informed that it would take nine months to repair the ship, and that all the work had to be done in the Kaafjord as the battleship would be extremely vulnerable if she tried to make the passage to any major port. As Soviet forces were advancing rapidly toward northern Norway at that time, Dönitz judged that it was not feasible either to restore the ship to ocean-going service or to retain her in the Kaafjord. He decided therefore to use Tirpitz as a floating artillery battery for the defence of Tromsø against amphibious landings and to bolster a defensive line which was being prepared in the Lyngenfjord area. Dönitz also expressed hope that retaining the ship in commission would 'continue to tie down enemy forces and by her presence…confound the enemies' intentions'.

Konteradmiral Rudolf Peters, the commander of the German naval forces in northern Norway, was ordered to base Tirpitz at a location near Tromsø where the water was shallow enough to prevent the battleship from sinking completely if she suffered further damage, and the anchorage selected was just off the coast of the small island of Håkøya, 3.5 miles (5.6 km) to the west of Tromsø. This location lacked the natural defences Tirpitz had enjoyed at her previous Norwegian bases where she had sheltered in fjords which were flanked by steep mountains rising from the sea: this had made it difficult for attacking aircraft to spot and target the battleship. Instead, the terrain around Håkøya was fairly flat, and it was near the open sea. To prepare Tirpitz for the 170-mile (275-km) voyage to the south-west, a repair ship was sent to the Kaafjord and helped the battleship’s crew to weld steel plates over the hole in her hull.[16]

The Allies were able to confirm by photo reconnaissance, signals intelligence and Norwegian agents that Tirpitz had been badly damaged in 'Paravane', but were unsure if this had rendered the battleship permanently unserviceable. Dönitz’s decision of 23 September was also not known. As a result, the Royal Navy continued to assign capital ships to the Home Fleet to guard against the possibility of Tirpitz putting to sea.

Tirpitz's passage to Tromsø took place during 15/16 October. The battleship departed the Kaafjord at 12.00 on 15 October under the escort of several warships. While Tirpitz was able to move under her own power, the flotilla included ocean-going tugs tasked with towing the battleship if her damaged bow broke off. The German force slowly proceeded south, and Tirpitz eventually arrived at her berth off Håkøya at 15.00 on 16 October. Soon after reaching Håkøya, 600 men most of them of the engine room crew, were removed from the ship: this left about 1,700 men on board.

The Allies responded with alacrity to Tirpitz's redeployment. Norwegian agents of the British Secret Intelligence Service in the Kaafjord and Tromsø areas provided reports during the battleship’s journey, Egil Lindberg radioing the UK on 16 October to confirm the ship’s arrival in the Tromsø area. In response to these reports, the British aircraft carrier Implacable was dispatched from the Home Fleet’s main base at Scapa Flow in the Orkney islands group on 16 October for the task of confirming Tirpitz's location, and the RAF also flew photo-reconnaissance sorties over the Tromsø area. As a precaution in case Tirpitz was able to conduct combat operations, the battleship King George V was diverted from an impending deployment to the Indian Ocean to reinforce the Home Fleet until repairs had been completed on its sole battleship, Duke of York.

British reconnaissance aircraft located Tirpitz during the afternoon of 18 October. The first aeroplane to arrive over the area was a de Havilland Mosquito of the RAF’s No. 540 Squadron operating from Dyce in north-eastern Scotland. The Mosquito’s crew took photographs of the battleship from a high altitude, and returned to base despite damage inflicted by Flak guns. Shortly after this, Implacable's Fairey Firefly aircraft reconnoitred the Tromsø area, several taking low-altitude photographs of Tirpitz off Håkøya; these aircraft also came under German Flak fire, but none was damaged. During the evening of 18 October, Implacable's commander sought permission to attack Tirpitz on the following day, but Admiral Sir Bruce Fraser, commander of the Home Fleet, refused on the grounds that the carrier had sailed without any of the Supermarine Seafire fighter-bombers needed to suppress the Flak guns. Fraser was also aware from the attacks on Tirpitz in the Kaafjord that the carrier’s two squadrons of Fairey Barracuda dive-bombers would probably not be able to inflict significant damage, and that further raids by RAF heavy bombers were required.

The RAF began preparations to attack Tirpitz again immediately after she was confirmed to be at Tromsø. As this was within range of Lancaster bombers flying from northern Scotland if they were fitted with extra fuel tanks and other modifications, this operation would be simpler to conduct than 'Paravane', but even so required a return flight of 2,252 miles (3624 km).

The Lancaster aircraft of Nos 9 and 617 Squadrons were modified to extend their range. All of the aircraft selected for the operation were fitted with uprated Rolls-Royce Merlin 24 engines, which were hurriedly obtained from maintenance units and airfields across the UK. Each Lancaster also carried two extra fuel tanks inside their fuselage: a tank usually fitted to Vickers Wellington bombers and a type of drop tank used by the Mosquito: the extra fuel put the aircraft greatly above their authorised maximum take-off weights. To address this, the bombers' forward and dorsal gun turrets were removed, along with 3,000 rounds of ammunition from the rear turret, the flare chute, the armour plating around the pilot’s seat and some of the oxygen and nitrogen bottles. The reduction in armament left the bombers very vulnerable to German fighter attack, and they would have to fly without escort as no British fighter possessed the range to reach and return from Tromsø.

The operational order for the attack on Tirpitz was issued by No. 5 Group on 24 October. It specified that the battleship was to be attacked by 36 Lancaster bombers, Nos 9 and 617 Squadrons each contributing 18 of the aircraft. Another Lancaster, in this instance of No. 463 Squadron, an RAAF film unit, would also take part but carry no bombs. As the 'Johnnie Walker' self-propelled mines had failed during 'Paravane', only Tallboy bombs were to be used, each bomber carrying one of the weapons. This 12,000-lb (5443-kg) bomb was the largest in service with the RAF, and capable of penetrating heavily armoured targets.

The two squadrons were to remain at their home bases until it was judged that weather conditions over Tromsø were likely to be suitable. Their aircraft would then be loaded and flown to the RAF bases at Kinloss, Lossiemouth and Milltown in northern Scotland. The operational order stated that the attack was required as 'it appears likely that the Germans may attempt to get the battleship back to a base in Germany, where the necessary repairs and refit can be carried out'.

After the decision was made to launch the attack, the bombers were to fly individually across the Norwegian Sea, and cross the Norwegian coast at a point between Mosjøen and Namos, where the electronic warfare aircraft of No. 100 Group had found a gap in German radar coverage. The bombers were then to rendezvous over the Torneträsk lake in northern Sweden despite the fact that this flight path involved a violation of Sweden’s neutrality, but was selected as it would allow the bombers to approach Tromsø from the south-east, which it was believed the Germans would not expect. After the rendezvous had been completed and if the weather conditions remained suitable, the bombers would proceed to Tromsø and attack Tirpitz if she could be visually spotted or, if obscured, her location confirmed relative to unobstructed landmarks. If these criteria were not met, the aircrew were not to bomb. Following the attack, the bombers were to fly directly back to Scotland.

Group Captain Colin McMullen was selected to command the ground crew and aircrew before the start of the attack, a role he had also played in 'Paravane', and the commanders of the two squadrons, Wing Commanders James Bazin of No. 9 Squadron and 'Willie' Tait of No. 617 Squadron, would control their units in the air. The attack was designated 'Obviate'.

Preparations were also made to recover aircraft or crews if any of the bombers ran short of fuel or suffered battle damage. The RAF base at Sumburgh in the Shetland islands group was selected as the emergency airfield for the return journey from Tromsø. If any of the bombers experienced engine problems or lacked sufficient fuel to return to the UK, they were to proceed to the Soviet airfields at Vaenga or Yagodnik. The Soviets were not informed of this until 29 October, the day of the attack. No. 5 Group also requested that three Royal Navy destroyers be stationed along the return route from Tromsø to rescue the crews of any bombers forced down over the Norwegian Sea.

The British were able to draw on two intelligence sources other than photo reconnaissance flights to monitor the German forces at Tromsø. Lindberg was based in that area and provided updates on Tirpitz by radio. As Lindberg worked in the local meteorological office, he also regularly reported on weather conditions. The other source of intelligence was German radio traffic decrypted by Allied codebreakers.

Tirpitz was particularly vulnerable to attack in the period immediately after she arrived at Håkøya. None of the many smoke generators and Flak guns, which had protected her against air attack in the Kaafjord, were initially available because they had not yet been shipped to the south. The only protection available was from the battleship’s own armament, two Flak ships anchored nearby, and several Flak batteries in the Tromsø area. British intelligence believed that there were 16 heavy and 16 light Flak guns in the area at the time of 'Obviate'. The battleship was also surrounded by torpedo nets as a protection against torpedoes and midget submarine attack. No fighter aircraft were stationed nearby. The depth of water below Tirpitz at her mooring was greater than anticipated, leaving the ship vulnerable to capsizing. Owing to the space needed by the torpedo nets, it was not possible to move Tirpitz closer to the shore. Instead, work began on building up the seabed using earth and gravel two weeks after she reached Håkøya.

The battleship’s crew expected further air attacks, and doubted that the ship would survive them. This, and a belief that Germany would lose the war, led to poor morale. The civilian population of Tromsø also expected air attacks after Tirpitz arrived, and were concerned about the prospect of being accidentally bombed.

On 26 October, Bomber Command advised the Admiralty that 'Obviate' would be launched as soon as weather conditions permitted after the night of 27 October. No. 5 Group also informed the two squadrons that day to make final preparations for the mission. This included loading the Tallboy bombs. In the evening of 27 October, the aircrew selected for 'Obviate' were briefed and told that they would fly to the designated Scottish airfields during the morning of the next day.

During the morning of 28 October, 20 Lancaster bombers from each squadron flew from their home bases to Kinloss, Lossiemouth and Milltown. A photo-reconnaissance Mosquito flew over Tromsø that morning, and confirmed that Tirpitz was still moored off Håkøya and that weather conditions remained favourable. As forecasts for the next day indicated that the good weather would continue, the attack was set for 29 October. Another Mosquito flew over the Tromsø area at midnight on 28 October, and reported that conditions remained clear.

The attack force departed Scotland in the early hours of 29 October. No. 9 Squadron dispatched 20 Lancasters, the aircraft taking off between 01.18 and 02.55, and No. 617 Squadron contributed 19 aircraft, which departed between 01.03 and 02.10. The No. 463 Squadron Lancaster accompanied the attack aircraft.

The approach flight was uneventful as the bombers flew individually across the Norwegian Sea at an altitude of 1,500 ft (460 m), and began climbing to 10,000 ft (3050 m) after they crossed the Norwegian coast. One of No. 9 Squadron’s aircraft experienced engine problems during the climb, and turned back. The bombers made rendezvous over the Torneträsk lake, formed up into their attack formations and headed toward Tromsø. During this stage of the flight the aircraft climbed to their bombing altitudes of between 13,000 and 16,000 ft (3960 and 4875 m).The aircraft were engaged by Swedish anti-aircraft guns when they passed near Abisko, but were not hit.

The attack was then frustrated by cloud cover. While the weather remained fine during the approach to Tromsø, the surrounding area was mainly covered by cloud. Tirpitz was visible when the bombers arrived over the Tromsø area, but was obscured before any of them were in position to release their bombs. Despite their orders to bring their Tallboys back to the UK if visual bombing was not possible, almost all the bombers attacked.

The first bombs were dropped at 07.49, No. 617 Squadron leading the way. Sixteen of this squadron’s aircraft released Tallboys aimed at Tirpitz's estimated position, several making several runs before attacking. One of the three Lancasters which did not bomb made four runs over Tromsø before Tait gave its pilot permission to break off the attack. No. 9 Squadron began its attack six minutes after No. 617 Squadron, 17 Lancaster bombers dropping Tallboys. Like No. 617 Squadron, several of No. 9 Squadron’s aircraft made several runs over Tromsø, one conducting five approaches before attacking. At least two of the No. 9 Squadron crews were able to take visual aim at Tirpitz through gaps in the cloud, but the others aimed at the battleship’s estimated position. Of the two No. 9 Squadron aircraft which did not bomb, one made two passes over Tromsø. The last bomb was dropped at 08.07.

The Germans started to fire at the bombers as the latter approached Tirpitz. Four of No. 9 Squadron;s aircraft, at least one of No. 617 Squadron, and the No. 463 Squadron film aeroplane were damaged by Flak fire. The No. 617 Squadron Lancaster lost so much fuel from two hits that its pilot judged that it would be impossible to reach either Sumburgh or the USSR. Instead, he decided to put his machine down in northern Sweden so that the crew could avoid being made prisoners of war. The aeroplane made a crash landing in a bog near Porjus: all of the crew survived, and were eventually repatriated to the UK by the Swedes. The damage inflicted on the other Lancaster aircraft was insignificant.

None of the Tallboy bombs struck Tirpitz, but several landed in the water near her. The detonation of one of these bombs damaged the battleship’s port-side propeller shaft and rudder, and caused flooding. Three of her crew were injured.

The other Lancaster bombers made uneventful return flights, all reaching the UK after completing flights of an average duration of 13 hours. An aeroplane of No. 617 Squadron made an emergency landing at Sumburgh after running short of fuel: this was one of the Lancaster machines which had not released its Tallboy. The damaged No. 463 Squadron aircraft successfully landed on one wheel at the RAF base at Waddington. The airmen were aware that Tirpitz had not been sunk, and were disappointed with the operation’s results.

Allied intelligence soon established that Tirpitz had been damaged only slightly. A Mosquito undertook a photo-reconnaissance flight over the Tromsø area at 12.10 on 29 October: its photographs revealed no visible damage to the battleship. German post-battle reports broadcast by radio which were intercepted and decoded confirmed that damage was limited to the propeller shaft and rudder.

The Germans believed that several Lancaster bombers had been shot down in the attack. Tirpitz's crew attributed the failure of 'Obviate' to their ship’s gunnery, and this improved morale. The Germans expected that further attacks would be made, and were frustrated that Luftwaffe fighters had not been available to protect the battleship. A force of 38 fighters was transferred to Bardufoss after 'Obviate' to bolster the region’s air defences.

The British remained determined to sink Tirpitz as soon as possible, and soon after the raid of 29 October decided to employ essentially the same plan for the following 'Catechism'. On 12 November, Nos 9 and 617 Squadrons once more set out from northern Scotland. The weather over the Tromsø area was clear when they arrived, and Tirpitz was hit by two Tallboy bombs. The damage from these bombs and several near misses caused the battleship to capsize.