'Percussion' was a British naval and air operation in the Bay of Biscay, off the south-west coast of France and comparatively close to the north coast of neutral Spain, against U-boats attempting to leave or reach French ports by means of a surfaced transit of the Bay of Biscay, and also to cover the passage of the OS.54 and SL.134 convoys (23/29 August 1943 and 2/8 January 1944).
The naval forces involved in the first stage of this extended undertaking were the 40th Escort Group and 5th Support Group strengthened by the light cruiser Bermuda, while the air element was provided by Air Vice Marshal B. E. Baker’s No. 19 Group of Air Marshal Sir John Slessor’s RAF Coastal Command 1.
As the first step in the operation, the 40th Escort Group (frigates Exe, Moyola and Waveney, and sloops Landguard, Bideford and Hastings) operated off Cape Finisterre and Cape Ortegal, but failed to intercept the returning U-510 and U-66 and the outbound U-123 and U-518, only Kapitänleutnant Hans-Günther Brosin’s U-134 being sunk, and then by a Leigh Light Wellington of No. 179 Squadron on 24 August. As the Canadian 5th Support Group (British frigates Nene and Tweed, and Canadian corvettes Calgary, Edmundston and Snowberry) relieved the 40th Escort Group, 14 Dornier Do 217E bombers of Major Franz Hollweg’s II/Kampfgeschwader 100 and seven Junkers Ju 88C heavy fighters attacked the support groups using, for the first time, the new Henschel Hs 293 air-to-surface missile. Landguard was damaged by four near-misses and Bideford by one.
On the same day, some 460 miles (740 km) to the west of Cape Finisterre, Kapitänleutnant Werner Pietzsch’s outbound U-523 met the combined OG.92 and KMS.24 convoys protected by the British Escort Group B1 (the destroyers Hurricane, Wanderer and Watchman, and corvettes Borage, Dahlia, Meadowsweet and Wallflower) and was sunk by the depth charges of Wallflower and Wanderer.
On 26 August the 5th Support Group was relieved by the 1st Support Group (sloops Pelican and Egret, and frigates Jed, Rother, Spey and Evenlode), which depth-charged and damaged the returning U-342. To relieve Bermuda, which was departing to refuel at Plymouth, the destroyers Grenville and Canadian Athabaskan arrived. The outbound U-305 and U-645, and the returning U-358, U-571, U-333, U-566 and U-757 all passed unharmed.
On 28 August there came a second German air attack on the support groups, in this instance by 18 Do 217 bombers, resulting in severe damage to Athabaskan and the destruction of Egret. The support groups retired to the west, and the U-boats' inbound and outbound movements were eased.
During later 'Percussion' operations against U-boat transit routes, on 2 January, a Halifax of No. 58 Squadron damaged the outbound U-445, which had to return for repairs. On 3 January a Wellington of No. 612 Squadron and Liberator of No. 224 Squadron damaged the outbound U-373. On 4 January a Wellington of No. 304 (Polish) Squadron damaged the returning U-629. On 5 January a Halifax of No. 58 Squadron damaged U-415 and on 8 January a Sunderland of No. 10 Squadron sank Kapitänleutnant Christian Reich’s returning U-426.