Mid-Atlantic gap
The Mid-Atlantic gap was a stretch of the Atlantic Ocean where no aircraft could reach, and where U-boats could operate almost freely during the Battle of the Atlantic. It sat beyond the range of every land-based aircraft in the RAF's Coastal Command. Merchant ships crossing that water did so without air cover, and they paid a terrible price in losses to German submarines. The gap would remain open for years, shaped not only by the limits of technology but by bitter rivalries between military commands, political decisions at the highest levels, and a desperate scramble for the right aircraft. What kinds of planes could have closed it sooner? Why did so many end up elsewhere? And how did a cat-and-mouse war of radar and countermeasures finally tip the balance?
When RAF Coastal Command was created in 1936, it was handed the job of antisubmarine warfare patrols but given almost nothing to do the work with. Its most common aircraft were the Anson and the Vildebeest, both of which were already obsolete before the Second World War even began. Shortages grew so severe that the command resorted to "scarecrow patrols" using Tiger Moths, which could do little more than signal a submarine's presence. The structural problem ran deeper than any individual shortage. RAF Bomber Command held higher priority for the best long-range aircraft, and Coastal Command received what Bomber Command no longer wanted. Wellingtons arrived only after Bomber Command converted to four-engined types and passed them down as cast-offs. The Lancaster, which could carry the standard 450 lb depth charge, entered service in early 1942 as Bomber Command's flagship aircraft and stayed firmly in Bomber Command's hands.
Coastal Command's most prized possession was an American-built aircraft, the Liberator GR.I, called the Very Long Range Liberator, or VLR. The Liberator B.I had proven too exposed to enemy fire for bombing runs over Europe, but its outstanding range and payload made it nearly ideal for hunting submarines far out at sea. Priority for the type, however, went to the U.S. Navy for Pacific reconnaissance, where the aircraft's long range was equally valued. Even by mid-1942, Coastal Command held only two squadrons of Liberators and B-17s combined. The nine Liberator GR.Is of 120 Squadron, based in Iceland, were significant enough to worry Admiral Karl Donitz, who commanded the German U-boat fleet. After patrols off Canada were added in 1942, convoy losses in that zone fell to just one ship. At the first sign of Coastal Command success against U-boats, Arthur Harris, head of Bomber Command, sought to redirect even those aircraft toward bombing German cities.
On the 11th of September 1942, Convoy ON 127 came under U-boat attack with a single VLR of 120 Squadron flying overhead. Fifteen U-boats converged on Convoy ON 131, and Coastal Command aircraft sank two of them. On the 12th of October 1942, 120 Squadron VLRs sank a U-boat while protecting Convoy ON 136. The pattern continued: on the 16th of October, VLRs guided by ship-borne high-frequency direction finding drove off three shadowing U-boats defending Convoy SC 104. On the 29th of October, five were driven off from Convoy HX 212, and seven more on the 6th of November around Convoy SC 107. The battle around SC 107 produced a sharp official observation that Newfoundland-based air support was inadequate, and that the early interception of the convoy had led to a bitter and costly fight. That finding prompted the RAF to belatedly shift more Coastal Command squadrons into the region.
Detecting surfaced U-boats at night was a top priority because submarines were most vulnerable on the surface, recharging their batteries, and they felt safest in the dark. The earlier AI.II radar was adapted into ASV.II, fitted to Coastal Command aircraft, but its 1.7-metre wavelength emissions meant a submarine could disappear into sea clutter before it came within visual range, usually at around one mile, by which time it had already dived. The Leigh light addressed the final detection problem; it overcame Air Ministry indifference and entered service in June 1941, proving very successful for illuminating surfaced submarines. Germans countered with the Metox receiver, which picked up ASV radar pulses at longer range than the radar could detect the submarine, making the system useless. The H2S three-gigahertz radar, operating at 10 cm, changed the equation, and paired with the Leigh light it proved lethal to U-boats. Arthur Harris blocked Coastal Command's access to H2S, arguing Bomber Command needed it more and that Coastal Command might lose it to the Germans. The first H2S-equipped system was fitted to a Coastal Command Wellington at Defford in December 1942. On the 2nd and the 3rd of February, a Stirling Pathfinder carrying H2S was shot down over the Netherlands on only its second operational use, and the Germans captured the device. Telefunken used the recovered equipment to produce the Rotterdam Gerat, named after where it was found.
German engineers working from the captured H2S built the Rotterdam Gerat into a defensive system for U-boats. Meanwhile, Coastal Command's first ASV.III-equipped patrol flew over the Bay of Biscay on the 1st of March. ASV.III made its first contact with a U-boat on the night of the 17th of March, but the Leigh light failed and the Wellington crew could not press the attack. The first successful attack using the combined system came the following night. German submariners began incorrectly believing that British aircraft were homing on emissions from the Metox receiver itself, which was no longer providing warnings. Scientists developed the FuMB7 Naxos U, a submersible version of the airborne Naxos radar detector used by night fighters. Naxos worked but it entered service on the same day that the 10-gigahertz H2X radar became operational in Coastal Command, and Naxos could not detect those frequencies. It was replaced by FuMB36 Tunis in May 1944 and supplemented by radar-absorbent material under the codename Schornsteinfeger, meaning "Chimneysweep."
Just before the TRIDENT Conference, Admiral Ernest J. King gained control of antisubmarine aircraft from the United States Army Air Forces, arranging a trade of B-24s for comparable types. This allowed Air Marshal John Slessor, head of Coastal Command, to borrow one squadron. After attacks on Convoy ON 166, the number of VLRs in Newfoundland finally began to increase. Canadians had been pressing for Liberators since autumn 1942, facing British doubts about the RCAF's ability to operate them effectively. Squadron Leader Bulloch, commanding officer of 120 Squadron, provided the confirmation that settled the question. By early March 1943, the number of VLRs in Newfoundland increased, though thirty-eight aircraft was still the total over the Mid-Atlantic Gap. The arrival of the 25th Antisubmarine Wing of the USAAF with medium-range B-24s equipped with H2S, probably built in Canada, allowed Coastal Command VLRs to be freed from other duties. The 25th Wing sank one U-boat over the Bay of Biscay before being redeployed to Morocco. A crisis in March 1943 came close to prompting Churchill and the Admiralty to abandon the convoy system altogether. The Mid-Atlantic Gap was finally closed in May 1943, when RCAF VLRs became operational in Newfoundland, at which point the Battle of the Atlantic was largely decided.
Common questions
What was the Mid-Atlantic gap in World War II?
The Mid-Atlantic gap was an area of the Atlantic Ocean that lay beyond the range of land-based RAF Coastal Command antisubmarine aircraft during the Battle of the Atlantic. Merchant ships crossing this zone had no air cover, leaving them exposed to German U-boat attacks. The gap was finally closed in May 1943.
When was the Mid-Atlantic gap closed?
The Mid-Atlantic gap was closed in May 1943, when RCAF Very Long Range Liberators became operational in Newfoundland. By that time, the Battle of the Atlantic was largely won.
What aircraft closed the Mid-Atlantic gap?
The gap was closed primarily by the Very Long Range Liberator, known as the VLR, an American-built aircraft operated by RAF Coastal Command and the Royal Canadian Air Force. As late as early 1943, only thirty-eight VLRs were covering the Mid-Atlantic Gap.
Why did RAF Coastal Command struggle to cover the Mid-Atlantic gap?
Coastal Command consistently received lower priority for long-range aircraft than RAF Bomber Command. By mid-1942 it held only two squadrons of Liberators and B-17s combined, and Arthur Harris sought to redirect even those to bomb German cities whenever Coastal Command showed signs of success against U-boats.
What role did the Leigh light play in fighting U-boats in the Mid-Atlantic gap?
The Leigh light was a powerful searchlight fitted to large aircraft like the Wellington and Liberator. It entered service in June 1941 and was used in combination with radar to illuminate surfaced U-boats at night, when submarines were most vulnerable while recharging their batteries.
How did Germany counter Allied radar used against U-boats in the Mid-Atlantic?
Germany developed the Metox receiver to detect ASV radar pulses before aircraft could spot a submarine, and after capturing an H2S radar from a downed Stirling Pathfinder in February 1943, Telefunken produced the Rotterdam Gerat. The FuMB7 Naxos U followed, but it entered service the same day that the H2X radar, which Naxos could not detect, became operational in Coastal Command.
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