Mid-Atlantic gap

Mid-Atlantic gap

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The Mid-Atlantic Gap was the gap in coverage by land-based Coastal Command
RAF Coastal Command
RAF Coastal Command was a formation within the Royal Air Force . Founded in 1936, it was the RAF's premier maritime arm, after the Royal Navy's secondment of the Fleet Air Arm in 1937. Naval aviation was neglected in the inter-war period, 1919–1939, and as a consequence the service did not receive...

Anti-submarine warfare
Anti-submarine warfare is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and deter, damage or destroy enemy submarines....

 (A/S) aircraft during the Battle of the Atlantic in the Second World War
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. It is frequently known as The Black Pit, as well as the Atlantic Gap, Air Gap, Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

 Gap, or just "the Gap". This resulted in heavy merchant shipping losses to U-boat
U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

s. The Black Pit, the most feared area where most losses occurred, was the area southeast of Greenland. The gap was eventually closed in May 1943, as growing numbers of VLR Liberators (Very Long Range models) and escort carriers became available, and as basing problems were addressed.


Coastal Command, when it was created in 1936, was given responsibility for A/S (or ASW) patrol. It was equipped only with small numbers of short-ranged aircraft, the most common being the Avro Anson
Avro Anson
The Avro Anson is a British twin-engine, multi-role aircraft that served with the Royal Air Force, Fleet Air Arm and numerous other air forces prior to, during, and after the Second World War. Named for British Admiral George Anson, it was originally designed for maritime reconnaissance, but was...

 (which was obsolescent by the start of World War Two) and Vickers Vildebeest
Vickers Vildebeest
The Vickers Vildebeest and the similar Vickers Vincent were two very large two- to three-seat single-engined British biplanes designed and built by Vickers and used as a light bomber, torpedo bomber and in the army cooperation roles...

 (which was obsolete); for a time, shortages of aircraft were so severe, "scarecrow patrols" using Tiger Moth
De Havilland Tiger Moth
The de Havilland DH 82 Tiger Moth is a 1930s biplane designed by Geoffrey de Havilland and was operated by the Royal Air Force and others as a primary trainer. The Tiger Moth remained in service with the RAF until replaced by the de Havilland Chipmunk in 1952, when many of the surplus aircraft...

s were even employed. Bomber Command
RAF Bomber Command
RAF Bomber Command controlled the RAF's bomber forces from 1936 to 1968. During World War II the command destroyed a significant proportion of Nazi Germany's industries and many German cities, and in the 1960s stood at the peak of its postwar military power with the V bombers and a supplemental...

 routinely got higher priority for the best, longest-ranged aircraft. Only as Bomber Command transitioned to four-engined aircraft did Coastal Command receive the castoffs, such as Vickers Wellington
Vickers Wellington
The Vickers Wellington was a British twin-engine, long range medium bomber designed in the mid-1930s at Brooklands in Weybridge, Surrey, by Vickers-Armstrongs' Chief Designer, R. K. Pierson. It was widely used as a night bomber in the early years of the Second World War, before being displaced as a...

s, which finally had adequate range for A/S patrol. Moreover, Coastal Command's motley assortment of Ansons, Whitley
Armstrong Whitworth Whitley
The Armstrong Whitworth A.W.38 Whitley was one of three British twin-engine, front line medium bomber types in service with the Royal Air Force at the outbreak of the Second World War...

s, and Hampden
Handley Page Hampden
The Handley Page HP.52 Hampden was a British twin-engine medium bomber of the Royal Air Force serving in the Second World War. With the Whitley and Wellington, the Hampden bore the brunt of the early bombing war over Europe, taking part in the first night raid on Berlin and the first 1,000-plane...

s were unable to carry the standard 450 lb (205 kg) depth charge
Depth charge
A depth charge is an anti-submarine warfare weapon intended to destroy or cripple a target submarine by the shock of exploding near it. Most use explosives and a fuze set to go off at a preselected depth in the ocean. Depth charges can be dropped by either surface ships, patrol aircraft, or from...

; that needed Wellingtons or Sunderland
Short Sunderland
The Short S.25 Sunderland was a British flying boat patrol bomber developed for the Royal Air Force by Short Brothers. It took its service name from the town and port of Sunderland in northeast England....

s. (The other aircraft capable of carrying it, the Avro Lancaster
Avro Lancaster
The Avro Lancaster is a British four-engined Second World War heavy bomber made initially by Avro for the Royal Air Force . It first saw active service in 1942, and together with the Handley Page Halifax it was one of the main heavy bombers of the RAF, the RCAF, and squadrons from other...

, was Bomber Command's crown jewel.)

Coastal Command's prize was the Liberator GR.I
Consolidated Liberator I
Consolidated Liberator I was the service name of the first Consolidated B-24 Liberator 4-engined bombers to see use with the Royal Air Force .-Service history:In August 1939, the USAAC ordered 38 examples of the Consolidated B-24A...

, commonly called the VLR Liberator or just VLR. The Liberator B.I proved too vulnerable for bombing missions over Europe, but had excellent range and payload, ideal for A/S patrol. Top priority for these was the U.S. Navy for reconnaissance operations in the Pacific, where their long legs were equally valuable, but where they generally carried out missions of lower priority than Coastal Command's. It remains unclear why the Short Stirling
Short Stirling
The Short Stirling was the first four-engined British heavy bomber of the Second World War. The Stirling was designed and built by Short Brothers to an Air Ministry specification from 1936, and entered service in 1941...

, which had better range and payload performance still, was not preferred.

VLRs were of particular importance in times when Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...

 was unable to read Kriegsmarine
The Kriegsmarine was the name of the German Navy during the Nazi regime . It superseded the Kaiserliche Marine of World War I and the post-war Reichsmarine. The Kriegsmarine was one of three official branches of the Wehrmacht, the unified armed forces of Nazi Germany.The Kriegsmarine grew rapidly...

Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...

Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by "breaking" high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School at Bletchley Park. "Ultra" eventually became the standard...

). When ON 127 was attacked U-584 on 11 September 1942, there was exactly one VLR of 120 Squadron overhead. Fifteen U-boats converged on ON 131, only to meet aircraft, and Coastal Command sank two, while in protecting ON 136, 120 Squadron's VLRs sank U-597 on 12 October 1942. Even then, VLRs proved invaluable in co-operation with shipborne "Huff Duff". Defending SC 104, VLRs guided by HF/DF drove off three shadowers in one day, 16 October. They bettered the performance on 29 October, for HX 212, driving off five, and seven on 6 November around SC 107. "...[T]he apparent inadequacy Newfoundland-based air support was highlighted by the early interception of SC 107 and the resultant bitter and costly battle." This led RAF to belatedly move a number of Coastal Command squadrons.

The paltry nine Liberator GR.Is operating over the Atlantic, members of 120 Squadron
No. 120 Squadron RAF
No. 120 Squadron of the Royal Air Force operated the Nimrod MR2, based at RAF Kinloss, Moray, Scotland until the type's withdrawal in March 2010.-Formation in WWI:...

 based in Iceland, were nevertheless a worry to Admiral Dönitz
Karl Dönitz
Karl Dönitz was a German naval commander during World War II. He started his career in the German Navy during World War I. In 1918, while he was in command of , the submarine was sunk by British forces and Dönitz was taken prisoner...

, BdU. As a measure of how valuable they were, after patrols off Canada were added in 1942, only one ship was lost in convoy. Even in mid-1942, Coastal Command only had two squadrons of Liberators and B-17s, and at the first sign of Coastal Command's success against U-boats, Harris sought to have their aircraft used in attacking German cities.

After Convoy SC 118
Convoy SC 118
Convoy SC-118 was the 118th of the numbered series of World War II Slow Convoys of merchant ships from Sydney, Cape Breton Island to Liverpool.-Prelude:...

, Professor Patrick M. S. Blackett
Patrick Blackett, Baron Blackett
Patrick Maynard Stuart Blackett, Baron Blackett OM CH FRS was an English experimental physicist known for his work on cloud chambers, cosmic rays, and paleomagnetism. He also made a major contribution in World War II advising on military strategy and developing Operational Research...

, Director of the Admiralty
The Admiralty was formerly the authority in the Kingdom of England, and later in the United Kingdom, responsible for the command of the Royal Navy...

's Operations Research
Operations research
Operations research is an interdisciplinary mathematical science that focuses on the effective use of technology by organizations...

 section, made several proposals, including diverting VLRs from Bomber Command to Coastal Command. "Despite the strength of Blackett's case, the Admiralty (not to mention the Air Ministry, Bomber Command, and the Americans) believed for some time yet that it could not afford to reduce the heavy air offensive in the Bay of Biscay or to abandon the bombing of German bases by the RAF." "The number of VLR aircraft operating in the North Atlantic in February [1943] was only 18, and no substantial increase was made until after the crisis of March." Nor were night air patrols, recognized as necessary, initiated until Fall 1943.

Bomber Command did not refuse entirely to offer assistance against U-boats. From 14 January 1943 through May, they flew seven thousand sorties against the U-boat pens in Lorient
Lorient, or L'Orient, is a commune and a seaport in the Morbihan department in Brittany in north-western France.-History:At the beginning of the 17th century, merchants who were trading with India had established warehouses in Port-Louis...

, Brest
Brest, France
Brest is a city in the Finistère department in Brittany in northwestern France. Located in a sheltered position not far from the western tip of the Breton peninsula, and the western extremity of metropolitan France, Brest is an important harbour and the second French military port after Toulon...

, and St. Nazaire
Saint-Nazaire , is a commune in the Loire-Atlantique department in western France.The town has a major harbour, on the right bank of the Loire River estuary, near the Atlantic Ocean. The town is at the south of the second-largest swamp in France, called "la Brière"...

, at a cost of 266 aircraft and crews. They accomplished no damage to the pens nor the submarines within them. Coastal Command strength never reached 266 VLRs. Missions flown against German U-boat building yards had similarly disappointing results.

Aircraft also had an important indirect role, by preventing even the formation of wolf packs. They limited the places U-boats could attack in safety, and (by reducing the ability of shadowers to find and track convoys) made shipping harder to find, thereby reducing losses. This also helped escorts, by enabling them to deal with one U-boat at a time. Despite a willingness of RCAF
Royal Canadian Air Force
The history of the Royal Canadian Air Force begins in 1920, when the air force was created as the Canadian Air Force . In 1924 the CAF was renamed the Royal Canadian Air Force and granted royal sanction by King George V. The RCAF existed as an independent service until 1968...

 aircraft to fly in (perennially bad) conditions off the Grand Banks
Grand Banks
The Grand Banks of Newfoundland are a group of underwater plateaus southeast of Newfoundland on the North American continental shelf. These areas are relatively shallow, ranging from in depth. The cold Labrador Current mixes with the warm waters of the Gulf Stream here.The mixing of these waters...

 Coastal Command would never have attempted, U-boats could trail convoy
A convoy is a group of vehicles, typically motor vehicles or ships, traveling together for mutual support and protection. Often, a convoy is organized with armed defensive support, though it may also be used in a non-military sense, for example when driving through remote areas.-Age of Sail:Naval...

s beginning very soon after departure from Halifax
City of Halifax
Halifax is a city in Canada, which was the capital of the province of Nova Scotia and shire town of Halifax County. It was the largest city in Atlantic Canada until it was amalgamated into Halifax Regional Municipality in 1996...

. Without ASV, the almost "perpetual fog of the Grand Banks also allowed pack operations to penetrate within a couple of hundred miles of Newfoundland, while aircraft patrolled harmlessly above", visual detection impossible.

A means of detecting surfaced submarines at night, when they were at their most vulnerable, recharging batteries, and felt most safe, was a top priority for Coastal Command. ASV gave it to them. The previous AI.II (Mark 2 Airborne Interception) radar became ASV.II (Air to Surface Vessel Mark 2) fitted in Coastal Command aircraft. Coastal Command priority for it, however, ranked behind Fighter Command
RAF Fighter Command
RAF Fighter Command was one of three functional commands of the Royal Air Force. It was formed in 1936 to allow more specialised control of fighter aircraft. It served throughout the Second World War, gaining recognition in the Battle of Britain. The Command continued until 17 November 1943, when...

's night fighter
Night fighter
A night fighter is a fighter aircraft adapted for use at night or in other times of bad visibility...

 units. ASV.II's 1½-metre wavelength (actually 1.7 m, 176 MHz), however, meant a submarine was usually lost in sea return before it came in visual range, at around a mile (1,850 m), by which time it was already diving. In response, the Leigh light
Leigh light
The Leigh Light was a British World War II era anti-submarine device used in the Second Battle of the Atlantic.It was a powerful carbon arc searchlight of 24 inches diameter fitted to a number of the British Royal Air Force's Coastal Command patrol bombers to help them spot surfaced...

 was developed. Though it had to overcome Air Ministry indifference, and only entered service in June 1941, it proved very successful. This, however, required a large aircraft, such as the Wellington or Liberator, to carry the generator needed to power the light, and most of Coastal Command's aircraft were incapable of it, nor were Bomber Command inclined to turn over anything better. Moreover, the Germans developed Metox
The Metox, named after its manufacturer, was a pioneering high frequency very sensitive radar warning receiver manufactured by a small French company in occupied Paris, which could detect ASV transmissions from patrolling Allied aircraft. It is not clear whether the design was German or French or...

, which picked up ASV's radar pulses before it was able to detect a submarine at all, rendering it useless.

The appearance of H2S
H2S radar
H2S was the first airborne, ground scanning radar system. It was developed in Britain in World War II for the Royal Air Force and was used in various RAF bomber aircraft from 1943 to the 1990s. It was designed to identify targets on the ground for night and all-weather bombing...

 (10 cm) radar changed that, and the combination of H2S (as ASV.III) and Leigh light would prove lethal to U-boats. Harris, however, denied Coastal Command H2S, claiming Bomber Command needed it to find targets, in preference to Gee
GEE (navigation)
Gee was the code name given to a radio navigation system used by the Royal Air Force during World War II.Different sources record the name as GEE or Gee. The naming supposedly comes from "Grid", so the lower case form is more correct, and is the form used in Drippy's publications. See Drippy 1946....

 and Oboe
Oboe (navigation)
Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. Oboe accurately measured the distance to an aircraft, and gave the pilot guidance on whether or not they were flying along a pre-selected circular route. The route was only 35 yards...

, while arguing Coastal Command might lose it to the Germans. Churchill backed him up. Marshal John Slessor
John Slessor
Marshal of the Royal Air Force Sir John Cotesworth Slessor GCB, DSO, MC was a senior commander in the Royal Air Force . A pilot in the Royal Flying Corps during World War I, he held operational commands in World War II and served in the RAF's most senior post, Chief of the Air Staff, from 1950 to...

, head of Coastal Command, countered Bomber Command also risked having it fall in enemy hands, and having the Germans produce a countermeasure against it, before Coastal Command ever got to use it. In the event, this was exactly what happened. The first ASV.III was fitted to a Coastal Command Wellington at Defford in December 1942, with twelve based at Chivenor by February 1943, while a copy of H2S was lost 2/3 February when a Stirling Pathfinder was shot down over the Netherlands, on only H2S's second operational use. Harris made similar objections to supplying 3 cm-wavelength H2X
H2X radar
H2X radar was an American development of the British H2S radar, the first ground mapping radar to be used in combat. It was used by the USAAF during World War II as a navigation system for daylight overcast and nighttime operations...

 to Coastal Command (which knew it as ASV.IV), again got higher priority, and again saw it fall into German hands, almost exactly a year later, in February 1944.

As Coastal Command predicted, the Germans captured the damaged H2S, which would have been next to impossible from a Coastal Command aircraft downed at sea, rather than over land, and Telefunken produced the Rotterdam Gerät (Rotterdam Device, named for where it was captured). Coastal Command's first ASV.III-equipped patrol took place over the Bay of Biscay 1 March. ASV.III made its first U-boat contact on the night of 17 March, but unfortunately the carrier Wellington suffered a malfunction of its Leigh Light and was unable to press home the attack. The first attack using the system occurred the next night. When ASV.III did enter service, German submariners, right up to Dönitz, began to mistakenly believe British aircraft were homing on emissions from the Metox receiver, which no longer gave warning. Meantime, German scientists were perfecting the Rotterdam Gerät to create FuMB7 Naxos U
Naxos radar detector
The FuG 350 Naxos radar warning receiver was a World War II German countermeasure to SHF band centimetric wavelength radar produced by a cavity magnetron...

 (commonly just Naxos) for U-boats. While fragile, Naxos worked. However, it entered service the same day as H2X (which Naxos could not detect) became operational in Coastal Command. Naxos was replaced by FuMB36 Tunis in May 1944, and was supplemented by Stumpf, what today would be called radar absorbent material
Radar absorbent material
Radar-absorbent material, or RAM, is a class of materials used in stealth technology to disguise a vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave depends upon its composition...

, under the codename Schornsteinfeger ("Chimneysweep").

Just before the TRIDENT Conference
Washington Conference (1943)
The Third Washington Conference was held in Washington, D.C. was a World War II strategic meeting from May 12 to May 27, 1943, between the heads of government of the United Kingdom and the United States. The delegations were headed by Winston Churchill and Franklin D...

, Admiral Ernest J. King got control of A/S aircraft from the Army Air Force
United States Army Air Forces
The United States Army Air Forces was the military aviation arm of the United States of America during and immediately after World War II, and the direct predecessor of the United States Air Force....

, arranging a trade of B-24s for comparable types. This enabled Slessor to make a deal with him to "borrow" one squadron. After attacks on ONS 166, the number of VLRs in Newfoundland finally increased. "Canadians had been pressing hard for Liberators since autumn 1942, against British doubts that the RCAF could employ them effectively, while RCAF, for its part, opposed RAF taking over a job RCAF saw as its own. The commanding officer of 120 Squadron, Squadron Leader
Squadron Leader
Squadron Leader is a commissioned rank in the Royal Air Force and the air forces of many countries which have historical British influence. It is also sometimes used as the English translation of an equivalent rank in countries which have a non-English air force-specific rank structure. In these...

 Bulloch, confirmed RCAF's ability, and in early March 1943, the number in Newfoundland belatedly increased (though it was not enough to constitute 10 Squadron
No. 10 Squadron RCAF
No. 10 Squadron RCAF was a Royal Canadian Air Force squadron that was active during the Second World War. It was primarily used in an anti-submarine role and was based on the east coast of Canada and Newfoundland. It established the RCAF record for attacks on U-boats with 22 and was successful in...

, RCAF, before 10 May), while 120 Squadron's strength doubled. This still only put all of thirty-eight VLRs over the Mid-Atlantic Gap. The arrival of 25th Antisubmarine Wing
25th Antisubmarine Wing
The 25th Anti-Submarine Wing is an inactive United States Air Force unit. Its last assignment was with the Army Air Forces Antisubmarine Command, based in New York City, New York. It was inactivated on 15 October 1943-History:...

, USAAF, with its medium-range B-24s (equipped with H2S, probably built by Canadians), made it possible to free up Coastal Command VLRs without it. The growth in numbers of escort carriers meant "a dramatic increase of USAAF Fortresses and medium-range Liberators" could be based in Newfoundland. 25h Wing flew over the Bay of Biscay, where they sank one U-boat before being redeployed to Morocco.

Increasing availability of escort carriers reduced the hazard of the Gap. After a crisis in March which nearly had Churchill and the Admiralty abandon convoy altogether, the Mid-Atlantic Gap was finally closed in May 1943, when RCAF VLRs became operational in Newfoundland, by which time the Battle of the Atlantic was largely won.