Avro Shackleton

The Avro Shackleton is a British long-range maritime patrol aircraft (MPA) which was used by the Royal Air Force (RAF) and the South African Air Force (SAAF). It was developed by Avro from the Avro Lincoln bomber, which itself had been a development of the famous wartime Avro Lancaster bomber.
The Shackleton was developed during the late 1940s as part of Britain's military response to the rapid expansion of the Soviet Navy, in particular its submarine force. Produced as the primary type equipping RAF Coastal Command, the "Type 696", as it was initially designated, incorporated major elements of the Lincoln, as well as the Avro Tudor passenger aircraft, and was furnished with extensive electronics suites in order to perform the anti-submarine warfare (ASW) mission along with a much-improved crew environment to accommodate the long mission times involved in patrol work. Being known for a short time as the "Lincoln ASR.3", it was decided that the Type 696 would be named "Shackleton" in service, after the polar explorer Sir Ernest Shackleton.
It entered operational service with the RAF in April 1951. The Shackleton was used primarily in the ASW and MPA roles, but it was also frequently deployed as an aerial search and rescue (SAR) platform and for performing several other secondary roles such as mail delivery and as a crude troop-transport aircraft. In addition to its service with the RAF, South Africa also elected to procure the Shackleton to equip the SAAF. In South African service, the type was operated in the maritime patrol capacity between 1957 and 1984. During March 1971, a number of SAAF Shackletons were used during the SS Wafra oil spill, intentionally sinking the stricken oil tanker using depth charges to prevent further ecological contamination.
During the 1970s, the Shackleton was replaced in the maritime patrol role by the jet-powered Hawker Siddeley Nimrod. During its later life, a small number of the RAF's existing Shackletons received extensive modifications in order to adapt them to perform the airborne early warning (AEW) role. The type continued to be used in this support capacity until 1991, when it was replaced by the Boeing E-3 Sentry AEW aircraft. These were the last examples of the type remaining in active service.
Development.
Origins.
The Battle of the Atlantic was a crucial element of the Second World War, in which Britain sought to protect its shipping from the German U-boat threat. The development of increasingly capable diesel-electric submarines had been rapid, in particular the snorkel virtually eliminated the need for submarines to surface while on patrol. Aircraft that had once been highly effective submarine-killers had very quickly become incapable in the face of these advances. In addition, lend-leased aircraft such as the Consolidated B-24 Liberator had been returned following the end of hostilities. Several Avro Lancasters had undergone rapid conversion – designated as Maritime Reconnaissance Mk 3 (MR3) – as a stopgap measure for maritime search and rescue and general reconnaissance duties; however, RAF Coastal Command had diminished to only a third of its size immediately after the Second World War.
In the emerging climate of the Cold War and the potential requirement to guard the North Atlantic from an anticipated rapid expansion of the Soviet Navy's submarine force, a new aerial platform to perform the anti-submarine mission was required. Work had begun on the requirement for a new maritime patrol aircraft in 1944, at which point there had been an emphasis for long-range platforms for Far East operations; however, with the early end of the war in the Pacific, the requirement was refined considerably. In late 1945, the Air Staff had expressed interest in a conversion of the Avro Lincoln as general reconnaissance and air/sea rescue aircraft; they formalised their requirements for such an aircraft under Air Ministry specification R.5/46. Avro's Chief Designer Roy Chadwick initially led the effort to build an aircraft to this requirement, designated as the Avro "Type 696".
The Type 696 was a significant development upon the Lincoln. Elements of the Avro Tudor airliner were also reused in the design; Lincoln and Tudor had been derivatives of the successful wartime Avro Lancaster bomber. Crucially, the new aircraft was to be capable of a range while carrying up to of weapons and equipment. In addition to featuring a large amount of electronic equipment, the Type 696 had a much-improved crew environment over other aircraft types to allow them to be more effective during the lengthy mission times anticipated. During development the Type 696 was provisionally referred to as the "Lincoln ASR.3" before the officially allocated name 'Shackleton' was selected.
The first test flight of the prototype Shackleton GR.1, serial "VW135", was made on 9 March 1949 from the manufacturer's airfield at Woodford, Cheshire in the hands of Avro's Chief Test Pilot J.H. "Jimmy" Orrell. The GR.1 was later redesignated "Maritime Reconnaissance Mark I" (MR 1). The prototype differed from subsequent production Shackletons in a number of areas; it featured a number of turrets and was equipped for air-to-air refuelling using the looped-line method. These did not feature on production aircraft due to judgments of ineffectiveness or performance difficulties incurred. However, the performance of the prototype had been such that, in addition to the go-ahead for the MR1's production, a specification for improved variant was issued in December 1949, before the first production Shackleton had even flown. By 1951, the MR1 had become officially considered as an interim type due to several shortcomings.
Further development.
The "MR 2" was an improved version of the Shackleton, featuring numerous refinements that had been proposed for the MR1. The radar was upgraded to ASV Mk 13, and the radome relocated from the aircraft's nose to a ventral position aft of the bomb bay, the radome was retractable and could be fully extended only with the bomb bay doors open. It had improved allround radar coverage and minimised the risk of bird strikes. Both the nose and tail section were lengthened, the tailplane was redesigned, the undercarriage was strengthened and twin-retractable tailwheels were fitted. The Bristol dorsal turret was initially retained, but was later removed from all aircraft after delivery. The prototype, VW 126, was modified as an aerodynamic prototype at the end of 1950 and first flew with the MR 2 modification on 19 July 1951.
VW 126 was tested at Boscombe Down in August 1951, particular attention being paid to changes made to improve its ground handling, such as the addition of toebrakes and a lockable rudder system. One production Mk 1 aircraft was modified on the line at Woodford with the Mk 2 changes and first flew on 17 June 1952. After trials were successful, it was decided to complete the last ten aircraft being built under the Mk 1 contract to MR 2 standard and further orders were placed for new aircraft. In order to keep pace with changing submarine threats, the Mk 2 force was progressively upgraded, with Phase I, II and III modifications introducing improved radar, weapons and other systems, as well as structural work to increase fatigue life. Production of the MR 2 ended in May 1954.
The "Type 716" Shackleton "MR 3" was another redesign in response to crew feedback and observations. A new tricycle undercarriage was introduced, the fuselage was increased in all main dimensions and had new wings with better ailerons and tip tanks. The weapons capability was also upgraded to include homing torpedoes and Mk 101 Lulu nuclear depth bombs. To reduce crew fatigue on 15-hour flights, the sound deadening was improved and a proper galley and sleeping space were included. Due to these upgrades, the takeoff weight of the RAF's MR 3s had risen by over 30,000 lb (13,600 kg) (Ph. III) and assistance from Armstrong Siddeley Viper Mk 203 turbojets was needed on takeoff with a 5-minute limit. The Griffons had to be run at high power for very long periods after a heavyweight take-off so the Vipers were later cleared to run for four hours continuously so lower Griffon power settings could be used which reduced the risk of failures. The extra strain took a toll on the airframe, and flight life of the RAF MR 3s was so reduced that they were outlived by the MR 2s. Due to the arms embargo against South Africa, the SAAF's MR 3s never received these upgrades but were maintained independently by the SAAF.
The "Type 719" Shackleton IV, later known as the "MR 4", was a projected variant intended to meet a Canadian requirement for a long-range patrol aircraft. The MR 4 would have been practically a new aircraft, sharing only the nose, cockpit, and outer wings with earlier variants; it would have also been powered by the Napier Nomad compound engine. The Shackleton IV was cancelled in 1955 and the Canadian requirement subsequently met by the "Britannia Maritime Reconnaissance" later to emerge as the Canadair Argus.
In 1967, ten MR 2s were modified as training aircraft to replace the T 4 in-service with the Maritime Operational Training Unit; known as T 2s, the crew rest areas were replaced by additional radar equipment and the original radar fittings removed.
Design.
The Shackleton was a purpose-built aircraft for the maritime patrol role; however, the legacy of Avro's preceding aircraft is present in many aspects of the overall design. The centre section of the Shackleton's wing originates from the Lincoln, while the outer wing and undercarriage were sourced from the Tudor outer wings; at one stage during development, the tailplane had closely resembled the Lincoln's, but was enlarged and changed soon after. An entirely new fuselage was adopted, being wider and deeper to provide a large space in which to accommodate the crew, their equipment, and a large bomb bay. Later variants of the Shackleton were substantially redesigned, adopting a new nosewheel undercarriage, redesigned wings and centre-section, and a larger fuel capacity for more range.
Various armaments and equipment were carried by the Shackleton in order to perform its missions. In ASW operations, the ASV Mk 13 radar was the primary detection tool; it could detect a destroyer at a range of , a surfaced submarine at , and a submarine's conning tower at , although rough seas considerably reduced the radar's effectiveness. Other equipment included droppable sonobuoys, electronic warfare support measures and an Autolycus diesel fume detection system. A special camera bay housed several reconnaissance cameras capable of medium altitude and nighttime vertical photography, and low-altitude oblique photography. The crew would also perform visual searches using various lookout positions that were provided for this purpose. Weapons carried included up to nine bombs, three homing torpedoes or depth-charges; the aircraft also had two 20 mm cannon in a Bristol dorsal turret. An in-flight refueling receptacle could be accommodated, but was not fitted on production aircraft.
The Merlin engines were replaced with the larger, more powerful and slower-revving Rolls-Royce Griffons with contra-rotating propellers. This engine's distinctive noise often caused crew members to develop high-tone deafness. The Griffon was needed because the Shackleton was heavier and had more drag than the Lincoln. The Griffon provided equivalent power to the Merlin but at lower engine speed, which led to reduced fuel consumption in the denser air at low altitudes; the Shackleton would often loiter for several hours at roughly or lower when searching for submarines. Lower-revving Griffons, compared to Merlins, reduced engine stress which improved their reliability. Using conventional propellers would have needed an increase in propeller diameter to absorb the engine's power and torque, this not being possible due to space limitations imposed by the undercarriage length and engine nacelle positioning; the contra-rotating propellers gave greater blade area within the same propeller diameter.
Numerous problems were encountered during the Shackleton's operational service. In practice, the diesel fume detection system was prone to false alarms and thus received little operational use. The engines, hydraulics, and elements of the avionics were known for their unreliability, and the aircraft proved to be fairly maintenance-intensive. The prototype MR 3 was lost due to poor stalling characteristics; this was rectified prior to production, although a satisfactory stall-warning device was not installed until 1969. The Shackleton is often incorrectly assigned the unfortunate distinction of holding the record for the highest number of aircrew killed in one type in peacetime in the RAF. The true figures suggest rather differently in that some of its contemporaries fared far worse, such as the Gloster Meteor with over 430 fatal losses of aircrew against the Shackleton's 156. Several programs to support and extend the fatigue life limits of the Shackleton's airframe were required; the fatigue life problems ultimately necessitated the rapid introduction of a whole new maritime patrol aircraft in the form of the Hawker Siddeley Nimrod, which began being introduced to RAF service in 1969.
Operational history.
Royal Air Force.
On 30 March 1951, the first Shackleton was delivered to No. 120 Squadron RAF; by the end of 1952 seven squadrons were operating the type. The first operational deployment of the Shackleton occurred in 1955 as a troop-transport for British Army movements to Cyprus; less than a year later, the type's first combat deployment took place during the Suez Crisis, codenamed Operation Musketeer. In 1957, British RAF Shackletons participated heavily during Jebel Akhdar War in Oman to expand the territory of the Sultanate, which was a "de facto" British colony, in order to gain access to oil wells in the interior parts of Oman. The RAF made 1,635 raids, dropping 1,094 tons and firing 900 rockets at the interior of Oman between July and December 1958 targeting insurgents, mountain top villages, water channels and crops in a war that remained under low profile.
During the 1960s, the typical Shackleton crew comprised two pilots, two navigators, a flight engineer, an air electronics officer, and four air electronics operators. During this period, equipment upgrades had become routine in order to keep pace with ever more capable submarines; problems with airframe fatigue were identified, leading to several programmes being carried out to strengthen the aircraft and thus extend its viable service life. In 1966, nuclear depth charges were introduced to the Shackleton's arsenal with the aim of countering the Soviets' development of deep-diving submarines.
Maritime reconnaissance was a large element of the Shackleton's service. This mission was often performed to identify and monitor naval and merchant shipping and to demonstrate sovereignty. During the Indonesia–Malaysia confrontation in the 1960s, Shackletons monitored the seas for vessels involved in arms smuggling. Similar operations were conducted in Cyprus, and Shackletons operating from bases in Madagascar cooperated with Royal Navy vessels to enforce a United Nation-mandated oil blockade of Rhodesia.
The Shackleton would often be used to perform search and rescue missions, at all times one crew being kept on standby somewhere across the UK for this role. The Shackleton had also replaced the Avro Lincoln in the colonial policing mission, aircraft often being stationed in the Aden Protectorate and Oman to carry out various support missions, including convoy escorting, supply dropping, photo reconnaissance, communication relaying, and ground-attack missions; the Shackleton was also employed in several short-term bombing operations. Other roles included weather reconnaissance and transport duties, in the latter role each Shackleton could carry freight panniers in the bomb bay or up to 16 fully equipped soldiers.
In 1969, a jet-powered replacement patrol aircraft, the Hawker Siddeley Nimrod, began to enter RAF service, which was to spell the end for the Shackleton in most roles. While radically differing in external appearance, the Shackleton and the initial version of the Nimrod shared many sensor systems and onboard equipment.
The intention to retire the Shackleton was thwarted by the need to provide AEW coverage in the North Sea and northern Atlantic following the withdrawal of the Fleet Air Arm's Fairey Gannet aircraft used in the AEW role in the 1970s. As an interim replacement, the existing AN/APS-20 radar was installed in modified Shackleton MR 2s, redesignated the AEW 2, as an interim measure from 1972. These were operated by No. 8 Sqn, based at RAF Lossiemouth. All 12 AEW aircraft were given names from "The Magic Roundabout" and "The Herbs" TV series. The intended replacement, the British Aerospace Nimrod AEW3, suffered considerable development difficulties which culminated in the Nimrod AEW 3 being cancelled in favour of an off-the-shelf purchase of the Boeing E-3 Sentry, which allowed the last Shackletons to be retired in 1991.
South African Air Force.
During the Second World War, the importance of securing the sea routes around the Cape of Good Hope had been made apparent, with over a hundred vessels being sunk in South African waters by enemy vessels between 1942 and 1945. In the postwar situation, the South African Air Force sought a large and capable platform to perform the maritime patrol role. After evaluating four RAF MR 2s in 1953, an order was placed for eight Shackletons as a replacement for the SAAF's aging Short Sunderland maritime patrol aircraft. Modifications were required to fulfill South African conditions and requirements, such as the ability to operate over the Indian Ocean, the resulting aircraft was designated the Shackleton MR 3.
On 18 August 1957, the first two Shackletons were delivered to D.F. Malan Airport, Cape Town. Two more followed on 13 October 1957 and the remainder arrived in February 1958. Delivered to the same basic standard as the RAF's MR 3s, they were assigned single letter codes between "J" and "Q" and operated by 35 Squadron SAAF. The type typically patrolled the sea lanes around the Cape of Good Hope, often monitoring Soviet vessels traversing between the Indian and Atlantic oceans. The Shackleton was briefly used in low-level overland patrols along the Southern Rhodesian border, but these duties ended following concerns of the disturbance of wildlife.
Often, the Shackleton would be called in to perform search and rescue operations in the treacherous waters around the Cape. In March 1971, Shackletons successfully intervened in the SS Wafra oil spill, deliberately sinking the stricken oil tanker with depth charges in order to prevent an ecological disaster. The only operational loss incurred was 1718 K, which crashed into the Wemmershoek mountains at night time on 8 August 1963 with the loss of all thirteen crew.
Due to an embargo imposed by the United Nations over South Africa's policy of apartheid, acquiring components for the Shackleton fleet became increasingly difficult and thus the aircraft's serviceability suffered. The fleet had been modified to Phase III standards prior to the implementation of the arms embargo, albeit without the auxiliary Viper engine. Two of the aircraft were re-sparred, 1716 J in the United Kingdom and 1717 O in South Africa by the SAAF, but the lack of engine spares and tyres, together with airframe fatigue, took a gradual toll. By November 1984, the fatigue lives of all but the two re-sparred aircraft had expired and the fleet was retired into storage. Although the joke has been applied to several aircraft, the Shackleton was often described as "a hundred thousand rivets flying in close formation." 
Variants.
Avro 696 Shackleton prototypes.
Three prototype Type 696s were ordered in May 1947 to meet specification R 5/46:
Avro 696 Shackleton Mk.2.
In 1971 Twelve MR 2s were converted at Woodford and Bitteswell as Airborne Early Warning aircraft, the first AEW.2 flew on 30 September 1971 and the type entered service with 8 Squadron on 1 January 1972.