2. The air defence of the Bristol area

2.1 Establishment of the Air Defences - 1937/39

2.1.1 Observer Corps

The first step towards providing an air defence system for the Bristol area was taken in 1937 when No.23 Observer Corps Group was established at Bristol. By June 1938 this formation, the principal task of which was the plotting and identification of enemy aircraft over land, had set up some 33 Observer Posts covering Bristol, North Somerset, South Gloucestershire and Wiltshire, the most local being at West Harptree, Weston-super-Mare, Winscombe, Clevedon, Keynsham, Long Ashton, Avonmouth and Almondsbury. The Corps also provided information from which Fighter Command initiated the Air Raid Warnings throughout the country, and in April 1941 received the title 'Royal' in recognition of the valuable work it was undertaking, often by men working in long hours in posts in exposed locations with little protection from the elements and with no opportunity to light a fire which would be seen from the air.

2.1.2 Balloon Barrage

In September 1938 the government announced that balloon barrage protection was to be provided for a number of provincial towns and cities including Bristol, where it was decided that the presence of Filton aerodrome prevented the use of balloons to cover the whole area. Two small independent layouts were therefore proposed to protect the harbour installations at Avonmouth and the Bristol City Docks. The system involved raising a lethal cable barrage into the air around the potential target compelling an enemy bomber to fly above the balloons at heights at which other anti-aircraft weapons could be used against it more effectively, and from which it would bomb less accurately. The balloons employed were flown from a mobile winch and were designed to be deployed at a maximum altitude of 5000 feet. When the barrage area was not directly threatened the balloons were grounded or kept close hauled at 500 feet to provide as little potential danger as possible to 'friendly' aircraft.

Provincial barrages were initially organised into Auxiliary Air Force Squadrons operating some 24 balloons each. In each locality depots, known as Balloon Centres, were formed to administer the balloon squadrons and to be responsible for the assembly and testing of balloons and the training of balloon crews in time of war. In the Bristol area February 1939 saw the start of recruiting for the three 'County of Gloucester' Squadrons, No.927, 928 and 929, with these moving to Pucklechurch on August 9th when the new camp was taken over as the permanent home for the local No.11 Balloon Centre. In order to supply sufficient hydrogen gas for the balloons in the western barrages Imperial Chemical Industries erected a new hydrogen plant at Weston-super-Mare Gas Works which by the end of 1939 was already producing 2,500,000 cubic feet of hydrogen a week.

2.1.3 Heavy Anti-Aircraft Guns

By late 1938 it had become obvious to the War Office that recruiting for their new anti-aircraft and searchlight formations was not proceeding fast enough to provide the necessary personnel and so they asked for existing Territorial Army units of other rôles and other arms to accept conversion to anti-aircraft duties. As a result in Bristol on at the beginning of November 1938 the 66th (South Midland) Field Brigade took over heavy anti-aircraft duties, becoming the 76th Heavy Anti-Aircraft Regiment, while the 4th (City of Bristol) Battalion, The Gloucestershire Regiment, was re-designated the 66th Searchlight Regiment.

At the outbreak of war heavy anti-aircraft guns for the close defence of potential targets in Britain were in very short supply as the first production model of the new and improved Vickers 3.7" gun with an effective engagement ceiling of 25,000 feet had only appeared in early 1938. In addition some of the lower performance 3" 20 cwt. semi-mobile guns were available as a stop-gap measure, but these were essentially little more than a World War I weapon on a modified carriage, with an effective ceiling of only 14,000 feet. Initially four-gun positions were built, except at airfields where two guns were acceptable, and at this time before suitable gunlaying radar became available, action with any degree of accuracy was only possible against visible targets, otherwise only blind, geographical, barrages could be fired by the guns.

2.1.4 Searchlights

To assist the guns and fighter aircraft an elaborate searchlight layout was prepared for Southern England, with each major Gun Defended Area, such as Bristol, being provided with a local system to provide illumination for night engagements. Here the searchlights were to be deployed in groups of 48 on a 3500 yard spacing, the 90 cm carbon-arc projectors provided for this purpose being capable of producing some 210 million candle power. By late 1939 improved sound locators to operate with searchlights had also been introduced and apart from directing the searchlight beams, the new locators were also intended to assist in the plotting of night raids.

2.1.5 Light Anti-Aircraft Guns

Light anti-aircraft guns were also provided to give protection from attacks carried out at altitudes of less than 3000 feet against certain important installations known as 'Vulnerable Points' and these included the Bristol Aeroplane Company's factory at Filton, Filton Aerodrome, the National Smelting plant at Avonmouth, the Electricity Power Station at Portishead and Parnall Aircraft at Yate.

In December 1938 the 23rd Light Anti-Aircraft Regiment was formed at Bristol to which a number of Territorial Army Reserve personnel were recruited, resulting, at the outbreak of war, in the guns often being manned by shifts of workers on sites near their places of employment. However, due to a severe lack of personnel and equipment many of the sites allocated to the regiment were initially occupied by men from heavy, rocket and searchlight batteries, and it was to be May 1940 before the situation was anything like being resolved, although the provision of adequate weapons remained a problem for several years.

Originally it had been assumed that 0.303" Lewis guns would be adequate, but it was soon realised that a specialised weapon would be necessary. The mobile single barrel Swedish Bofors 40 mm gun was selected as the ideal equipment, but due to manufacturing problems by late 1939 few had been issued, and not until 1942 did production overtake demand. In an attempt to rectify this situation during the first three years of war the shortfall was mainly made up by using Vickers Mark VIII naval two-pounders with specially designed land mountings, old World War One 3" heavy anti-aircraft guns fitted with deflection sights, and Hispano 20 mm cannons, but all of these proved to be far from ideal.

2.1.6 Fighter Command

At the beginning of December 1938 Bristol's own No.501 (County of Gloucester) Squadron, Auxiliary Airforce, at that time flying bombers from Filton airfield was finally re-designated a fighter squadron with a complement of some 20 pilots and 16 aircraft, 12 of which were to be at operational readiness in time of war to take part in the defence of the Bristol area. The squadron's first Hawker Hurricane arrived in early March 1939, and this was one of two similar single engined monoplane fighters available to the RAF at the outbreak of war, both of which were armed with eight 0.303" machine guns. The Hurricane, which was a very stable gun platform well suited to destroying bombers, had a maximum speed of some 328 mph, while the Supermarine Spitfire, the most advanced of the single engined interceptors available at the time, was able to achieve a very respectable 355 mph.

By mid-August 1939 the main components of the country's air defences were assembled under RAF Fighter Command which for operational purposes was divided into a number of geographical Groups, each sub-divided into Sectors. The Sector Operations Room at Filton was responsible for the local fighters, balloon barrage and searchlights, as well as the anti-aircraft guns which it controlled via the Army's Gun Operations Room at Worrall Road, Clifton. The Sector Operations Room also received information about the movement of hostile aircraft from the Observer Corps and the searchlight sites, while Filton also maintained direct communication with adjacent Sectors, Groups and with HQ Fighter Command.

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