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List Entry Summary

This building is listed under the Planning (Listed Buildings and Conservation Areas) Act 1990 as amended for its special architectural or historic interest.


List entry Number: 1395116



The building may lie within the boundary of more than one authority.

County: Greater London Authority

District: Hillingdon

District Type: London Borough


National Park: Not applicable to this List entry.

Grade: II

Date first listed: 26-Oct-2010

Date of most recent amendment: Not applicable to this List entry.

Legacy System Information

The contents of this record have been generated from a legacy data system.

Legacy System: LBS

UID: 508437

Asset Groupings

This list entry does not comprise part of an Asset Grouping. Asset Groupings are not part of the official record but are added later for information.

List entry Description

Summary of Building

Legacy Record - This information may be included in the List Entry Details.

Reasons for Designation

Yes list


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804/0/10095 Building 27 at RAF Northolt 26-OCT-10

II Former Operations Block (Building 27), RAF Northolt, designed in September 1924 by the Works and Buildings architect C. H. Andrews and completed in 1928. Extended and modified in the late 1930s; entrance & toilet block added probably 1950s; 1960s re-roofing; windows replaced post-1980.

MATERIALS: Yellow stock brick in English bond; hipped asbestos-tiled roof; uPVC replacement windows in original openings with stone heads and cills; weatherboarding to 1930s extension and felt roof.

PLAN: The building is oriented west-east with the main entrance in the south elevation through a mid C20 entrance extension. There is a west-east corridor with short flights of steps both up and down along the southern side of the building providing access to a series of small rooms to the west and north and to the large Operations Room to the east. To the east of the Operations Room is a late 1930s extension. At the south-west corner is the original plant room.

EXTERIOR: Building 27 is a single storey rectangular building in English bond yellow stock brick, oriented west-east. The hipped asbestos tiled roof was re-tiled in the 1960s; the original tiles were diamond-shaped asbestos tiles and the current ones are square. As built there was an external earth traverse to protect the building (now removed). There is a mid-C20 flat roofed entrance and post-war toilet block to the south, main elevation, and the original low plant-room projection with chimney at the south-west corner. The north elevation has a run of ten windows. There is a late 1930s extension to the east end which is constructed at right angles to the main building with yellow stock brick walls in stretcher bond to window cill level, and weatherboarded and painted dark green above. It has a pitched felt roof. Windows throughout are modern replacements of post 1980 date but within the original openings. (The originals were multi-paned steel Crittall casements.) Those to the main building, including the mid C20 extension, have stone heads and concrete cills; those to the 1930s extension have very slight timber heads. External doors are also modern replacements. There are ducts through the north wall of the central wireless room which allowed cables to pass through the wall to link to the masts, formerly located on the traverse to the north of the building.

INTERIOR: The 1920s plan and modifications of the Second World War are clearly legible. As built the layout consisted of, from west to east, a store to the north and a W/T (Wireless Telegraphy) workshop to the south, then the PBX Room (the telephone exchange), Battery Room, Wireless Room and Signals Office along the northern side, with the internal access corridor to the south, and finally the Operations Room to the east which has exposed roof trusses.

In the mid 1930s the interior was modified by the insertion of a staircase to the east of the entrance, allowing the former Signals Office room floor to be raised such that it could be used as an observation room with three wireless booths overlooking the main Operations Room. At this time, the windows in the eastern wall of the observation room were inserted, the frame for which survives in part, as does evidence for the position of the booths. Initially the staircase descended into the Operations Room at its south-west corner, the scar of which is visible on the south wall. Following the construction of the eastern extension, the Searchlight Room, in the late 1930s a raised walkway was constructed along the southern side of the Operations Room allowing access through a now blocked raised doorway. The Searchlight Room appears to have a later inserted dividing partition wall.

Original features from this 1920s configuration survive including its paint scheme of cream and brown, floor boards, internal four panelled doors with Bakelite handles, and internal borrow-light windows between the rooms along the north of the building and the internal corridor. These are multi-paned windows with hoppers. There is also a grill in the battery room ceiling, to allow battery venting (cooling), cable ducts in the wireless room and also a hatch into the roof void from the W/T workshop. In the roof void are surviving porcelain insulators for aerial wires as well as a water tank.

Investigations by the Station in partnership with the Air Historical Branch (RAF) have identified many features which survive from its key 1930s to 1940 phase including the paint scheme of this era (cream, light and dark green), the original flooring and position of the raised walkways, under floor cable conduit positions, the position and shape of the plotting (map) table. Also that the orientation of the operations room was longitudinal with the position of the electronic tote boards, one for each of Northolt's five squadrons, identified on the east wall of the operations room, overlooked by the raised observation booths and the ops room dais to the west.

HISTORY: The outbreak of the First World War in 1914 and the rapid expansion of the Royal Flying Corps (the predecessor of the RAF) led to the selection and requisition of a number of new airfields in England. One such was at Northolt where land was acquired by the War Office from February 1914 with the landing ground operational by the following month although accommodation was initially temporary and basic. The airfield was laid out on a north-south alignment with the aircraft sheds aligned west-east to the north of the flying field. Further land was requisitioned as the airfield expanded. During the First World War Northolt was primarily a training airfield for pilots to be then deployed to the Western Front. Latterly it housed fighters to intercept Zeppelin airship bombing raids. Northolt became one of ten such 'air defence stations' surrounding London, this arrangement being known as the 'London Air Defence System'.

In the inter-war period Northolt functioned as both a military and civilian airfield with a flying school. The site was occupied by both a bombing and a fighter squadron as part of the 1920s RAF expansion plans, and considerable enhancement of the buildings took place. Further expansion, between 1928 and 1934, resulted in the construction of further buildings including the Pilots' Room and Watch Office (building 23). The Operations Block (building 27), which is under assessment here, was also completed by 1928, having been designed in 1924 although it is not clear how rapidly construction began. During the late 1920s to early 1930s Northolt took on the role of the transportation of senior government officials and the royal family, and this traffic increased when it became London's wartime airport. The station's most significant role at this time however, was its participation in numerous trials and air exercises associated with the development of the 'Dowding' system; the Integrated Air Defence System or IADS (see below).

Northolt was a key fighter station as part of 11 Group, RAF Fighter Command which covered the south-east of England and was commanded by Air Chief Marshall Sir Keith Park. (Fighter Command had been established in 1936 to organise the country's fighter stations in a number of regional groups with a central command based at RAF Bentley Priory. Other Commands covered Bomber, Transport and Training.) Northolt, acting in association with Uxbridge and RAF Bentley Priory, was very much in the front-line during the Second World War. The station's fighter squadrons played a key role throughout and were notable for their success in destroying enemy aircraft, with limited losses, during the 1940 Battle of Britain. Northolt was the first airfield to house operational Hurricanes and was also the home to Polish Fighter Squadrons. In 1943 it became the main terminal for RAF Transport Command but also remained a fighter station until March 1944. In the same year it became the home of a number of air photographic reconnaissance detachments from RAF Benson charged with the gathering of imagery of key events in the battle for the liberation of north-west Europe, namely the D-Day landings, Arnheim and the Rhine crossings. Inevitably Northolt continued to expand during the Second World War, the most significant development being the construction of two concrete runways. The accommodation was enhanced also, but all post-1940 buildings were either of a temporary brick construction or were prefabricated.

Building 27 was the 'Z' Sector Operations Block, constructed in the inter-war period. It was designed in September 1924 by the Works and Buildings architect C H Andrews (Francis 2008, 116) and completed in 1928. It was the prototype Second World War Station Sector Operations Room, designed by Air Marshall Dowding in the mid 1930s as part of the development of Fighter Command and the 'Dowding System': a method of communication to allow the various elements of the command chain to communicate efficiently and effectively in the understanding and intercept of enemy aircraft; the first such system in the world (see below). Building 27 functioned as the Sector Operation Block until mid September 1940 when the functions were dispersed off-station for safety, following an Air Ministry directive, to a temporary operations room near Ruislip Gardens Underground Station. From this time onwards Building 27 functioned as both a training facility and as the Night Intercept Room, prototyping a methodology for night fighter direction.

The 'Dowding' system, officially known as the Integrated Air Defence System (IADS), was developed and tested in the mid to late 1930s between a triangle of key sites: Bentley Priory, Stanmore, London, Fighter Command HQ; RAF Uxbridge, Group HQ for 11 Group Fighter Command (the country was divided into Groups) and then to Sector level (a subdivision of the groups) to RAF Northolt (Building 27). 11 Group was the key Fighter Command Group as it included London and the South-east and was therefore in the front-line of enemy attack, hence why 11 Group was instrumental in devising an appropriate system of response. Each Sector (in this case 'Z' Sector) had a main fighter station (here RAF Northolt), which therefore had a Sector Operations Room to direct operations for its own and other fighter stations within the Sector. The Sector Operations Room was therefore in communication via telephone, teleprinter and wireless with other Operations Blocks, the Observer Corps and many other operations to allow the spotting and tracking of enemy aircraft and to co-ordinate an appropriate response. Building 27 was therefore the third stage in the chain of command, responsible for deploying and directing the Sector's squadrons to meet an enemy attack.

While RAF Northolt was in the lead in terms of developing the processes of the Dowding System at Sector level, it was RAF Hornchurch (where there was also an early operations block, now demolished) which was to shadow Northolt's activities and lead on the development of the most appropriate design for such operations buildings. This included their internal layout, with a view to the Hornchurch scheme being rolled out nationally. However the resulting Hornchurch design concept was considered far too expensive, at £3,000 per station, and the Northolt layout, which was deemed to be working successfully and was also much cheaper, was adopted by Fighter Command nationally. Building 27 is therefore the prototype for all Second World War Operations Rooms. Building 27 was also associated with two significant historical figures: Wing Commander Keith Park, station commander in the early 1930s and later Air Chief Marshall in charge of 11 Group (after whom the building has recently been renamed), and also Wing Commander Vincent who commanded at Northolt during the Battle of Britain.

Building 27 was extended to the east in the late 1930s, when the Searchlight Room was added. This housed a variety of functions over its operational life-time including accommodation for anti-aircraft and searchlight liaison officers; co-ordinating the movement of aircraft at night; facilities for establishing the position of 'friendly' aircraft, and subsequently, after the Battle of Britain, was used to prototype a method of directing Night Fighter intercepts. Such processes had been found lacking during the Battle of Britain. Northolt had a sector wide function for Night (as well as for Day) Intercept, and again the systems devised here were rolled out nationally. Following the return, later in the war, of the Sector Operations function from Ruislip Gardens to Northolt (to a new hardened operations block; building 43), Building 27 was used as a training school for Sector officers and as a filtering facility for night operations. In 1940 the height of the traverses surrounding the building were increased to eaves height for added protection but were subsequently removed.

A flat-roofed entrance and toilet block extension was added, possibly in the 1950s and in the 1960s the diamond asbestos roof slates were replaced with square ones. At some time after 1980 the external windows were also replaced with double-glazed units. Modern exterior windows and doors and the 1960s slates are not of special interest and the mid C20 entrance block is of lesser interest.

SOURCES: Francis P, British Military Airfield Architecture: from Airships to the Jet Age, Patrick Stephens Limited (1996) Francis P, RAF Northolt: Historical Report & Building Survey, draft unpublished report dated 18 Feb 2008 for Defence Estates (2008) Gill, J, Operations Block (Building 27) RAF Northolt, Hillingdon, Greater London: Historic Building Recording, unpublished client report of March 2008 (OA reference 3557) Lloyd, S, The Sir Keith Park Building: Building 27 Sector Z Operations Block, No.11 Group, Fighter Command. An in-depth examination. (unpublished report, 2010).

REASON FOR DESIGNATION: The former Operations Block (Building 27), RAF Northolt, designed in September 1924 by the Works and Buildings architect C. H. Andrews and completed in 1928 and subsequently extended and modified in the late 1930s, has been considered for designation and is recommended for listing at grade II for the following principal reasons: * Design Interest: a building which evokes through its fabric the layout and function of a key late 1920s to 1940s Operations Block; * Intactness: although it has experienced post-war alteration including replacement windows, recent investigations have revealed that the inter-war and wartime fabric survives well; * Historic Interest: A building which contributed to victory in the Battle of Britain and the Second World War, and therefore has significance nationally; * Technological Interest: A building of national if not international significance in the development of the 'Dowding System' of Integrated Air Defence; the world's first such system which remains in use by the RAF today and which has been adopted by the majority of air forces around the world.

Selected Sources

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National Grid Reference: TQ 10028 85500


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