Building 27 At Raf Northolt

Building 27 at RAF Northolt is a former operations block designed in September 1924 by Works and Buildings architect C. H. Andrews and completed in 1928. The building was extended and modified in the late 1930s, with an entrance and toilet block added probably in the 1950s. The roof was re-tiled in the 1960s and windows were replaced after 1980.

Construction and Materials

The building is constructed of yellow stock brick laid in English bond with a hipped asbestos-tiled roof. The windows are modern uPVC replacements set within the original openings, which have stone heads and concrete cills. The late 1930s extension features weatherboarding. The building is oriented west to east, with the main entrance in the south elevation accessed through a mid-20th-century entrance extension. A west-east corridor runs along the southern side of the building with short flights of steps both up and down, providing access to a series of small rooms to the west and north and to the large operations room to the east. A late 1930s extension lies to the east of the operations room, and the original plant room occupies the south-west corner.

Exterior

Building 27 is a single-storey rectangular structure in yellow stock brick laid in English bond. The hipped asbestos-tiled roof was re-tiled in the 1960s when the original diamond-shaped asbestos tiles were replaced with square ones. When first built, the building was protected by an external earth traverse which has since been removed. The south elevation, which is the main front, features a mid-20th-century flat-roofed entrance and post-war toilet block, with the original low plant-room projection and chimney at the south-west corner. The north elevation has a run of ten windows.

The late 1930s extension at the east end is constructed at right angles to the main building, with yellow stock brick walls in stretcher bond up to window cill level, and weatherboarding painted dark green above. It has a pitched felt roof. Throughout the building, windows are modern replacements dating from after 1980, though they occupy the original openings. The originals were multi-paned steel Crittall casements. Windows in the main building, including those in the mid-20th-century extension, have stone heads and concrete cills, while those in the 1930s extension have very slight timber heads. External doors are also modern replacements. Ducts pass through the north wall of the central wireless room, which allowed cables to connect to masts formerly located on the traverse north of the building.

Interior

The 1920s plan and Second World War modifications remain clearly legible. The original layout consisted, from west to east, of a store to the north and a wireless telegraphy workshop to the south, followed by the PBX Room (telephone exchange), battery room, wireless room and signals office along the northern side, with an 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 with the insertion of a staircase to the east of the entrance. This allowed the former signals office floor to be raised so it could serve as an observation room with three wireless booths overlooking the main operations room. At this time, windows were inserted in the eastern wall of the observation room, 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 this arrangement is visible on the south wall. Following construction of the eastern extension (the searchlight room) in the late 1930s, a raised walkway was built 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 the 1920s configuration survive, including the paint scheme of cream and brown, floorboards, 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. A grill in the battery room ceiling allows for battery venting (cooling), and there are cable ducts in the wireless room and a hatch into the roof void from the wireless telegraphy workshop. In the roof void are surviving porcelain insulators for aerial wires and a water tank.

Investigations by the station in partnership with the Air Historical Branch (RAF) have identified many features surviving from the 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, underfloor cable conduit positions, and the position and shape of the plotting (map) table. 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 operations room dais to the west.

Historical Context

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. Land at Northolt 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 aircraft sheds aligned west-east to the north of the flying field. During the First World War, Northolt was primarily a training airfield for pilots to be deployed to the Western Front. Later it housed fighters to intercept Zeppelin airship bombing raids. Northolt became one of ten such 'air defence stations' surrounding London, an arrangement 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 additional buildings including the Pilots' Room and Watch Office (Building 23). Building 27, the operations block, was designed in 1924 and completed by 1928, though it is not clear how rapidly construction began. During the late 1920s to early 1930s, Northolt took on the role of transporting 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.

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 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 gathering imagery of key events in the battle for the liberation of north-west Europe, namely the D-Day landings, Arnhem and the Rhine crossings. Northolt continued to expand during the Second World War, the most significant development being the construction of two concrete runways. Accommodation was also enhanced, but all post-1940 buildings were either of 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 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 understanding and intercepting enemy aircraft, the first such system in the world. 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, as the country was divided into Groups); and then to Sector level (a subdivision of the groups) at 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 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 lifetime, 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. The modern exterior windows and doors and the 1960s slates are not of special interest, and the mid-20th-century entrance block is of lesser interest.