The British Overseas Airways Corporation (BOAC) maintenance complex comprises four large aircraft hangers with dramatic folding doors, built around an internal cross-shaped structure that accommodates offices and workshops. Owen Williams was both architect and engineer. Each hanger was designed to hold two of the largest size of planes in use in the 1950s. The buildings were commissioned by the Ministry of Civil Aviation but it was clear from an early date that BOAC were to be the end users.

Williams had been interested in hanger design from before WWII and had previously presented plans for hexagonal hangers to the Ministry. After considering these, the Ministry decided on the more conventional rectangular structure built around an internal cross plan. William's reinforced concrete design won out over a steel framed building proposed by the Ministry's own engineers, on the grounds of cost.

The quality and scope of Wlliams's work here attests to a lifetime's experience with concrete. Up until then, most hangars in the UK had been built of steel to standardised designs. This project is made up of large scale elements, each of which required a fresh engineering approach. Williams was able to draw on his early experience of constructing large span bridges. Each of the four hangars measures some 102m (335 ft) by 43m (141 ft). Planes access the building via four symmetrically positioned doorways - two at the front and two at the back. The cross-shaped central area, with one long axis and one short, both separates and connects the hangars - engineering and service areas run along the internal sides of each.

To make sufficiently wide door openings, Williams devised double-cantilevered arches built in situ of reinforced concrete. Each door consists of two bracketed arches, one springing from each side of the opening and bridged by a centrally-positioned V-shaped beam. The arches are offset so that the sliding door movement is unobstructed. To allow the arches to reach the width needed, a counterbalance technique was used. Huge elevated chambers were built on each side of the openings. These were gradually filled with balast material to balance the weight of the roof as construction progressed. In the end, each chamber contained about 1000 tonnes of balast. This technique required a sophisticated use of reinforced concrete - Williams had first used deadweight in superstructure design in 1934 for the Empire Pool (now Wembley Arena).

The main engineering hall is a structure within a structure. It is 264m (866 ft) long and lies between the front and back hangar pairs, forming the long axis of the cross plan. Its raked cylindrical columns sit at 5.5m (18 ft) centres, with 23.2m (76 ft) clear spans across the space. Overhead beams run parallel to the roof structure in the storey-height below the roof. The columns support a series of galleries jettied out progressively from one floor to the next. Gantry cranes run on rails bracketed to the columns.

The hangar roofs are constructed from 3m (10 ft) deep V-shaped concrete trusses (using pre-cast and in situ members) set on a 5.5m (18 ft) grid and spanning 42.7m (140 ft). A similar design of lesser span covers the engineering hall, making a total depth of 128m (420 ft). Williams was always interested in providing good levels of daylight and here roof glazing does the job. Patent glazing fills the angled spaces in the trusses, and the gutters are integrated into the lower truss chords. Comparison can be made with Joseph Paxton's 'ridge and furrow' glazing developed for the Great Exhibition building of 1851, exactly a century earlier.

For the side walls of the building, Williams used continuous horizontal bands of simple framed windows with bands of brickwork between. The side elevations contrast with the highly-engineered symmetrical composition of the front and back elevations. Although perhaps monotonous, the side elevations show us that Williams wasn't looking for applied architectural style but thought engineering by itself displayed architectural quality.

The maintenance building is still in use and can be seen from the approach road to Terminal 4, driving west. Some of William's immense folding hanger doors have more recently been obscured by a steel-framed extension designed to accommodate large modern jets.