Former RAF Alconbury: TR-1A Hardened Aircraft Shelter (Building Number 4112)

Former RAF Alconbury: Hardened Aircraft Shelter (Building Number 4112)

This is a hardened aircraft shelter built between 1987 and 1989 at RAF Alconbury for the United States Air Force. It is one of 13 contemporary shelters designed to accommodate the Lockheed TR-1A reconnaissance aircraft, which has a wingspan of 103 feet (31 metres). The design was undertaken by the Property Services Agency Directorate of Defence Works, based on research conducted by the New Mexico Engineering Research Institute at the University of New Mexico. The PSA's Directorate of Civil Engineering Services developed the design to withstand weapons threats. Detailed designs were produced by Turner Wright and Partners, civil engineers, and the shelter was constructed by Fairclough Civil Engineering Limited.

The shelter has a reinforced-concrete foundation and foundation edge beams, with a reinforced-concrete arch superstructure lined with galvanised steel, and steel front and rear doors infilled with concrete. It forms a pair with HAS Building Number 4111, which stands opposite across a Y-shaped hardstanding. Earthen berms planted with trees partly conceal the shelter on its north-west, south-east and south-west sides.

The shelter is rectangular on plan, aligned roughly north-east to south-west, measuring 42 metres wide and 35 metres long with a maximum height of 10 metres. On its left-hand side is a single-storey personnel access door.

The shelter has a three-centred arched profile in cross section, with the front face of the arch featuring a continuous 50-millimetre thick steel rebound plate. The double-leaf front doors each have 12-millimetre thick armour-plated steel skin and are stabilised by a rolled hollow steel section framework. Each leaf is constructed from six prefabricated full-height panels that were welded together on site after being fabricated in Italy. Two rail tracks set within the reinforced-concrete foundation slab allow the doors to move laterally, with the framework transferring equal weight to each rail. At the rear is a single-leaf sliding door, also of 12-millimetre thick armour-plated steel, through which jet efflux was vented to a blast deflector fence. The doors are operated by rack and pinion gears driven by hydraulic motors located in the mezzanine plant room. On the left-hand side there is a single-storey personnel door protected by a reinforced-concrete blast wall and roof. To the right-hand side of the door is an entrance telephone, and on the external face of the blast wall is a steel door concealing a diesel generator refuelling panel. The roof has T-shaped ventilators.

Inside, the floor is of exposed reinforced concrete and the walls are of exposed corrugated steel. The shelter retains 13 supply and extract ventilation systems for normal use, dedicated smoke extract and standby generator cooling, and two further systems for vehicle exhaust extraction. The original shelter lighting, comprising high bay and underwing units, also survives. At the rear left-hand corner is a pair of stacked rooms: the lower one is the sensor readiness area, which retains no original equipment, and the upper one is a plant room accessed by a metal staircase, which contains a hydraulic power unit for operating the doors.