Former RAF Alconbury: TR-1A Hardened Aircraft Shelter (Building Number 4111)

Hardened Aircraft Shelter (Building Number 4111), Former RAF Alconbury

This hardened aircraft shelter was built between 1987 and 1989 at RAF Alconbury to house United States Air Force Lockheed TR-1A reconnaissance aircraft. It is one of 13 identical shelters constructed at the base to accommodate the aircraft's 103-foot wingspan. The design was developed by the Property Services Agency (PSA) Directorate of Defence Works, based on research from the New Mexico Engineering Research Institute at the University of New Mexico. The PSA's Directorate of Civil Engineering Services adapted the research to withstand specific weapons threats. Turner Wright and Partners, civil engineers, produced the detailed designs, and the shelter was constructed by Fairclough Civil Engineering Limited.

The shelter has a reinforced-concrete foundation with edge beams and a reinforced-concrete arch superstructure with galvanised steel lining. The front and rear doors are steel infilled with concrete.

The shelter is rectangular in plan, aligned roughly east to west, measuring 42 metres wide and 35 metres long with a maximum height of 10 metres. It stands as one of a pair with Building Number 4112, positioned on opposite sides of a Y-shaped hardstanding. Earthen berms planted with trees partially conceal the shelter on its north, north-east and south-west sides.

The shelter features a three-centred arched profile in cross-section, with a continuous 50-millimetre thick steel rebound plate on the front face of the arch. The double-leaf front doors are each faced with 12-millimetre thick armour-plated steel and stabilised by rolled hollow steel section (RHS) framework. Each leaf comprises six prefabricated full-height panels, fabricated in Italy and welded together on site. Two rail tracks set within the reinforced-concrete foundation slab permit lateral door movement, with the RHS framework distributing weight equally to each rail. At the rear, a single-leaf sliding door of 12-millimetre thick armour-plated steel allows venting of jet efflux to a blast deflector fence. Both door sets are operated by rack and pinion gearing driven by hydraulic motors in the mezzanine plant room. On the left-hand side, a single-storey personnel door is protected by a reinforced-concrete blast wall and roof. Originally open on both sides, this has since been infilled with a secondary steel door and rear wall. An entrance telephone is mounted to the right of the original door, and a steel door on the blast wall's external face conceals a diesel generator refuelling panel. The roof carries T-shaped ventilators.

The interior has an exposed reinforced-concrete floor and exposed corrugated-steel walls. 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. Original high bay and underwing lighting also survives. At the rear left-hand corner, a pair of stacked rooms comprise a lower sensor readiness area (now without original equipment) and an upper plant room, accessed by a metal staircase, which contains a hydraulic power unit for operating the doors.