Hydraulic Dampers manufactured by Ace Controls International have been used to automatically shut trap doors on stairwells in tower crane mast sections. The doors were intended to be a last-ditch line of safety should other fall restraints such as ropes or harnesses fail, or in case they were not used. After field testing, it was found that the original manually-operated trap doors were being left open, which defeated their purpose. Ace fitted model HB 28 to every door so that they would close automatically. There are five models in the range, with strokes ranging from 25mm-800mm.