Hydraulic control unit with hand pump controls. Based on contemporary photographs from the Los Angeles times, it appears this unit was added after the first flight. According to the Docent that conducted the tour, when Hughes first attempted to land the craft after the first flight he cut the engines back to idle at which point he lost all hydraulic preasure and was unable to control the aircraft he quickly applied minimum power and the preasure came up. This unit appears to have been added so that the pilot had better awarness of the preasures in each of the lines.

The H-4 was the first aircraft to use Hydraulic contols to move the flight controls. Due to the size of the aircraft and the size of the control surfaces the preasure that would have been needed to move the surfaces by cable would have been more than one person could exert.