The compartment between SWB3 and the front spar was originally equipped with a bladder cell for water and plumbing to inject that water into the engines to increase engine thrust on takeoff. When the original engines were replaced with higher thrust engines, water for water injection was not needed, and the compartment between SWB3 and the front spar was converted to a dry bay. The accident airplane's water system had been deactivated, and the bladder cell and plumbing had been removed. In some later models of the 747 and in the military (E-4B) version, this dry bay has been modified to carry fuel.