The carts are used by Boeing to test the fuselage pressurization leakage rates and various aircraft hydraulic systems during final assembly. The air carts are supplied with shop compressed air and regulate it to a carefully controlled and measured flow and pressure. The cart output air is used to check leakage rates of the final assembly fuselage. The hydraulic cart is used to test operation of landing gear and other aircraft hydraulic functions. It is designed to provide 6000psi pressure using aircraft hydraulic fluid. All the carts were designed, built, tested by LEI and delivered to the 737 plant in Renton WA.