To: 411A
I am, not at all familiar with the system architecture of the 747 however many commercial jets have more than three systems. The primary systems use engine driven pumps and most secondary systems are electrically driven and can be used to directly power one or more of the primary systems providing it is only because of a pump failure. In some cases there is a power transfer system which can use the electrically or engine driven pump in one system to power a hydraulically driven pump in the effected system. However in the case of the Japanese 747 the systems were compromised in that the plumbing had carried away which would negate the use of an alternate system. The only means left to the pilot to control the 747 was differential thrust just like on the United Sioux City DC-10 only the DC-10 still had an intact rudder to maintain longitudinal stability. The Japanese 747 just wandered about the sky until it hit a mountain.
