B747-400
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
Flight Crew Diverted; Flight Crew Landed As Precaution
Aircraft
While in cruise on [airway]; we got a HYD QTY LOW 1 EICAS (Engine Indication And Crew Alerting System) message. We ran the checklist. At this time we had normal pressure in all HYD (Hydraulic) systems. I believe the system 1 quantity at this time was approximately .23. It steadily decreased before our very eyes; with the pressure holding. I contacted Dispatch on the SAT (Satellite) phone; who patched in MX (Maintenance) Control. MX Control recommended continuing to ZZZ; and keep an eye on it. We figured we were going to lose the pressure when we started with the flaps and gear; but that could happen in one hour; or in 8. We agreed to keep an eye on it. Further along; we step climbed from 310 to 330. Upon movement of the elevator to climb; the HYD SYS pressure 1 dropped briefly; then came back up. We leveled off; then shortly thereafter; the pressure bottomed out; and we got the HYD PRESS SYS 1 EICAS message. We ran that checklist. Concurrently; the quantity on SYS 4 dropped from .9; to .63. Again I called Dispatch; and MX Control. I asked if there was a reason system 4 should be dropping; while in cruise. They had no definitive answer. At this time I determined it wasn't prudent to cross the Pacific. Between Dispatch; MX; [Maintenance Operations Manager]; and whoever else; we made the decision to divert into ZZZZ. We advised ATC) of such; and that we needed to dump fuel for approximately 45 minutes. We developed an arrival plan; dumped approximately 50.0 fuel; and proceeded inbound to land. I had Dispatch advise ZZZZ we would be needing a tug; due to no nose wheel steering; and I advised ATC of such; also. I had ZZZZ have the emergency equipment standby as a precaution; but declined to declare an emergency; unless further issues dictated. We made an uneventful arrival/approach/landing. I was able to taxi clear of the runway using a combination of A/S steering effectiveness; differential braking/power; and a convenient taxiway. We stopped; shut down; and were towed into the gate. MX advised that there was a major hydraulic leak coming from the tail. [Both First Officers] did an excellent job throughout this event; as did the Dispatcher; MX (Maintenance) Control; and whoever else participated behind the scenes. There was good communication between myself and Dispatch; MX Control; and Operations personnel. We had an initial plan; then when circumstances dictated; we adjusted the plan; and executed it. Communication between all concerned was excellent; and that showed in the end result. I let it be known to the crew that we had gas (plenty of gas); and presently; we had time. So we will be slow; deliberate; and make sure we cross all the t's; and dot all the i's; in all phases of what we were doing. FO's did an excellent job; and to me were an excellent reflection on the job that the training center does. It all went like it was supposed to.
B747-400 flight crew encountered a hydraulic leak and diverted.