B777 Undifferentiated or Other Model
Flight Crew Total 24000; Flight Crew Type 1197
Human-Machine Interface; Communication Breakdown; Confusion
Aircraft Equipment Problem Less Severe
Flight Crew Overcame Equipment Problem; General Maintenance Action
Chart Or Publication; Manuals; Procedure; Aircraft
Lost Center [C] Hydraulic System quantity and pressure after landing gear and flap extension on final approach. I deemed it safer to land than to go around. My crew agreed. We accomplished associated checklists on the ground. Of note regarding the checklists; the HYD PRESS SYS 'C' Checklist yielded an unexpected result. The system evidently sensed at least momentary valid [hyd] pressure after we turned on an AIR [Driven] DEMAND pump. We infer this because the remaining checklist actions were disabled by the Electronic Checklist logic. However; the Center Electric and [Air Driven] Hyd Demand pumps all showed faults on the Overhead panel. While we might have missed a momentary pressure fluctuation on the Hydraulic System Synoptic [page]; for all intents and purposes the synoptic quantity and pressure remained at zero for the Center Hydraulic System. The Disabled checklist items would have directed us to secure all the hydraulic pumps. This was not done and the pumps operated absent indicated hydraulic quantity and pressure for over 40-minutes.  Aircraft System Malfunction Hydraulic Power.
Reporter stated when he called for Intermediate flaps position; he noticed a momentary flicker of the Center Hyd System pressure. So he decided to fully extend the flaps and lower the Landing gear. That's when he noticed the loss of 'C' System quantity and pressure. The Checklist for loss of Center 'C' Hyd System directs pilot to select one or the other Air Driven (ADP) (Demand) Hyd pumps. When he did switch to one of the ADP's; there was apparently enough fluid to provide momentary hyd pressure to the 'C' System.  As a result; the aircraft's Electronic Checklist logic prevented any further continuation of the Disabled Checklist items; even though all the Hyd pumps for the Center 'C' Hyd System on the Overhead panel were showing fault lights. The Hyd Synoptic page for the Center System also continued to indicate zero Hydraulic quantity and pressure.Reporter stated he thought of shutting off all the hyd pumps associated with the Center Hyd System; but since the Electronic Checklist logic stopped any further procedures from being continued by graying/blueing out the remaining section of the Disabled list and notifying the crew the Checklist had been accomplished; he decided to leave the pumps on. The loss of Center 'C' Hydraulic System fluid was due to a slow; continuous hyd leak from a hyd line at the Nose Wheel Steering Bypass pin area.
Captain reported their Disabled Checklist for loss of Center (C) Hydraulic System quantity and pressure directed the pilot to select an Air Driven (Demand) Pump (ADP) on B777 aircraft. The Electronic Checklist logic stopped the continuation of the Disabled checklist due to a momentary spike in the 'C' System pressure. As a result; the Center System hyd pumps were left on even with the Center Hyd quantity and pressure at zero.