B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
General Maintenance Action
Aircraft
Flying at [cruise] altitude we received an EICAS message C HYD QTY. After checking our status page we realized we were down [to] 49 percent. Realizing we had a leak in the center system we elected to depressurize the system by turning off both electric pumps and the center demand pump and switching autopilot from center to left. This stopped the loss of fluid. We now had one hour and 20 minutes to plan for a total loss of the center system and our plan of action. Since we had 49 percent left in the center system and this was holding we decided we would turn the pumps on when on approach and see if gear and flaps would extend normally. We discussed our option if they did not and we both were well versed on HYDRAULIC SYSTEM PRESSURE (C ONLY) checklist. On approach flaps and gear did extend; however fluid loss was increasing. We had discussed that the loss of all center fluid down to the standpipe was a possibility; so we could use reserve brakes and steering if needed. Approach and landing were normal and we had 11 percent fluid left with steering commands normal. We taxied in leaving the flaps down and had enough fluid to steer to parking. The cause of the leak was determined to be an o-ring the size of a dime that had deteriorated.
B767 pilot reported landing safely with a leak in the C hydraulic system that ultimately reduced quantity to 11%.