B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Physiological - Other; Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; Flight Crew FLC Overrode Automation; Flight Crew Landed As Precaution
Aircraft
I was the Relief Pilot on this trip.  While the Captain was on his break; we got a CABIN Automatic (AUTO) Inoperative (INOP) light followed by AUTO INOP light.  We followed the procedures laid out in the 767 QRH on page 2.9 titled CABIN AUTOMATIC INOPERATIVE; and as directed; began controlling the pressurization using the CABIN ALTITUDE MANUAL control.  We determined that BOTH 1 and 2 were inoperative.  We woke the Captain up.  The manual pressurization became difficult to properly maintain; and started getting away from us.  Both our Flight Attendants and passengers began complaining about ear discomfort from pressurization; and I felt discomfort as well.  With about 4 1/2 hours left of flying to get to our destination (MIA); we decided that the best course of action was to divert to our nearest suitable alternate with maintenance capability.  On my post flight walk around; I discovered a massive water leak from the water drain near the tail of the aircraft.  This was a continuous steady leak of ice cold water that continued right up until well AFTER we left the aircraft when our flight attendants became no longer legal to fly. This was at least 3 hours of continuous steady flow.  Maintenance discovered that the leak originated from the aft galley coffee maker; and determined that BOTH AUTO CABIN controllers had been frozen to the point that they became inoperative.  Pictures were shown to me (taken by maintenance) of large chunks of ice found in the aft sections of the aircraft as they attempted to trouble shoot the problem and find the water leak.  [The flight] was continued later that day after our crew was rested and the flight attendants were once again legal to fly.  We took off at XA:30 PM and landed in MIA at XE:48PM without any further issues with our Auto Cabin Controllers nor any further problems with water leaks.  It was flown with the leak isolated with the aft galley water system turned off.
A B767 flight crew while enroute experienced a cabin automatic pressurization failure of systems 1 and 2. The standby pressurization system was difficult to control and they elected to divert and landed. Post flight revealed that a water leak had frozen both auto cabin pressurization controllers.