Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 28000; Flight Crew Type 7000
Confusion
Aircraft Equipment Problem Critical
Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
The preflight briefing indicated the right wing fuel tank gauge will be inoperative for the flight.  When we got to the airplane; the maintenance paperwork indicated everything had been fixed.  Two hours into the flight; the left wing tank showed a 20;000 LB imbalance with the right wing tank showing more fuel.  The 'fuel disagree' checklist popped up and we started to complete it.  Before completion; the total fuel indication disappeared along with the left wing fuel indications.  We now; only had a right wing fuel indication.  The 'Fuel Disagree' checklist was inadequate for this situation.  We then elected to do the 'Fuel Leak' checklist and found it also inadequate; without a total fuel reading.  The same was true for the 'Fuel Imbalance' checklist.  After contact with Dispatch and Maintenance; it was concluded that a fuel leak was possible; but could not be verified visually; or by checklist.  Without being sure the airplane was not leaking fuel; an immediate diversion was planned.  Under this situation; we found the 'Fuel Disagree;' 'Fuel Leak;' and the 'Fuel Imbalance' checklists; inadequate to determine with certainty; whether the airplane was leaking fuel; or if the fuel sensors were the problem.  Making the wrong assumption could lead to a serious problem.
B777 Captain experiences a left fuel tank quantity indication failure in cruise; which also causes the failure of the total fuel indication.  A fuel imbalance indication had briefly annunciated before the quantity failure and without any method to determine if a fuel leak existed the crew elects to divert.