B767-300 and 300 ER
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; Flight Crew Landed As Precaution; General Declared Emergency
Aircraft
Approx 2 hours into the flight at cruise altitude [we] experienced a loss of the Captian's ADI. Screen went blank (Flat Panel) along with PITCH and ROLL flags on the First Officer's ADI. There were no other associated failures that could pinpoint the root cause of the failure. We were able to 'bootstrap' the Captain's ADI via the IRS alt switch; but due to a concern over the unknown cause of our degraded directional and attitude reference; we declared an emergency with Center and diverted. Overweight landing at 329;000; min sink rate landing; no auto brakes; min braking with a landing rollout of 11;000 plus FT. As a side note; a possible cause of the gyro failure was a lighting storm in the vicinity. The failure was coincidental with a period of St. Elmo's fire visible in the cockpit.
B767 pilot reports loss of Captain's ADI at cruise altitude on an ETOPS flight.  No associated faults are noted and the ADI is regained using the IRS alternate switch; however the crew elects to divert for maintenance.