Date: 201902
Local time of day: 0601-1200
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Weather elements / visibility: 5
Light: Dawn
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Air Data Computer
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While at cruise at FL300 and approaching oceanic waypoint ZZZZZ; we received an EICAS message for IAS DISAGREE. Split between left and right airspeed indicators was 11 knots. Left read 308 and right read 297 knots. First Officer [FO1] was in the PRM for his rest period so First Officer [FO2] and I attempted to diagnose the problem. I was the flying pilot while the First Officer and I read and discussed the checklist. We were maintaining altitude and airspeed with automation on while we continued to diagnose the situation. After some time; maybe 5 minutes; we got a RUDDER RATIO and an AILERON LOCKOUT EICAS. It was starting to become clear we had a pitot static/ADC problem. We started the Unreliable Airspeed memory items and began to run the checklist. A few minutes later we received some 5 or so stall warnings/stick shaker a LEFT AND RIGHT EEC MODE EICAS messages. We then brought First Officer [FO1] into the event by calling him to the flight deck to help with checklists and to help us with pitch and power settings and to help interpret the many malfunctions we were experiencing. We were fairly confident at this point we had a problem with the right side ADC/ pitot static system as the left continued to remain at 308 while the standby airspeed indicator and right airspeed slowly began to bleed off speed. After completing the checklist we regained the malfunctioning right airspeed indicator and realized we were down to a single ADC with no automation or Flight Directors. We then called our Dispatcher via SAT COM and with the assistance of the Dispatcher; the SME (System Matter Expert) on duty and [Maintenance Control] concluded that with only a single ADC system continuing to [our destination] was not a safe course of action with 3 hours of flight remaining and the [destination are being] IFR. The checklist called for a flaps 20 landing and an ILS if available so we concluded the best course of action with the SME and Dispatcher in agreement would be to land at ZZZZ where the weather was 12000 foot ceilings with unlimited visibility. We then [coordinated] with [Center] asked for Crash Fire and Rescue to be standing by in the event our airspeed problem became worse. We elected to land with a 5 knot tailwind because it was the only ILS available and the runway was 11000 feet. Our landing distance remaining indicated we had plenty of runway. We landed slightly overweight at about 328;000 pounds. Center fuel was empty so dumping fuel was not an option.While commencing our descent from FL300; we were getting conflicting information from all three airspeed indications. We established a pitch and power setting from the Airspeed Unreliable checklist which didn't give us the performance we expected from the Left side Airspeed indicator; the side we thought was reliable. We confirmed ground speed and altitude with Gander multiple times to confirm the left side information was inaccurate. Standby was most accurate in the initial descent. Descending through 14;000 feet; the ND (Navigation Display) and Standby Airspeed indicator began to behave erratically. The ND was showing winds aloft at over 200 miles an hour and dropping to less than 50 with the direction changing over 180 degrees in less than 1000 altitude change in the descent. Within a few more thousand feet; the Standby Airspeed was normal and all of the EICAS messages all went away except the Right EEC. We landed without incident and were given a follow me car to parking. Throughout the event my two fellow crew members performed flawlessly and I couldn't have been more impressed with their calm; professionalism; and expertise. It was a very confusing situation and I am happy to have had such a great crew to work the problem and resolve it.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B767-300 flight crew reported experiencing an Air Data Computer malfuntion while on a international flight.