Date: 201210
Local time of day: 1201-1800
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Aircraft: Boeing Company Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Aircraft component: Airspeed Indicator
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Communication Breakdown; Troubleshooting; Human-Machine Interface; Distraction; Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Weight And Balance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Diverted; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: During takeoff roll; after the 'set thrust' call; I noticed that the EGT on one engine was in the upper amber arc while the other was indicating 175 degrees cooler. All other engine data [and] performance was normal. This distracted me for a few extra seconds. I looked to my EASI (tape airspeed display) to see what our speed was in order to make the '100 knots' call out. The display read 30 knots; and I knew we were well above that. I checked the analog display and it indicated 30 knots. I checked the standby ASI and it indicated 0. The Captain's EASI was increasing from 82 to 83 knots; and I called out 'reject; reject; reject.' The Captain did not respond after a few seconds so I stated 'I have no airspeed.' The Captain scanned his airspeed indicator and said 'continuing.' We both verbalized V1 and VR using his airspeed data; and I also crosschecked it with the standby which seemed to be working properly at the time. We continued straight ahead and cleaned up the airplane on schedule using his airspeed information. I completed the after takeoff checklist. By now several EICAS messages were uninhibited and IAS DISAGREE was one of them. There were a couple others that I do not recall. The Captain directed me to ask Control for a level-off at FL050 so we could sort this out and run checklists. It seemed to me that the Captain's airspeed indicator was not reading correctly several times during climb out; plus we were getting new EICAS messages. My airspeed had now began to indicate grossly inaccurate values; ranging from 30-180 knots; well below what we were actually flying. At times the EASI would revert to a 'flagged' status; generating more EICAS messages. I pulled out the Airspeed Unreliable Checklist and asked the Captain if this was the checklist we should use; as it seemed to me that his airspeed was not steady. Unbeknown to me at the time; he was selecting the right air data computer back and forth on his side; which led to his displays [alternately] indicating the same incorrect information as mine. I therefore; had every reason to believe that both ADCs were grossly unreliable; and as such I chose not to select the Left ADC on my side. I reviewed the unreliable airspeed table that lists altitudes; power settings; pitch attitudes; etc.; and everything seemed to compute for our phase of flight and power settings. We elected to return to our departure airport as per the QRH. Operating in a non-radar environment meant that my attention was spent three ways between the QRH; communicating with ATC; and navigating via DME and VOR radials. We eventually finished the Unreliable Airspeed Checklist and table; and proceeded with the Overweight Landing Checklist and its requirement to check approach climb limits and landing distance. By now my airspeed data generally matched that of the Captain's; though it often would drop 20-30 knots and slowly catch back up. By the time we had turned onto final approach my airspeed constantly matched the Captain's. We would land some 23;000 pounds overweight and; thus; had declared an emergency about 8 minutes prior to landing so that the emergency equipment would be available should the brakes get hot or catch fire. The Captain had set the autobrakes to level 4.  The touchdown was very soft; and the hottest brake temperature indicated was a five on five separate wheels. I cancelled the emergency after we turned off the runway and we taxied to the gate.




Synopsis: The flight crew of an unidentified Boeing aircraft continued their takeoff despite the failure of the right ADC and airspeed indicator late in the takeoff roll.  They returned to their takeoff airport for an uneventful overweight landing.