Date: 201304
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Air Data Computer
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On the takeoff roll at 100 KTS I noted the Captain's airspeed hung up while the First Officer's was increasing. We took off and shortly afterward got an Airspeed Unreliable EICAS with a Mach Trim EICAS and Rudder Ratio EICAS. We told Tower to keep us with them since we would be returning to the field for landing. After getting to initial altitude and after completing the After Takeoff Checklist I compared all three [airspeed indicators] and the First Officer and standby [airspeed indicators] matched while the Captain's [airspeed] was off at least 30-40 KTS. I sent an ACARS message to Dispatch to call me and started the Airspeed Unreliable QRH checklist. When I received Dispatch's frequency I had no luck contacting him on either radio. I asked for another frequency and he gave me [another] one and I also had no luck with either radio for that frequency. In the meantime we declared an emergency and had [Approach] give us vectors; so we could finish the checklists. I completed the Airspeed Unreliable checklist and went to ALTN Air Data on the Captain's side and all three airspeeds now matched up. We got the ATIS and landing performance and I also did the Mach Trim QRH as well as the Rudder Ratio QRH. Since my radio contact with Dispatch never worked I sent him an ACARS message telling him we had declared an emergency because of unreliable airspeed and were returning to [departure airport] and to notify the Company. Shortly after takeoff I informed the purser that we would be returning and to please give the passengers that information and that I would talk to them further when I was finished running all of the required checklists. After all of the irregular checklists were done as well as the Approach Descent Checklist I gave the passengers an update on our return and information about the nature of our emergency. We had a light fuel load and made a normal weight landing with no further problems.


Second narrative: At liftoff; we received multiple MC and EICAS messages - Mach Speed Trim; IAS Disagree; and Rudder Ratio are the three that I can remember. The jet was flying fine and I maintained control. The only flight characteristic that was of note was a slight nose heavy feel.


Synopsis: B757 flight crew reported loss of Captain's air data computer on takeoff roll. After liftoff they ran the procedures; declared an emergency; and returned to departure airport.