B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Confusion; Troubleshooting; Workload
Aircraft Equipment Problem Less Severe
Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Declared Emergency; General Maintenance Action
Aircraft
After takeoff; First Officer's (FO's) flight director gave erroneous commands and the heading triangle was not displayed on the FO's ND. The FO's flight director only gave normal indications when L was selected on his INSTR SOURCE SEL panel. There were no other abnormal indications in the cockpit. No EICAS messages were displayed; no amber lights came on anywhere. After some investigation we discovered there was no information available from the center and right IRU on the IRS Mode Select panel or either FMC. Concerned that we were operating on only 1 IRU; after consulting with Tech and Dispatch; we declared an emergency; went direct to ZZZ [our departure airport].  Accomplished QRH Overweight Landing checklist; made required AML entries.
As a crew; we don't know why 2 IRUs could apparently fail (records indicate maintenance found them both faulted and replaced them) without any cockpit failure annunciations. Each IRU has a FAULT light and they are monitored by the EICAS.
After takeoff; First Officer's flight director gave erroneous commands with two IRUs inoperative.  Emergency was declared and crew returned to departure airport.