B777 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Total 17600; Flight Crew Type 14000
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Returned To Clearance
Human Factors; Aircraft
While in cruise with the autopilot and seatbelt on the TCAS initiated a RA indicating traffic. There was no TA preceding the event. I scanned the instruments and a target was displayed at our altitude in our immediate vicinity. I tried to acquire the target visually as well. The TCAS issued the aural warning 'Descend! Descend!' and the pilot flying initiated a descent and turned the autopilot off. I initiated a radio call broadcasting our position and actions on frequency 123.45. The TCAS then issued an additional warning of 'Increase descent! Increase descent!' and we descended approximately 500 feet below our assigned altitude at which time it indicated we were clear of the conflict. We returned to our assigned altitude and the pilot flying engaged the autopilot. We contacted Gander Center and explained our actions. They confirmed our altitude and that they showed no traffic in our vicinity at the time. We reviewed the maintenance history showing the TCAS had indicated false RA reports as early as the same day of our departure. I contacted Dispatch and Maintenance and wrote it up. I suggested we switch from the left transponder to the right and Maintenance agreed. We fully briefed the Captain and flying First Officer when they returned from break. The flight was completed without further occurrence or incident.
At cruise altitude; B-777 TCAS issued a RA and; as a result; the flight crew deviated from assigned altitude by 500 ft. TCAS issued a verbal 'Clear of conflict' and the crew returned the aircraft to assigned altitude and notified Center of the deviation. Neither ATC nor the crew observed traffic in the area. Review of the aircraft maintenance history revealed an earlier-reported false RA.