B777 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Total 22000; Flight Crew Type 10000
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
On North Atlantic Track E; in VHF contact with Gander Domestic. Level cruise at FL380. RA event with 'Descend; Descend!' We did at approximately at FL378 the warning went away. We were stable at FL377. Notified Gander they had no traffic close. Only traffic was at FL390 at about 5 NM in our 11 o'clock position. We reengaged autopilot and claimed back to FL380. We notified the Captain who was on break. The event was written up in the maintenance log with a '0' for FRM item. On descent the flying crew experienced the same event at a lower altitude. Again no traffic in the vicinity.
During Atlantic crossing at FL380 in Ganders airspace; an RA was commanded by the TCAS system. The pilot flying immediately followed the command to descend. Aircraft descended about 300 feet before the command steering went away. At no time did the system verbalize 'clear of conflict' as is the norm for an effective RA maneuver. Informed Gander Radio of the avoidance maneuver and they informed the crew that there was no traffic closer than 5 NM away at FL390. Climbed back up to assigned altitude of FL380. We believed at the time that it is was a system malfunction. Happened a second time while flying the arrival. The aircraft was at FL280 when another RA was commanded. Pilot flying immediately disconnected the autopilot and followed the corrective steering to descend. Descended to approximately FL277 when RA commands ended. Again; no 'clear of conflict' was announced from the system. Climbed back up to FL280 and informed Center of the RA maneuver. They also claim no traffic anywhere near our position. Wrote-up the malfunctioning system and sent to Maintenance via ACARS.
B777 flight crew experiences several false TCAS RAs for co-altitude traffic; commanding a descent. In each incident ATC advised there was no traffic nearby.