B777 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 190; Flight Crew Total 17300; Flight Crew Type 1100
Confusion; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Aircraft Equipment Problem Dissipated; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
On descent we were passing through FL210 when we got a TCAS RA. The TCAS showed a target at our precise location and at our exact altitude. The Captain responded to the RA and descended. When we got to about FL203 we got another TCAS RA that told us to climb. The TCAS showed a target again at our exact location and altitude. The Captain again responded to the RA. The RA and target disappeared after a climb of about 100-200 feet. We reported all of this to the Center Controller who said that he showed no aircraft in our vicinity. He said he would write it up on his end but that he recommended we check our equipment. It appeared to us that our TCAS was responding to our own aircraft as an intruder aircraft. The Captain said that he had heard of this happening during the early days of TCAS use. After landing the Captain wrote up the TCAS and called Maintenance to explain the writeup.
During descent on the STAR descending through FL210 we got a TCAS RA red symbol on nose of our aircraft ND symbol and 'descend' aural command. I disconnected autopilot and increased rated outside of red bar. Subsequently we got a pitch reversal TCAS RA to 'Climb' aural message at FL202; which I smoothly complied with. We informed ATC; he said no traffic in our area. He said 'May want to get our equipment checked.' We saw no traffic visually. Upon block in; we wrote up the TCAS and verbally informed Maintenance.
During descent a B777 TCAS generated an apparent false traffic target and issued a descend command followed by a climb command. ATC reported no traffic in the area so this is an assumed null bus fault.