A B777 pilot reported receiving erroneous TCAS TA and RA warnings which he believed were generated b...

Narrative:

During level flight at FL280; received a TCAS aural traffic advisory (TA) 'traffic; traffic' which was depicted on navigation display at 12 o'clock position and FL285. This was followed shortly (within 4-5 seconds) by a TCAS aural resolution advisory (RA) 'descend; descend'. This was promptly complied with by the pilot flying while I advised ATC. ATC advised us that they had no traffic at FL285; but a navy aircraft was at our 12 o'clock position at FL250 with a malfunctioning mode C transponder. After climbing back and maintaining FL280; it became apparent that following the TCAS RA was actually descending us into the traffic that was creating the TA/RA due to an admitted malfunctioning mode C transponder. Due to the rapid onset of the RA; neither visual nor ATC confirmation of the traffic could be made prior to actually executing the RA maneuver.

Original NASA ASRS Text

Title: A B777 pilot reported receiving erroneous TCAS TA and RA warnings which he believed were generated by an aircraft below them with a malfunctioning Transponder Mode C.

Narrative: During level flight at FL280; received a TCAS aural Traffic Advisory (TA) 'TRAFFIC; TRAFFIC' which was depicted on NAV display at 12 o'clock position and FL285. This was followed shortly (within 4-5 seconds) by a TCAS aural Resolution Advisory (RA) 'DESCEND; DESCEND'. This was promptly complied with by the pilot flying while I advised ATC. ATC advised us that they had no traffic at FL285; but a Navy aircraft was at our 12 o'clock position at FL250 with a malfunctioning Mode C Transponder. After climbing back and maintaining FL280; it became apparent that following the TCAS RA was actually descending us INTO the traffic that was creating the TA/RA due to an admitted malfunctioning Mode C Transponder. Due to the rapid onset of the RA; neither visual nor ATC confirmation of the traffic could be made prior to actually executing the RA maneuver.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	882681
Time
Date	201004
Local Time Of Day	1201-1800
Place
Locale Reference	ZBW.ARTCC
State Reference	NH
Aircraft 1
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Traffic Collision Avoidance System (TCAS)
Person 1
Function	Pilot Not Flying First Officer
Experience	Flight Crew Last 90 Days 180 Flight Crew Total 13000 Flight Crew Type 1100
Events
Anomaly	ATC Issue All Types Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude

A B777 pilot reported receiving erroneous TCAS TA and RA warnings which he believed were generated by an aircraft below them with a malfunctioning transponder mode C.