Narrative:

This is a report of a type of TCASII failure which is not normally seen. No training in the failure mode was given to any of our crews. It will happen again. I took no evasive action. En route at cruise altitude in rvsm/mnps we received a TCASII resolution to descend. This was followed with a TCASII command to increase descent rate. The rates commanded were 1500 FPM followed by the rate of 2500 FPM. The problem was: the target which was creating the resolution was a false target. The threat was a target which appeared from behind our aircraft and showed zero altitude difference from our aircraft. I did not respond to the warning. It appeared to me to be a false target. I have seen false targets in the past on the B767 fleet. In those cases the target would appear and only show the yellow ball symbol. In this case the target changed from the yellow ball symbol to the red square as the resolution command was issued. When the ball symbol appeared the 'traffic, traffic' warning was issued. With the red square display the 'descend' and 'increase descent' commands were given. My concern is the training we are giving to the crews is to respond immediately to TCASII resolutions. I was not aware that there is a failure mode which could create a resolution. A TCASII response in rvsm airspace is the most serious part of this problem. In attempting to understand this problem I have been working with captain a and mr B, manager of aircraft reliability. Mr B also asked that we speak with commercial radio. Commercial radio is the point of contact for logging rvsm problems. This failure mode is not unknown to this unit. It can be seen if there is a failure of the suppression bus. That bus is used to make the TCASII not respond to the aircraft itself. My air carrier has asked rockwell collins to evaluate the TCASII computer and the transponder. I spoke with mr C of rockwell collins about this type of failure. He concurs with the theory that the suppression bus is the problem. Mr C also advised that neither the transponder nor the TCASII computer will normally be the problem. Several yrs ago, in the early TCASII installations wiring in the suppression bus caused the problems. I believe we need to advise ICAO of this failure. Someone in ICAO is maintaining a database of problems in rvsm airspace. There is a definite potential for a safety problem with this failure. Callback conversation with reporter revealed the following information: captain reporter states that postflt revealed that the TCASII unit had a suppression bus failure which resulted in the aircraft picking up its own transponder as an intruder. Reporter contacted rockwell collins who is the manufacturer and they reported that, though relatively rare, false TCASII targets will be generated if the suppression bus has a fault. Reportedly, the suppression bus failures are usually found on older aircraft when corrosion causes the problem. Reporter states that trans-ocean flying involves other factors which, in conjunction with this type of failure could result in future ATC problems, especially in the rvsm separation situation. The flight crew flying over to europe is on the back side of the clock and when they return they are fatigued. Then, if the captain is out of the cockpit for crew rest and 2 first officer's in the seats, with a false TCASII and traffic in the vicinity, could easily result in near midair collision. The specific TCASII failure of picking up its own transponder always shows the intruder target superimposed at the aircraft position. Reporter's concern is that aircraft with only 1000 ft separation, a faster aircraft overtaking the lead aircraft and passing either below or over lead aircraft could result in a TCASII RA which could put the 2 aircraft together.

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Original NASA ASRS Text

Title: B767-200 ACFT ON OCEANIC RTE RECEIVED A TCASII WARNING. RPTR CAPT ASSUMED IT WAS FALSE AND TOOK NO EVASIVE ACTION. THE TCASII PROB IS REPORTEDLY CAUSED BY FAILURE OF THE SUPPRESSION BUS WHICH INHIBITS TCASII FROM RESPONDING TO ITS OWN ACFT.

Narrative: THIS IS A RPT OF A TYPE OF TCASII FAILURE WHICH IS NOT NORMALLY SEEN. NO TRAINING IN THE FAILURE MODE WAS GIVEN TO ANY OF OUR CREWS. IT WILL HAPPEN AGAIN. I TOOK NO EVASIVE ACTION. ENRTE AT CRUISE ALT IN RVSM/MNPS WE RECEIVED A TCASII RESOLUTION TO DSND. THIS WAS FOLLOWED WITH A TCASII COMMAND TO INCREASE DSCNT RATE. THE RATES COMMANDED WERE 1500 FPM FOLLOWED BY THE RATE OF 2500 FPM. THE PROB WAS: THE TARGET WHICH WAS CREATING THE RESOLUTION WAS A FALSE TARGET. THE THREAT WAS A TARGET WHICH APPEARED FROM BEHIND OUR ACFT AND SHOWED ZERO ALT DIFFERENCE FROM OUR ACFT. I DID NOT RESPOND TO THE WARNING. IT APPEARED TO ME TO BE A FALSE TARGET. I HAVE SEEN FALSE TARGETS IN THE PAST ON THE B767 FLEET. IN THOSE CASES THE TARGET WOULD APPEAR AND ONLY SHOW THE YELLOW BALL SYMBOL. IN THIS CASE THE TARGET CHANGED FROM THE YELLOW BALL SYMBOL TO THE RED SQUARE AS THE RESOLUTION COMMAND WAS ISSUED. WHEN THE BALL SYMBOL APPEARED THE 'TFC, TFC' WARNING WAS ISSUED. WITH THE RED SQUARE DISPLAY THE 'DSND' AND 'INCREASE DSCNT' COMMANDS WERE GIVEN. MY CONCERN IS THE TRAINING WE ARE GIVING TO THE CREWS IS TO RESPOND IMMEDIATELY TO TCASII RESOLUTIONS. I WAS NOT AWARE THAT THERE IS A FAILURE MODE WHICH COULD CREATE A RESOLUTION. A TCASII RESPONSE IN RVSM AIRSPACE IS THE MOST SERIOUS PART OF THIS PROB. IN ATTEMPTING TO UNDERSTAND THIS PROB I HAVE BEEN WORKING WITH CAPT A AND MR B, MGR OF ACFT RELIABILITY. MR B ALSO ASKED THAT WE SPEAK WITH COMMERCIAL RADIO. COMMERCIAL RADIO IS THE POINT OF CONTACT FOR LOGGING RVSM PROBS. THIS FAILURE MODE IS NOT UNKNOWN TO THIS UNIT. IT CAN BE SEEN IF THERE IS A FAILURE OF THE SUPPRESSION BUS. THAT BUS IS USED TO MAKE THE TCASII NOT RESPOND TO THE ACFT ITSELF. MY ACR HAS ASKED ROCKWELL COLLINS TO EVALUATE THE TCASII COMPUTER AND THE XPONDER. I SPOKE WITH MR C OF ROCKWELL COLLINS ABOUT THIS TYPE OF FAILURE. HE CONCURS WITH THE THEORY THAT THE SUPPRESSION BUS IS THE PROB. MR C ALSO ADVISED THAT NEITHER THE XPONDER NOR THE TCASII COMPUTER WILL NORMALLY BE THE PROB. SEVERAL YRS AGO, IN THE EARLY TCASII INSTALLATIONS WIRING IN THE SUPPRESSION BUS CAUSED THE PROBS. I BELIEVE WE NEED TO ADVISE ICAO OF THIS FAILURE. SOMEONE IN ICAO IS MAINTAINING A DATABASE OF PROBS IN RVSM AIRSPACE. THERE IS A DEFINITE POTENTIAL FOR A SAFETY PROB WITH THIS FAILURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CAPT RPTR STATES THAT POSTFLT REVEALED THAT THE TCASII UNIT HAD A SUPPRESSION BUS FAILURE WHICH RESULTED IN THE ACFT PICKING UP ITS OWN XPONDER AS AN INTRUDER. RPTR CONTACTED ROCKWELL COLLINS WHO IS THE MANUFACTURER AND THEY RPTED THAT, THOUGH RELATIVELY RARE, FALSE TCASII TARGETS WILL BE GENERATED IF THE SUPPRESSION BUS HAS A FAULT. REPORTEDLY, THE SUPPRESSION BUS FAILURES ARE USUALLY FOUND ON OLDER ACFT WHEN CORROSION CAUSES THE PROB. RPTR STATES THAT TRANS-OCEAN FLYING INVOLVES OTHER FACTORS WHICH, IN CONJUNCTION WITH THIS TYPE OF FAILURE COULD RESULT IN FUTURE ATC PROBS, ESPECIALLY IN THE RVSM SEPARATION SIT. THE FLC FLYING OVER TO EUROPE IS ON THE BACK SIDE OF THE CLOCK AND WHEN THEY RETURN THEY ARE FATIGUED. THEN, IF THE CAPT IS OUT OF THE COCKPIT FOR CREW REST AND 2 FO'S IN THE SEATS, WITH A FALSE TCASII AND TFC IN THE VICINITY, COULD EASILY RESULT IN NMAC. THE SPECIFIC TCASII FAILURE OF PICKING UP ITS OWN XPONDER ALWAYS SHOWS THE INTRUDER TARGET SUPERIMPOSED AT THE ACFT POS. RPTR'S CONCERN IS THAT ACFT WITH ONLY 1000 FT SEPARATION, A FASTER ACFT OVERTAKING THE LEAD ACFT AND PASSING EITHER BELOW OR OVER LEAD ACFT COULD RESULT IN A TCASII RA WHICH COULD PUT THE 2 ACFT TOGETHER.

Data retrieved from NASA's ASRS site as of July 2007 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.