Narrative:

Air carrier Y, at 13000 ft, was in the vicinity of the hnk VOR. Air carrier X, at 14000 ft, was approximately directly over air carrier Y. Somebody asked 'is that traffic descending?' I advised him 'air carrier Y is level at 13000 ft.' air carrier X advised 'we got a TCASII resolution...' and descended directly through air carrier Y. Air carrier X advised 'we have him in sight.' air carrier X went from 14000 ft to 12000 ft in approximately 12 seconds. Upon further conversation with the pilot of air carrier X he advised the other aircraft was 'real close.' my estimation of separation was 0 ft vertical and less than 1/4 mi at the time of the incident. As a side note, we had picked up a bad mode C altitude of 13900 ft on air carrier Y approximately 3-4 mins before the incident. The pilot of air carrier X advised me he believed the bad mode C from air carrier Y was probably the cause of the RA. Supplemental information from acn 474495: air carrier X, level at 14000 ft, questioned if air carrier Y was descending. When advised that air carrier Y was level at 13000 ft, air carrier X responded that he had an RA. Air carrier X then began a rapid descent to 12000 ft. Air carrier Y also said he had an RA to climb, but this climb was not observed on the radar. Air carrier Y also stated that he had the traffic in sight. Air carrier X did not have the air carrier Y in sight until the last min. Air carrier X climbed back to 14000 ft when air carrier Y was several mi behind him. It is possible that the mode C on air carrier Y was not working properly. The mode C on air carrier Y showed 13900 ft when the 2 airplanes were 40 mi apart. Air carrier Y stated he was level at 13000 ft. The next couple of updates reflected that air carrier Y was level at 13000 ft. When air carrier X asked if air carrier Y was descending, air carrier Y mode C read 130 north instead of 130 C. Callback conversation with reporter acn 474496 revealed the following information: reporter said he had issued traffic when the 2 aircraft were approximately 5 mi apart. He said both aircraft saw each other during the avoidance maneuver. At that time the radar controller observed 130 north (north = not receiving data from aircraft) on the aircraft's automated data block. He said the altitude was reading correctly about 75% of the time and he continued to use the information.

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Original NASA ASRS Text

Title: A DH8 IN ZBW AIRSPACE AT 14000 FT DSNDS IMMEDIATELY ON RECEIVING A TCASII RA ON TFC, AN ATR43, RPTED AT 13000 FT BUT SHOWING 13900 FT.

Narrative: ACR Y, AT 13000 FT, WAS IN THE VICINITY OF THE HNK VOR. ACR X, AT 14000 FT, WAS APPROX DIRECTLY OVER ACR Y. SOMEBODY ASKED 'IS THAT TFC DSNDING?' I ADVISED HIM 'ACR Y IS LEVEL AT 13000 FT.' ACR X ADVISED 'WE GOT A TCASII RESOLUTION...' AND DSNDED DIRECTLY THROUGH ACR Y. ACR X ADVISED 'WE HAVE HIM IN SIGHT.' ACR X WENT FROM 14000 FT TO 12000 FT IN APPROX 12 SECONDS. UPON FURTHER CONVERSATION WITH THE PLT OF ACR X HE ADVISED THE OTHER ACFT WAS 'REAL CLOSE.' MY ESTIMATION OF SEPARATION WAS 0 FT VERT AND LESS THAN 1/4 MI AT THE TIME OF THE INCIDENT. AS A SIDE NOTE, WE HAD PICKED UP A BAD MODE C ALT OF 13900 FT ON ACR Y APPROX 3-4 MINS BEFORE THE INCIDENT. THE PLT OF ACR X ADVISED ME HE BELIEVED THE BAD MODE C FROM ACR Y WAS PROBABLY THE CAUSE OF THE RA. SUPPLEMENTAL INFO FROM ACN 474495: ACR X, LEVEL AT 14000 FT, QUESTIONED IF ACR Y WAS DSNDING. WHEN ADVISED THAT ACR Y WAS LEVEL AT 13000 FT, ACR X RESPONDED THAT HE HAD AN RA. ACR X THEN BEGAN A RAPID DSCNT TO 12000 FT. ACR Y ALSO SAID HE HAD AN RA TO CLB, BUT THIS CLB WAS NOT OBSERVED ON THE RADAR. ACR Y ALSO STATED THAT HE HAD THE TFC IN SIGHT. ACR X DID NOT HAVE THE ACR Y IN SIGHT UNTIL THE LAST MIN. ACR X CLBED BACK TO 14000 FT WHEN ACR Y WAS SEVERAL MI BEHIND HIM. IT IS POSSIBLE THAT THE MODE C ON ACR Y WAS NOT WORKING PROPERLY. THE MODE C ON ACR Y SHOWED 13900 FT WHEN THE 2 AIRPLANES WERE 40 MI APART. ACR Y STATED HE WAS LEVEL AT 13000 FT. THE NEXT COUPLE OF UPDATES REFLECTED THAT ACR Y WAS LEVEL AT 13000 FT. WHEN ACR X ASKED IF ACR Y WAS DSNDING, ACR Y MODE C READ 130 N INSTEAD OF 130 C. CALLBACK CONVERSATION WITH RPTR ACN 474496 REVEALED THE FOLLOWING INFO: RPTR SAID HE HAD ISSUED TFC WHEN THE 2 ACFT WERE APPROX 5 MI APART. HE SAID BOTH ACFT SAW EACH OTHER DURING THE AVOIDANCE MANEUVER. AT THAT TIME THE RADAR CTLR OBSERVED 130 N (N = NOT RECEIVING DATA FROM ACFT) ON THE ACFT'S AUTOMATED DATA BLOCK. HE SAID THE ALT WAS READING CORRECTLY ABOUT 75% OF THE TIME AND HE CONTINUED TO USE THE INFO.

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.