Narrative:

Approximately 5 mins prior to the event, ZTL issued a TA that an small transport was in our 2 O'clock position so many mi away (do not recall the distance) at FL280. 2-3 mins later, the traffic appeared on TCASII display unit consistent with the ATC TA and with a minus 8 indication (800 ft below our altitude). The target track was converging with ours. After approximately another 2 mins, the TCASII issued a TA. Shortly after the TA, TCASII commanded an RA to 'climb.' the TCASII display unit commanded a 1500 FPM climb. A climb was immediately commenced. Passing FL293 TCASII indicated 'clear of traffic.' we leveled at FL295 and then returned to assigned FL280. ATC was advised of the altitude deviation as it occurred and the subsequent return to the assigned flight level. At no time did either pilot visually see the 'intruder' aircraft. Flight conditions at the time were VMC although layers of thin broken stratus clouds were above and below our level as well as occasionally at our level. In my judgement, absent visual acquisition of the other aircraft, I am obliged to comply with the TCASII resolution. The 'evasive action' notwithstanding, in this case I do not believe a genuine conflict occurred. ATC advised us that the aircraft was at (assigned and cruising altitude is our inference) FL280 and our TCASII display unit indicated a constant difference (800 ft) in vertical separation until the RA event occurred. I think the problem, if it is a problem, is probably technical and lay within the parameters and sensitivity of the equipment.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION FROM ASSIGNED AFTER RECEIVING TCASII RA FROM OPPOSITE DIRECTION TFC.

Narrative: APPROX 5 MINS PRIOR TO THE EVENT, ZTL ISSUED A TA THAT AN SMT WAS IN OUR 2 O'CLOCK POS SO MANY MI AWAY (DO NOT RECALL THE DISTANCE) AT FL280. 2-3 MINS LATER, THE TFC APPEARED ON TCASII DISPLAY UNIT CONSISTENT WITH THE ATC TA AND WITH A MINUS 8 INDICATION (800 FT BELOW OUR ALT). THE TARGET TRACK WAS CONVERGING WITH OURS. AFTER APPROX ANOTHER 2 MINS, THE TCASII ISSUED A TA. SHORTLY AFTER THE TA, TCASII COMMANDED AN RA TO 'CLB.' THE TCASII DISPLAY UNIT COMMANDED A 1500 FPM CLB. A CLB WAS IMMEDIATELY COMMENCED. PASSING FL293 TCASII INDICATED 'CLR OF TFC.' WE LEVELED AT FL295 AND THEN RETURNED TO ASSIGNED FL280. ATC WAS ADVISED OF THE ALTDEV AS IT OCCURRED AND THE SUBSEQUENT RETURN TO THE ASSIGNED FLT LEVEL. AT NO TIME DID EITHER PLT VISUALLY SEE THE 'INTRUDER' ACFT. FLT CONDITIONS AT THE TIME WERE VMC ALTHOUGH LAYERS OF THIN BROKEN STRATUS CLOUDS WERE ABOVE AND BELOW OUR LEVEL AS WELL AS OCCASIONALLY AT OUR LEVEL. IN MY JUDGEMENT, ABSENT VISUAL ACQUISITION OF THE OTHER ACFT, I AM OBLIGED TO COMPLY WITH THE TCASII RESOLUTION. THE 'EVASIVE ACTION' NOTWITHSTANDING, IN THIS CASE I DO NOT BELIEVE A GENUINE CONFLICT OCCURRED. ATC ADVISED US THAT THE ACFT WAS AT (ASSIGNED AND CRUISING ALT IS OUR INFERENCE) FL280 AND OUR TCASII DISPLAY UNIT INDICATED A CONSTANT DIFFERENCE (800 FT) IN VERT SEPARATION UNTIL THE RA EVENT OCCURRED. I THINK THE PROB, IF IT IS A PROB, IS PROBABLY TECHNICAL AND LAY WITHIN THE PARAMETERS AND SENSITIVITY OF THE EQUIP.

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.