B737 at FL260 with ZJX experienced TCAS RA with traffic not observed on ATC radar.

Attributes
ACN	634127
Time
Date	200410
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : chs.vortac
State Reference	SC
Altitude	msl bound lower : 24500 msl bound upper : 26000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zjx.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 230 flight time total : 13000 flight time type : 8100
ASRS Report	634127
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 195 flight time total : 6000 flight time type : 2700
ASRS Report	634128
Events
Anomaly	conflict : airborne critical
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action
Miss Distance	horizontal : 300 vertical : 700
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft ATC Human Performance
Primary Problem	Ambiguous

Narrative:

Cruising at FL260 noted TCAS target at FL265 at 12:30 O'clock position closing. The TCAS soon called out 'traffic' shortly followed by an RA 'descend.' advised ZJX, who said that they had no one near us on their scope and that our descent path was also clear. As we descended our separation with the TCAS target increased proportionally to our descent. The RA resolved to 'clear of conflict' as the TCAS target passed nearly overhead. We bottomed at about FL250 and returned to FL260. TCAS target exited our scope at 6 O'clock position still indicating separation appropriate for an aircraft at FL265. We were in thick clouds and never saw the other aircraft. We advised ZJX of all the above and they again said they never saw a return in our vicinity. Supplemental information from acn 634128: the 'unknown contact' (which we never saw) passed approximately 900 ft above us and about 500-700 ft past our starboard side (according to the TCAS display). After descending about 1500 ft, the advisory went away and we informed ATC we were climbing back to FL260. ATC again said he didn't have anyone around us on radar and cleared us back up to FL260.

Original NASA ASRS Text

Title: B737 AT FL260 WITH ZJX EXPERIENCED TCAS RA WITH TFC NOT OBSERVED ON ATC RADAR.

Narrative: CRUISING AT FL260 NOTED TCAS TARGET AT FL265 AT 12:30 O'CLOCK POS CLOSING. THE TCAS SOON CALLED OUT 'TFC' SHORTLY FOLLOWED BY AN RA 'DSND.' ADVISED ZJX, WHO SAID THAT THEY HAD NO ONE NEAR US ON THEIR SCOPE AND THAT OUR DSCNT PATH WAS ALSO CLR. AS WE DSNDED OUR SEPARATION WITH THE TCAS TARGET INCREASED PROPORTIONALLY TO OUR DSCNT. THE RA RESOLVED TO 'CLR OF CONFLICT' AS THE TCAS TARGET PASSED NEARLY OVERHEAD. WE BOTTOMED AT ABOUT FL250 AND RETURNED TO FL260. TCAS TARGET EXITED OUR SCOPE AT 6 O'CLOCK POS STILL INDICATING SEPARATION APPROPRIATE FOR AN ACFT AT FL265. WE WERE IN THICK CLOUDS AND NEVER SAW THE OTHER ACFT. WE ADVISED ZJX OF ALL THE ABOVE AND THEY AGAIN SAID THEY NEVER SAW A RETURN IN OUR VICINITY. SUPPLEMENTAL INFO FROM ACN 634128: THE 'UNKNOWN CONTACT' (WHICH WE NEVER SAW) PASSED APPROX 900 FT ABOVE US AND ABOUT 500-700 FT PAST OUR STARBOARD SIDE (ACCORDING TO THE TCAS DISPLAY). AFTER DSNDING ABOUT 1500 FT, THE ADVISORY WENT AWAY AND WE INFORMED ATC WE WERE CLBING BACK TO FL260. ATC AGAIN SAID HE DIDN'T HAVE ANYONE AROUND US ON RADAR AND CLRED US BACK UP TO FL260.

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.