Air carrier X TCASII RA had less than standard separation from Y. System error.

Attributes
ACN	307550
Time
Date	199506
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mol
State Reference	VA
Altitude	msl bound lower : 29000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 6100 flight time type : 400
ASRS Report	307550
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe inflight encounter : weather non adherence : published procedure non adherence : required legal separation
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 12000 vertical : 900
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X at FL295 while descending from FL330 to FL290 assigned. At 290 KIAS (56 mi southwest mol) we experienced a TCASII RA event. We were talking to ZDC on 133.02 on an approved WX deviation between cumulo nimbus with good in-flight visibility when we observed a yellow traffic symbol on TCASII display at approximately 1 O'clock position, 6 mi, plus 100 ft above our altitude. ZDC then called that traffic to us and ordered an immediate descent to FL250. PF (captain) disconnected the autoplt and initiated an immediate descent while I visually searched for traffic. We received a TCASII RA to 'climb, crossing climb' with a commanded (plus 2500 FPM climb) when already established in the descent, so we continued to descend. The descent rate became approximately 4000 FPM. The altitude difference of aircraft Y indicated on the TCASII display went from plus 100 ft to plus 900 ft during the maneuver and never showed level with us. The traffic past directly in front of us and then continued over to our 9 O'clock position at about 2 mi, plus 900 ft on TCASII display. We never visually spotted the traffic when the 'clear of conflict' was announced by TCASII. The entire even lasted about 30 seconds. We shallowed the descent rate and leveled at FL250 assigned. We never heard any other instructions or knew who the other aircraft was.

Original NASA ASRS Text

Title: ACR X TCASII RA HAD LTSS FROM Y. SYS ERROR.

Narrative: ACR X AT FL295 WHILE DSNDING FROM FL330 TO FL290 ASSIGNED. AT 290 KIAS (56 MI SW MOL) WE EXPERIENCED A TCASII RA EVENT. WE WERE TALKING TO ZDC ON 133.02 ON AN APPROVED WX DEV BTWN CUMULO NIMBUS WITH GOOD INFLT VISIBILITY WHEN WE OBSERVED A YELLOW TFC SYMBOL ON TCASII DISPLAY AT APPROX 1 O'CLOCK POS, 6 MI, PLUS 100 FT ABOVE OUR ALT. ZDC THEN CALLED THAT TFC TO US AND ORDERED AN IMMEDIATE DSCNT TO FL250. PF (CAPT) DISCONNECTED THE AUTOPLT AND INITIATED AN IMMEDIATE DSCNT WHILE I VISUALLY SEARCHED FOR TFC. WE RECEIVED A TCASII RA TO 'CLB, XING CLB' WITH A COMMANDED (PLUS 2500 FPM CLB) WHEN ALREADY ESTABLISHED IN THE DSCNT, SO WE CONTINUED TO DSND. THE DSCNT RATE BECAME APPROX 4000 FPM. THE ALT DIFFERENCE OF ACFT Y INDICATED ON THE TCASII DISPLAY WENT FROM PLUS 100 FT TO PLUS 900 FT DURING THE MANEUVER AND NEVER SHOWED LEVEL WITH US. THE TFC PAST DIRECTLY IN FRONT OF US AND THEN CONTINUED OVER TO OUR 9 O'CLOCK POS AT ABOUT 2 MI, PLUS 900 FT ON TCASII DISPLAY. WE NEVER VISUALLY SPOTTED THE TFC WHEN THE 'CLR OF CONFLICT' WAS ANNOUNCED BY TCASII. THE ENTIRE EVEN LASTED ABOUT 30 SECONDS. WE SHALLOWED THE DSCNT RATE AND LEVELED AT FL250 ASSIGNED. WE NEVER HEARD ANY OTHER INSTRUCTIONS OR KNEW WHO THE OTHER ACFT WAS.

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.