TCASII changed its mind in mid situation.

Attributes
ACN	270455
Time
Date	199405
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : msy
State Reference	LA
Altitude	msl bound lower : 9000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	arrival other enroute airway : msy
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 250
ASRS Report	270455
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe conflict : airborne less severe non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Miss Distance	horizontal : 500 vertical : 1000
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were cleared for a descent maintaining visual separation from another aircraft. The sighted traffic seemed to be far enough away that it would not be a factor. While descending at about 1500 - 2000 FPM, the TCASII issued a climb (about 1200-1500 FPM) RA. The descent was slowed as we began to comply with the TCASII advisory, and indeed the sighted traffic did seem much closer -- and closing fast. However, it clearly was already above our altitude. We were just about in level flight when the TCASII changed its advisory to descend. That area being visually clear, we resumed the descent at about 1500-2000 FPM and the traffic passed us, in the opposite direction, off our right about 500 ft and 1000 ft above. The traffic's lights were always clearly visible but ground lights below and behind them may have added to the illusion that the aircraft was not moving toward us and thus was not a factor. However, since the traffic was moving directly in the opposite direction. The closure was very fast. That, coupled with the fast descent, may have momentarily outpaced the TCASII, which then issued the climb advisory. At that point, we should have been descending faster. Our starting to comply with the TCASII climb advisory actually kept us nearer the threat.

Original NASA ASRS Text

Title: TCASII CHANGED ITS MIND IN MID SIT.

Narrative: WE WERE CLRED FOR A DSCNT MAINTAINING VISUAL SEPARATION FROM ANOTHER ACFT. THE SIGHTED TFC SEEMED TO BE FAR ENOUGH AWAY THAT IT WOULD NOT BE A FACTOR. WHILE DSNDING AT ABOUT 1500 - 2000 FPM, THE TCASII ISSUED A CLB (ABOUT 1200-1500 FPM) RA. THE DSCNT WAS SLOWED AS WE BEGAN TO COMPLY WITH THE TCASII ADVISORY, AND INDEED THE SIGHTED TFC DID SEEM MUCH CLOSER -- AND CLOSING FAST. HOWEVER, IT CLRLY WAS ALREADY ABOVE OUR ALT. WE WERE JUST ABOUT IN LEVEL FLT WHEN THE TCASII CHANGED ITS ADVISORY TO DSND. THAT AREA BEING VISUALLY CLR, WE RESUMED THE DSCNT AT ABOUT 1500-2000 FPM AND THE TFC PASSED US, IN THE OPPOSITE DIRECTION, OFF OUR R ABOUT 500 FT AND 1000 FT ABOVE. THE TFC'S LIGHTS WERE ALWAYS CLRLY VISIBLE BUT GND LIGHTS BELOW AND BEHIND THEM MAY HAVE ADDED TO THE ILLUSION THAT THE ACFT WAS NOT MOVING TOWARD US AND THUS WAS NOT A FACTOR. HOWEVER, SINCE THE TFC WAS MOVING DIRECTLY IN THE OPPOSITE DIRECTION. THE CLOSURE WAS VERY FAST. THAT, COUPLED WITH THE FAST DSCNT, MAY HAVE MOMENTARILY OUTPACED THE TCASII, WHICH THEN ISSUED THE CLB ADVISORY. AT THAT POINT, WE SHOULD HAVE BEEN DSNDING FASTER. OUR STARTING TO COMPLY WITH THE TCASII CLB ADVISORY ACTUALLY KEPT US NEARER THE THREAT.

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.