Aircraft descends to satisfy TCASII RA.

Attributes
ACN	238593
Time
Date	199304
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zau
State Reference	IL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Light Transport
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time total : 11500
ASRS Report	238593
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 18000 vertical : 0
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	other

Narrative:

TCASII TA alert of an intruder rapidly descending toward our cruise altitude of 35000 ft, 12:30 position, approximately 8 mi. We asked center what he was doing and were told there was no aircraft in that position at that altitude. He then asked a small outfit operating out of mid-west his altitude and was told approximately 35900 ft descending to 29000 ft. ATC told him to immediately return to 39000 ft which was his assigned altitude and then left to 360 degrees. In the meantime, we responded to our TCASII RA and began a descent and left turn. No visual contact was made and the intruder was clear of any conflict very quickly. Our concern was that our TCASII instruction was to descend into a rapidly descending aircraft at our 12 O'clock position. Had we not both initiated opposite turns we would most likely have passed very close to each other. This will hopefully be ammunition to use in the plan to develop 'multi-directional' TCASII as compared to the 'vertical only' RA mode we have at present.

Original NASA ASRS Text

Title: ACFT DSNDS TO SATISFY TCASII RA.

Narrative: TCASII TA ALERT OF AN INTRUDER RAPIDLY DSNDING TOWARD OUR CRUISE ALT OF 35000 FT, 12:30 POS, APPROX 8 MI. WE ASKED CTR WHAT HE WAS DOING AND WERE TOLD THERE WAS NO ACFT IN THAT POS AT THAT ALT. HE THEN ASKED A SMALL OUTFIT OPERATING OUT OF MID-WEST HIS ALT AND WAS TOLD APPROX 35900 FT DSNDING TO 29000 FT. ATC TOLD HIM TO IMMEDIATELY RETURN TO 39000 FT WHICH WAS HIS ASSIGNED ALT AND THEN L TO 360 DEGS. IN THE MEANTIME, WE RESPONDED TO OUR TCASII RA AND BEGAN A DSCNT AND L TURN. NO VISUAL CONTACT WAS MADE AND THE INTRUDER WAS CLR OF ANY CONFLICT VERY QUICKLY. OUR CONCERN WAS THAT OUR TCASII INSTRUCTION WAS TO DSND INTO A RAPIDLY DSNDING ACFT AT OUR 12 O'CLOCK POS. HAD WE NOT BOTH INITIATED OPPOSITE TURNS WE WOULD MOST LIKELY HAVE PASSED VERY CLOSE TO EACH OTHER. THIS WILL HOPEFULLY BE AMMUNITION TO USE IN THE PLAN TO DEVELOP 'MULTI-DIRECTIONAL' TCASII AS COMPARED TO THE 'VERT ONLY' RA MODE WE HAVE AT PRESENT.

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.