Air carrier Y descent through occupied altitude had less than standard separation from air carrier X TCASII RA evasive action taken. System error.

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Attributes
ACN	257569
Time
Date	199311
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : cxr
State Reference	OH
Altitude	msl bound lower : 29000 msl bound upper : 29000
Environment
Flight Conditions	Mixed
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zob tower : msy
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 12000 flight time type : 150
ASRS Report	257569
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	controller : issued new clearance flight crew : took evasive action other
Consequence	faa : investigated faa : reviewed incident with flight crew
Miss Distance	horizontal : 18000 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X TCASII RA at FL290. Initial indications approximately 10 degrees right of nose, 10 NM plus 09 (900 ft), descending. ATC assigned 30 degree left turn. First officer (PF) responded immediately but continued descent of conflicting traffic caused 'descend now' TCASII warning command. First officer initiated descent while in left turn (bottom of descent 28700 ft) while I (PNF) started lookout for traffic. Traffic never sighted, but last display showed plus 03 (300 ft) approximately 2-3 mi right of our aircraft, with us at 28700 ft. Called ZOB upon arrival in phl and discussed incident with ATC supervisor. Apparently air carrier Y was expediting descent to FL280 (through our altitude) and left turn. He claimed the 'snitch' alarm activated. Supervisor said controller's last look at situation seemed ok, then realized ensuing conflict, thus the associated left turns. I think it's safe to assume that the 'expedite descent' given to air carrier Y contrasted with his TCASII indications.

Original NASA ASRS Text

Title: ACR Y DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR X TCASII RA EVASIVE ACTION TAKEN. SYS ERROR.

Narrative: ACR X TCASII RA AT FL290. INITIAL INDICATIONS APPROX 10 DEGS R OF NOSE, 10 NM PLUS 09 (900 FT), DSNDING. ATC ASSIGNED 30 DEG L TURN. FO (PF) RESPONDED IMMEDIATELY BUT CONTINUED DSCNT OF CONFLICTING TFC CAUSED 'DSND NOW' TCASII WARNING COMMAND. FO INITIATED DSCNT WHILE IN L TURN (BOTTOM OF DSCNT 28700 FT) WHILE I (PNF) STARTED LOOKOUT FOR TFC. TFC NEVER SIGHTED, BUT LAST DISPLAY SHOWED PLUS 03 (300 FT) APPROX 2-3 MI R OF OUR ACFT, WITH US AT 28700 FT. CALLED ZOB UPON ARR IN PHL AND DISCUSSED INCIDENT WITH ATC SUPVR. APPARENTLY ACR Y WAS EXPEDITING DSCNT TO FL280 (THROUGH OUR ALT) AND L TURN. HE CLAIMED THE 'SNITCH' ALARM ACTIVATED. SUPVR SAID CTLR'S LAST LOOK AT SIT SEEMED OK, THEN REALIZED ENSUING CONFLICT, THUS THE ASSOCIATED L TURNS. I THINK IT'S SAFE TO ASSUME THAT THE 'EXPEDITE DSCNT' GIVEN TO ACR Y CONTRASTED WITH HIS TCASII INDICATIONS.

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.