An air carrier medium large transport descended in response to a false TCASII alert.

Attributes
ACN	229364
Time
Date	199212
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : gsh
State Reference	IN
Altitude	msl bound lower : 20000 msl bound upper : 20000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zob
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 15000 flight time type : 200
ASRS Report	229364
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : took evasive action
Miss Distance	horizontal : 2000 vertical : 700
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

In cruise at 320 KTS at FL200 assigned we got a 'traffic, traffic' call off TCASII before the yellow half circle appeared at 1 O'clock with plus 4 shown. At same time, ATC called traffic at FL190 12 O'clock but I didn't hear the call on altitude of FL190. I at first assumed TCASII was ATC traffic until first officer confirmed with ATC that traffic he called was at FL190 which by now we see in and out of cloud below us. By then we got a RA 'descend, descend' call on TCASII and with all the chatter in the cockpit between ATC, first officer and TCASII, I had no choice but to descend below TCASII intruder while missing ATC traffic at FL190 which I lost sight of by then. My decision to descend 300 ft to split the altitudes between TCASII intruder and ATC called traffic at FL190 was my only option except turn left which I started. Total elapsed time from first traffic call on TCASII to clear of conflict was no more than 30 seconds.

Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A FALSE TCASII ALERT.

Narrative: IN CRUISE AT 320 KTS AT FL200 ASSIGNED WE GOT A 'TFC, TFC' CALL OFF TCASII BEFORE THE YELLOW HALF CIRCLE APPEARED AT 1 O'CLOCK WITH PLUS 4 SHOWN. AT SAME TIME, ATC CALLED TFC AT FL190 12 O'CLOCK BUT I DIDN'T HEAR THE CALL ON ALT OF FL190. I AT FIRST ASSUMED TCASII WAS ATC TFC UNTIL FO CONFIRMED WITH ATC THAT TFC HE CALLED WAS AT FL190 WHICH BY NOW WE SEE IN AND OUT OF CLOUD BELOW US. BY THEN WE GOT A RA 'DSND, DSND' CALL ON TCASII AND WITH ALL THE CHATTER IN THE COCKPIT BTWN ATC, FO AND TCASII, I HAD NO CHOICE BUT TO DSND BELOW TCASII INTRUDER WHILE MISSING ATC TFC AT FL190 WHICH I LOST SIGHT OF BY THEN. MY DECISION TO DSND 300 FT TO SPLIT THE ALTS BTWN TCASII INTRUDER AND ATC CALLED TFC AT FL190 WAS MY ONLY OPTION EXCEPT TURN L WHICH I STARTED. TOTAL ELAPSED TIME FROM FIRST TFC CALL ON TCASII TO CLR OF CONFLICT WAS NO MORE THAN 30 SECONDS.

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.