Air carrier has TCASII RA, descends.

Attributes
ACN	191649
Time
Date	199110
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 10600 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute airway : ord
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time total : 12000
ASRS Report	191649
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 1000 vertical : 300
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	other

Narrative:

Onboard TCASII commanded a descent from assigned altitude after giving an aural RA warning 'descend, descend now'. Aircraft descended to 10600 MSL where TCASII advisory ceased. Aircraft promptly returned to assigned altitude. Chicago approach immediately notified by crew of altitude deviation and reason for it. Estimate of near miss is from TCASII display since crew did not see other aircraft, if there was one. Also chicago approach did not notify us of a traffic conflict.

Original NASA ASRS Text

Title: ACR HAS TCASII RA, DSNDS.

Narrative: ONBOARD TCASII COMMANDED A DSCNT FROM ASSIGNED ALT AFTER GIVING AN AURAL RA WARNING 'DSND, DSND NOW'. ACFT DSNDED TO 10600 MSL WHERE TCASII ADVISORY CEASED. ACFT PROMPTLY RETURNED TO ASSIGNED ALT. CHICAGO APCH IMMEDIATELY NOTIFIED BY CREW OF ALT DEV AND REASON FOR IT. ESTIMATE OF NEAR MISS IS FROM TCASII DISPLAY SINCE CREW DID NOT SEE OTHER ACFT, IF THERE WAS ONE. ALSO CHICAGO APCH DID NOT NOTIFY US OF A TFC CONFLICT.

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.