An air carrier large transport descended in response to a TCASII command.

Attributes
ACN	256165
Time
Date	199311
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	None
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 15000 flight time type : 5000
ASRS Report	256165
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : vfr in imc non adherence : clearance non adherence : far other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action other
Consequence	Other
Miss Distance	horizontal : 0 vertical : 600
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Level flight at 11000 ft 320 KIAS assigned. TCASII set at 10 mi range. I was handed off from center to ord feeder approach control. I noticed a target on TCASII at our altitude. Because I first saw this aircraft directly ahead and it quickly became a TA, I told the first officer to follow the RA and descend. I told the controller that we're descending out of 11000 ft. The controller told us to turn to 090 degree heading because he also saw the traffic, which was squawking VFR with altitude reporting. The VFR passed directly above us by 600 ft (according to TCASII) we never saw him! WX was solid IMC with light mixed icing. My personal estimate was less than 1 mi visibility, but since we were head-on with the traffic, and less than 600 ft below, I would say visibility was less than 600 ft. I do not think we reduced our IFR separation below limits with any other IFR aircraft.

Original NASA ASRS Text

Title: AN ACR LGT DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: LEVEL FLT AT 11000 FT 320 KIAS ASSIGNED. TCASII SET AT 10 MI RANGE. I WAS HANDED OFF FROM CTR TO ORD FEEDER APCH CTL. I NOTICED A TARGET ON TCASII AT OUR ALT. BECAUSE I FIRST SAW THIS ACFT DIRECTLY AHEAD AND IT QUICKLY BECAME A TA, I TOLD THE FO TO FOLLOW THE RA AND DSND. I TOLD THE CTLR THAT WE'RE DSNDING OUT OF 11000 FT. THE CTLR TOLD US TO TURN TO 090 DEG HDG BECAUSE HE ALSO SAW THE TFC, WHICH WAS SQUAWKING VFR WITH ALT RPTING. THE VFR PASSED DIRECTLY ABOVE US BY 600 FT (ACCORDING TO TCASII) WE NEVER SAW HIM! WX WAS SOLID IMC WITH LIGHT MIXED ICING. MY PERSONAL ESTIMATE WAS LESS THAN 1 MI VISIBILITY, BUT SINCE WE WERE HEAD-ON WITH THE TFC, AND LESS THAN 600 FT BELOW, I WOULD SAY VISIBILITY WAS LESS THAN 600 FT. I DO NOT THINK WE REDUCED OUR IFR SEPARATION BELOW LIMITS WITH ANY OTHER IFR ACFT.

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.