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

Attributes
ACN	233323
Time
Date	199302
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 9700 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
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
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : flight engineer pilot : commercial pilot : atp
Experience	flight time last 90 days : 200 flight time total : 5300 flight time type : 300
ASRS Report	233323
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

TCASII sounded 'traffic,' followed by 'descend' command. We were level at 10000 ft, 30 NM northwest of ord. We left 10000 ft, leveled, then reversed and climbed back to 10000 ft when TCASII sounded all clear. Altitude was about 9700 ft. The traffic was never acquired visually, and never pointed out by ATC, until we advised them of our descent due to a TCASII RA. They then said we had a VFR at 10500 ft but he was no longer a factor. TCASII target said plus 4 descending, that was when the RA came, and that was as close as the target got. In my opinion, it was not an near midair collision, it presented no operational problems for us, nor did our RA maneuver cause a problem/conflict for ord ATC.

Original NASA ASRS Text

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

Narrative: TCASII SOUNDED 'TFC,' FOLLOWED BY 'DSND' COMMAND. WE WERE LEVEL AT 10000 FT, 30 NM NW OF ORD. WE LEFT 10000 FT, LEVELED, THEN REVERSED AND CLBED BACK TO 10000 FT WHEN TCASII SOUNDED ALL CLR. ALT WAS ABOUT 9700 FT. THE TFC WAS NEVER ACQUIRED VISUALLY, AND NEVER POINTED OUT BY ATC, UNTIL WE ADVISED THEM OF OUR DSCNT DUE TO A TCASII RA. THEY THEN SAID WE HAD A VFR AT 10500 FT BUT HE WAS NO LONGER A FACTOR. TCASII TARGET SAID PLUS 4 DSNDING, THAT WAS WHEN THE RA CAME, AND THAT WAS AS CLOSE AS THE TARGET GOT. IN MY OPINION, IT WAS NOT AN NMAC, IT PRESENTED NO OPERATIONAL PROBS FOR US, NOR DID OUR RA MANEUVER CAUSE A PROB/CONFLICT FOR ORD ATC.

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.