Air carrier descended below assigned altitude and came in close proximity to another aircraft.

Attributes
ACN	126342
Time
Date	198910
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : den
State Reference	CO
Altitude	msl bound lower : 10000 msl bound upper : 10400
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Controlling Facilities	tracon : den
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Route In Use	departure other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
ASRS Report	126342
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Air carrier X was level at 10000' MSL cruising east awaiting to pass traffic southbound descending to 11000' MSL. I called traffic to air carrier X and he had visibility on him. The traffic then descended west/O a clearance and I had to turn air carrier X to avoid him. The plot shows 1 mi and 400', then 8/10 mi and 600' when the other aircraft involved was instructed to climb. Supplemental information from acn 126572: during vectors to a visibility approach at den and cleared to 11000' MSL, approach control called traffic in our right front quarter. We were approximately 12000', descending. I looked through the first officer's window in an attempt to spot the traffic, which was a 2-ENG jet at our 1:30 position, perhaps 1000' low, climbing in our direction. As I picked him up visually he started a left turn, behind us. I then looked back to the captain's instrument panel and noted that we had transitioned to a climb and were passing through 10800' MSL. My last view of the traffic was about 500-700' low at about 1 mi (estimated position). I was then surprised to see us in a climb, 200' low. I heard no altitude warning horn, nor did no feel any abrupt pitch change (increase in G's).

Original NASA ASRS Text

Title: ACR DESCENDED BELOW ASSIGNED ALT AND CAME IN CLOSE PROX TO ANOTHER ACFT.

Narrative: ACR X WAS LEVEL AT 10000' MSL CRUISING E AWAITING TO PASS TFC SBND DSNDING TO 11000' MSL. I CALLED TFC TO ACR X AND HE HAD VIS ON HIM. THE TFC THEN DSNDED W/O A CLRNC AND I HAD TO TURN ACR X TO AVOID HIM. THE PLOT SHOWS 1 MI AND 400', THEN 8/10 MI AND 600' WHEN THE OTHER ACFT INVOLVED WAS INSTRUCTED TO CLB. SUPPLEMENTAL INFO FROM ACN 126572: DURING VECTORS TO A VIS APCH AT DEN AND CLRED TO 11000' MSL, APCH CTL CALLED TFC IN OUR RIGHT FRONT QUARTER. WE WERE APPROX 12000', DSNDING. I LOOKED THROUGH THE F/O'S WINDOW IN AN ATTEMPT TO SPOT THE TFC, WHICH WAS A 2-ENG JET AT OUR 1:30 POS, PERHAPS 1000' LOW, CLBING IN OUR DIRECTION. AS I PICKED HIM UP VISUALLY HE STARTED A LEFT TURN, BEHIND US. I THEN LOOKED BACK TO THE CAPT'S INSTRUMENT PANEL AND NOTED THAT WE HAD TRANSITIONED TO A CLB AND WERE PASSING THROUGH 10800' MSL. MY LAST VIEW OF THE TFC WAS ABOUT 500-700' LOW AT ABOUT 1 MI (ESTIMATED POS). I WAS THEN SURPRISED TO SEE US IN A CLB, 200' LOW. I HEARD NO ALT WARNING HORN, NOR DID NO FEEL ANY ABRUPT PITCH CHANGE (INCREASE IN G'S).

Data retrieved from NASA's ASRS site as of August 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.