Air carrier descended below assigned altitude resulting in less than standard separation.

Attributes
ACN	136232
Time
Date	199002
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : obk
State Reference	IL
Altitude	msl bound lower : 12200 msl bound upper : 13000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau artcc : zjx
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller non radar : 9 controller radar : 7
ASRS Report	136232
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 200
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Air carrier X was at 16000 and air carrier Y at 12000. Both aircraft inbound to ord via V84 pappi. Air carrier X had questioned a clearance and after I answered his question I told him to plan crossing story intersection at 8000. The pilot read back 'story at eight'. I then stated 'air carrier X for now, descend and maintain 13000'. The pilot read back 'out of 16 thousand for 13 thousand'. When I saw air carrier X's mode C indicate 12800 I thought at first it was a bad mode C but then it showed 12400. I told air carrier X to maintain 13,000 immediately and to turn immediately to a 180 degree heading and to descend to 11000. Then I cleared air carrier X to fly a 360 degree heading immediately. Both pilots complied and separation was restored within a few seconds. The pilot of air carrier X stated that he thought the clearance was to descend through 13000 then at pilot's discretion to cross story at 8000.

Original NASA ASRS Text

Title: ACR DESCENDED BELOW ASSIGNED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: ACR X WAS AT 16000 AND ACR Y AT 12000. BOTH ACFT INBND TO ORD VIA V84 PAPPI. ACR X HAD QUESTIONED A CLRNC AND AFTER I ANSWERED HIS QUESTION I TOLD HIM TO PLAN XING STORY INTXN AT 8000. THE PLT READ BACK 'STORY AT EIGHT'. I THEN STATED 'ACR X FOR NOW, DSND AND MAINTAIN 13000'. THE PLT READ BACK 'OUT OF 16 THOUSAND FOR 13 THOUSAND'. WHEN I SAW ACR X'S MODE C INDICATE 12800 I THOUGHT AT FIRST IT WAS A BAD MODE C BUT THEN IT SHOWED 12400. I TOLD ACR X TO MAINTAIN 13,000 IMMEDIATELY AND TO TURN IMMEDIATELY TO A 180 DEG HDG AND TO DSND TO 11000. THEN I CLRED ACR X TO FLY A 360 DEG HDG IMMEDIATELY. BOTH PLTS COMPLIED AND SEPARATION WAS RESTORED WITHIN A FEW SECONDS. THE PLT OF ACR X STATED THAT HE THOUGHT THE CLRNC WAS TO DSND THROUGH 13000 THEN AT PLT'S DISCRETION TO CROSS STORY AT 8000.

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.