ZAU controller does not hear air carrier readback error which results in a loss of separation.

Attributes
ACN	507711
Time
Date	200104
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : olk.vor
State Reference	IN
Altitude	msl single value : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc tower : abq.tower
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	descent : intermediate altitude
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zau.artcc
Operator	general aviation : corporate
Make Model Name	IAI1124/1124A/Westwind
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 19 controller time certified in position1 : 15
ASRS Report	507711
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 : required legal separation non adherence : published procedure non adherence : clearance other anomaly
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	none taken : detected after the fact
Consequence	faa : investigated
Miss Distance	horizontal : 10200 vertical : 700
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Aircraft #1 was being vectored for ord descending to FL250. Aircraft #2 was sbound climbing to FL240, requesting FL390. As aircraft converged, traffic was issued and aircraft #1 was turned from 240 degree heading to 280 degree heading and issued 230 KTS. Pilot did not use standard phraseology, instead read back 'two three oh on the altitude.' if pilot had stated FL230, we would not have had a misunderstanding. Aircraft #1 descended through FL250 after aircraft passed each other, resulting in loss of separation.

Original NASA ASRS Text

Title: ZAU CTLR DOES NOT HEAR ACR READBACK ERROR WHICH RESULTS IN A LOSS OF SEPARATION.

Narrative: ACFT #1 WAS BEING VECTORED FOR ORD DSNDING TO FL250. ACFT #2 WAS SBOUND CLBING TO FL240, REQUESTING FL390. AS ACFT CONVERGED, TFC WAS ISSUED AND ACFT #1 WAS TURNED FROM 240 DEG HDG TO 280 DEG HDG AND ISSUED 230 KTS. PLT DID NOT USE STANDARD PHRASEOLOGY, INSTEAD READ BACK 'TWO THREE OH ON THE ALT.' IF PLT HAD STATED FL230, WE WOULD NOT HAVE HAD A MISUNDERSTANDING. ACFT #1 DSNDED THROUGH FL250 AFTER ACFT PASSED EACH OTHER, RESULTING IN LOSS OF SEPARATION.

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.