ZOB controller reports loss of separation due to descent clearance misunderstanding.

Attributes
ACN	710937
Time
Date	200609
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : aco.vor
State Reference	OH
Altitude	msl single value : 16000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	Saab-Scania Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zob.artcc
Operator	general aviation : corporate
Make Model Name	Citation V
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Experience	controller military : 9 controller time certified in position1 : 2.5
ASRS Report	710937
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	controller : issued new clearance controller : separated traffic flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance ATC Facility
Primary Problem	Ambiguous

Narrative:

Aircraft Y was on frequency inbound to agc airport at FL210. Aircraft X was on the same route inbound to pittsburgh at 150. I gave aircraft Y a descent to 170. He read back 120 and was corrected immediately. A few mins later; aircraft Y was given a descent to 160. He read back an altitude of 60 and the readback was not caught. Meanwhile; aircraft X was given a descent out of 150 to 90. Aircraft X was given the traffic call of the citation descending to 160. Aircraft X reported aircraft Y in sight passing off his right side. Aircraft Y was noticed descending out of 156. I asked aircraft Y his altitude. He replied that he was descending to 60. I immediately climbed him back up to 160 and expedited aircraft X out of 140 to separate the 2 planes. The conflict alert had gone off. Aircraft Y was in front of aircraft X by 3.96 mi 150 KTS faster but too late to establish required separation. Both aircraft were on frequency 128.15 which has been documented for some time to have problems of 'dead spots' in the aco area which could have been a possible factor for the aircraft Y hearing the wrong altitude 2 times. Or the pilot just wasn't paying attention. In my opinion; the pilot's missed readback of 60 sounded ambiguous on the tapes.

Original NASA ASRS Text

Title: ZOB CTLR RPTS LOSS OF SEPARATION DUE TO DSCNT CLRNC MISUNDERSTANDING.

Narrative: ACFT Y WAS ON FREQ INBOUND TO AGC ARPT AT FL210. ACFT X WAS ON THE SAME RTE INBOUND TO PITTSBURGH AT 150. I GAVE ACFT Y A DSCNT TO 170. HE READ BACK 120 AND WAS CORRECTED IMMEDIATELY. A FEW MINS LATER; ACFT Y WAS GIVEN A DSCNT TO 160. HE READ BACK AN ALT OF 60 AND THE READBACK WAS NOT CAUGHT. MEANWHILE; ACFT X WAS GIVEN A DSCNT OUT OF 150 TO 90. ACFT X WAS GIVEN THE TFC CALL OF THE CITATION DSNDING TO 160. ACFT X RPTED ACFT Y IN SIGHT PASSING OFF HIS R SIDE. ACFT Y WAS NOTICED DSNDING OUT OF 156. I ASKED ACFT Y HIS ALT. HE REPLIED THAT HE WAS DSNDING TO 60. I IMMEDIATELY CLBED HIM BACK UP TO 160 AND EXPEDITED ACFT X OUT OF 140 TO SEPARATE THE 2 PLANES. THE CONFLICT ALERT HAD GONE OFF. ACFT Y WAS IN FRONT OF ACFT X BY 3.96 MI 150 KTS FASTER BUT TOO LATE TO ESTABLISH REQUIRED SEPARATION. BOTH ACFT WERE ON FREQ 128.15 WHICH HAS BEEN DOCUMENTED FOR SOME TIME TO HAVE PROBS OF 'DEAD SPOTS' IN THE ACO AREA WHICH COULD HAVE BEEN A POSSIBLE FACTOR FOR THE ACFT Y HEARING THE WRONG ALT 2 TIMES. OR THE PLT JUST WASN'T PAYING ATTN. IN MY OPINION; THE PLT'S MISSED READBACK OF 60 SOUNDED AMBIGUOUS ON THE TAPES.

Data retrieved from NASA's ASRS site as of January 2009 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.