Y90 controller has operational error when C208 descends at a slower rate than a B350.

Attributes
ACN	707899
Time
Date	200608
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : y90.tracon
State Reference	CT
Altitude	msl bound lower : 7400 msl bound upper : 8000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : y90.tracon
Operator	common carrier : air taxi
Make Model Name	Caravan 1 208A
Operating Under FAR Part	Part 135
Flight Phase	descent : intermediate altitude
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : y90.tracon
Operator	general aviation : personal
Make Model Name	Super King Air 350
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller non radar : 2 controller radar : 22 controller supervisory : 9 controller time certified in position1 : 16 controller time certified in position2 : 10
ASRS Report	707899
Person 2
Affiliation	company : air taxi
Function	flight crew : single pilot
Events
Anomaly	conflict : airborne less severe
Independent Detector	atc equipment : conflict alert
Resolutory Action	none taken : detected after the fact
Miss Distance	horizontal : 12000 vertical : 600
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was working an en route/arrival sector at Y90 and had several wbound en route aircraft and several bdl arrs under my control. The C208 was at 8000 ft wbound and I needed to get him to 6000 ft to comply with the LOA with new york approach. I issued the C208 descent to 6000 ft. The B350 was 5 mi behind the C208 and faster; and was required to be at 8000 ft before handoff to new york by LOA. The C208 left 8000 ft and I expected it to descend at least half as fast as the B350. About 20 seconds later; I descended the B350 from 10000 ft to 8000 ft. After making numerous xmissions to other aircraft; the conflict alert activated; and I instructed the C208 to expedite descent because he was only at 7400 ft. The C208 had only descended 600 ft in 2 mins; while the B350 descended nearly 2000 ft in about 1 min 38 seconds. Had the pilot of the C208 descended at a reasonable rate; this would not have occurred.

Original NASA ASRS Text

Title: Y90 CTLR HAS OPERROR WHEN C208 DSNDS AT A SLOWER RATE THAN A B350.

Narrative: I WAS WORKING AN ENRTE/ARR SECTOR AT Y90 AND HAD SEVERAL WBOUND ENRTE ACFT AND SEVERAL BDL ARRS UNDER MY CTL. THE C208 WAS AT 8000 FT WBOUND AND I NEEDED TO GET HIM TO 6000 FT TO COMPLY WITH THE LOA WITH NEW YORK APCH. I ISSUED THE C208 DSCNT TO 6000 FT. THE B350 WAS 5 MI BEHIND THE C208 AND FASTER; AND WAS REQUIRED TO BE AT 8000 FT BEFORE HDOF TO NEW YORK BY LOA. THE C208 LEFT 8000 FT AND I EXPECTED IT TO DSND AT LEAST HALF AS FAST AS THE B350. ABOUT 20 SECONDS LATER; I DSNDED THE B350 FROM 10000 FT TO 8000 FT. AFTER MAKING NUMEROUS XMISSIONS TO OTHER ACFT; THE CONFLICT ALERT ACTIVATED; AND I INSTRUCTED THE C208 TO EXPEDITE DSCNT BECAUSE HE WAS ONLY AT 7400 FT. THE C208 HAD ONLY DSNDED 600 FT IN 2 MINS; WHILE THE B350 DSNDED NEARLY 2000 FT IN ABOUT 1 MIN 38 SECONDS. HAD THE PLT OF THE C208 DSNDED AT A REASONABLE RATE; THIS WOULD NOT HAVE OCCURRED.

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.