An EMB135 flight crew on approach to bos passed within 400 ft of a light aircraft.

Attributes
ACN	702048
Time
Date	200607
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bos.airport
State Reference	MA
Altitude	msl bound lower : 3900 msl bound upper : 4500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : a90.tracon
Operator	common carrier : air carrier
Make Model Name	EMB ERJ 135 ER&LR
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : vacating altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : a90.tracon
Make Model Name	Small Aircraft
Operating Under FAR Part	Part 91
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	702048
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	conflict : nmac
Independent Detector	aircraft equipment : tcas other flight crewa
Resolutory Action	controller : separated traffic flight crew : took evasive action
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

On downwind; 30 NM from bos; we were given a descent from 5000 ft to 4000 ft. At about 4500 ft; we got a descending RA which we complied with promptly descending to 3900 ft. At about 4100 ft the single engine low wing GA aircraft passed directly over our position with the TCAS showing +400 ft. ATC acknowledged that they had a target and that he was indicating 4500 ft unverified. Their traffic alert came during our RA. The assessment of the GA altitude was in my opinion correct.

Original NASA ASRS Text

Title: AN EMB135 FLT CREW ON APCH TO BOS PASSED WITHIN 400 FT OF A LIGHT ACFT.

Narrative: ON DOWNWIND; 30 NM FROM BOS; WE WERE GIVEN A DSCNT FROM 5000 FT TO 4000 FT. AT ABOUT 4500 FT; WE GOT A DSNDING RA WHICH WE COMPLIED WITH PROMPTLY DSNDING TO 3900 FT. AT ABOUT 4100 FT THE SINGLE ENG LOW WING GA ACFT PASSED DIRECTLY OVER OUR POS WITH THE TCAS SHOWING +400 FT. ATC ACKNOWLEDGED THAT THEY HAD A TARGET AND THAT HE WAS INDICATING 4500 FT UNVERIFIED. THEIR TFC ALERT CAME DURING OUR RA. THE ASSESSMENT OF THE GA ALT WAS IN MY OPINION CORRECT.

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.