Ont tower controller experienced near midair collision with air carrier arrival and VFR BE58.

Attributes
ACN	586601
Time
Date	200307
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ont.airport
State Reference	CA
Altitude	msl single value : 4400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon tower : ont.tower
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : sct.tracon tower : ont.tower
Operator	general aviation : personal
Make Model Name	Baron 58/58TC
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	VFR
Person 1
Affiliation	government : faa
Function	controller : local
Person 2
Affiliation	government : faa
Function	controller : approach controller : departure
Events
Anomaly	conflict : nmac conflict : airborne critical non adherence : published procedure non adherence other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Miss Distance	horizontal : 3000 vertical : 400
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Inter Facility Coordination Failure other

Narrative:

Air carrier on approach to ont reported visual contact with beechcraft baron as pilot checked onto tower frequency. Pilot states he was not issued traffic by approach control. The baron was on approach control's frequency and ARTS data tag.

Original NASA ASRS Text

Title: ONT TWR CTLR EXPERIENCED NMAC WITH ACR ARR AND VFR BE58.

Narrative: ACR ON APCH TO ONT RPTED VISUAL CONTACT WITH BEECHCRAFT BARON AS PLT CHKED ONTO TWR FREQ. PLT STATES HE WAS NOT ISSUED TFC BY APCH CTL. THE BARON WAS ON APCH CTL'S FREQ AND ARTS DATA TAG.

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.