Air carrier first officer reported an near midair collision during arrival to ont; and the flying ca...

Narrative:

Descending on the SETER4 arrival into ont; we were given a vector direct petis from about bands. Shortly after making the turn to direct petis we were informed by socal that there was a target on the screen with no altitude or type and was not talking to them and he gave us a more westerly heading to avoid the traffic. About a minute or so on the westerly heading he had informed us that the primary target popped up on radar at which time I started looking outside and looking at our TCAS to see if there were any information. Nothing was showing on the TCAS and I saw the airplane in a turn from west to north wing up and level off northward. Both the [second officer] and I called out the traffic to the captain and he said that he had the traffic. My first instinct was to fly behind the airplane and keep him in my sight and to keep us separated. The captain then turned to the right bringing us to a parallel or less flight path at the same altitude. It appeared to me that the cessna was conducting flight training maneuvers. The captain then leveled off the airplane and the cessna was at our twelve o'clock and closing fast. The [second officer] and I started telling the captain to take action and he was not making any correction to avoid the traffic. Both of us had to tell the captain to descend immediately and I was reading myself to take control of the aircraft and the captain did start a slow and steady descent and the I watched the cessna passing us going northward clearing us approximately 50 feet above us and about 250-300 feet off the right wing. I informed ATC that we had cleared the conflicting aircraft and gave the type; unable to make out the tail number we continued the approach to landing at ont.

Original NASA ASRS Text

Title: Air Carrier First Officer reported an NMAC during arrival to ONT; and the flying Captain failed to make a timely turn to avoid VFR traffic.

Narrative: Descending on the SETER4 arrival into ONT; we were given a vector direct PETIS from about BANDS. Shortly after making the turn to direct PETIS we were informed by SoCal that there was a target on the screen with no altitude or type and was not talking to them and he gave us a more westerly heading to avoid the traffic. About a minute or so on the westerly heading he had informed us that the primary target popped up on radar at which time I started looking outside and looking at our TCAS to see if there were any information. Nothing was showing on the TCAS and I saw the airplane in a turn from west to north wing up and level off northward. Both the [Second Officer] and I called out the traffic to the captain and he said that he had the traffic. My first instinct was to fly behind the airplane and keep him in my sight and to keep us separated. The captain then turned to the right bringing us to a parallel or less flight path at the same altitude. It appeared to me that the Cessna was conducting flight training maneuvers. The captain then leveled off the airplane and the Cessna was at our twelve o'clock and closing fast. The [Second Officer] and I started telling the captain to take action and he was not making any correction to avoid the traffic. Both of us had to tell the captain to descend immediately and I was reading myself to take control of the aircraft and the captain did start a slow and steady descent and the I watched the Cessna passing us going northward clearing us approximately 50 feet above us and about 250-300 feet off the right wing. I informed ATC that we had cleared the conflicting aircraft and gave the type; unable to make out the tail number we continued the approach to landing at ONT.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1471130
Time
Date	201708
Local Time Of Day	0601-1200
Place
Locale Reference	ONT.Airport
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Vectors STAR SETER4
Flight Plan	IFR
Aircraft 2
Make Model Name	Cessna Twin Piston Undifferentiated or Other Model
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 135 Flight Crew Total 6040 Flight Crew Type 460
Events
Anomaly	Conflict NMAC Deviation - Track / Heading All Types
Miss Distance	Horizontal 0 Vertical 300

Air carrier first officer reported an near midair collision during arrival to ont; and the flying captain failed to make a timely turn to avoid VFR traffic.