B737 captain reported near midair collision with a cherokee on final approach; resulting in a go-aro...

Narrative:

We were cleared for a visual approach after [transitioning from an instrument approach]. We were told to follow a caravan and were number two for the airport. I heard the tower talk to a cherokee and instructed them to enter the downwind and follow a 737 on final (us). As we got closer to the runway I queried the tower about our traffic. They said we were now number one for the runway and company 737 was departing ahead of us. As we were lining up with the runway at about 500 ft. AGL I saw a cherokee on a close in base leg to our left. He was joining final in front of us. I called out the traffic to the first officer who was flying. He saw the traffic and pitched down to avoid the aircraft. The cherokee passed directly above us by about 100-200 ft. Our approach was now unstable so I called for a go around. The first officer asked if we were clear of the traffic (we now were) and then initiated the go around at 200 ft. AGL. We maneuvered to rejoin a visual approach to a landing. I reported a near miss to the tower. I later found out the cherokee did not have a transponder which explains why there was no TCAS alert. This also effected the ability of the tower to track them. If I had known the cherokee didn't have a transponder I would have been more vigilant about making sure we acquired visual contact.

Original NASA ASRS Text

Title: B737 Captain reported NMAC with a Cherokee on final approach; resulting in a go-around.

Narrative: We were cleared for a visual approach after [transitioning from an instrument approach]. We were told to follow a Caravan and were number two for the airport. I heard the Tower talk to a Cherokee and instructed them to enter the downwind and follow a 737 on final (us). As we got closer to the runway I queried the Tower about our traffic. They said we were now number one for the runway and company 737 was departing ahead of us. As we were lining up with the runway at about 500 ft. AGL I saw a Cherokee on a close in base leg to our left. He was joining final in front of us. I called out the traffic to the First Officer who was flying. He saw the traffic and pitched down to avoid the aircraft. The Cherokee passed directly above us by about 100-200 ft. Our approach was now unstable so I called for a go around. The First Officer asked if we were clear of the traffic (we now were) and then initiated the go around at 200 ft. AGL. We maneuvered to rejoin a visual approach to a landing. I reported a near miss to the Tower. I later found out the Cherokee did not have a transponder which explains why there was no TCAS alert. This also effected the ability of the Tower to track them. If I had known the Cherokee didn't have a transponder I would have been more vigilant about making sure we acquired visual contact.

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	1681161
Time
Date	201909
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	None
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Conflict NMAC Inflight Event / Encounter Unstabilized Approach

B737 captain reported near midair collision with a cherokee on final approach; resulting in a go-around.