Narrative:

Upon entering the traffic pattern; I had a potential near miss situation with a vintage T-6 texan. The T-6 was on a downwind leg for runway xx; while I called a 2-mile left base for xx. My internal flight following service reported no traffic. My TCAS did not show the T-6 or give a traffic alert. So I question whether T-6 had a transponder? I just happened to pick out the aircraft in my scan at 10 o'clock position converging at same altitude. I did not hear the T-6 give a position report; perhaps they did and I did not hear. Upon sighting the T-6 I gave way by making an evasive right 360. I announced that I had downwind traffic in sight and was giving way. I rejoined on the downwind leg and landed without further incident.

Google
 

Original NASA ASRS Text

Title: Helicopter pilot entering on the base leg reported a conflict with vintage military trainer on the downwind. The trainer apparently had no radios or transponder and the helicopter took evasive action.

Narrative: Upon entering the traffic pattern; I had a potential near miss situation with a Vintage T-6 Texan. The T-6 was on a downwind leg for Runway XX; while I called a 2-mile left base for XX. My internal flight following service reported no traffic. My TCAS did not show the T-6 or give a traffic alert. So I question whether T-6 had a transponder? I just happened to pick out the aircraft in my scan at 10 o'clock position converging at same altitude. I did not hear the T-6 give a position report; perhaps they did and I did not hear. Upon sighting the T-6 I gave way by making an evasive right 360. I announced that I had downwind traffic in sight and was giving way. I rejoined on the downwind leg and landed without further incident.

Data retrieved from NASA's ASRS site as of April 2012 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.