Narrative:

Inbound to ZZZ airport cancelled IFR when center switched us to CTAF. Was told there was another aircraft inbound to airport; and that we were much faster. Announced 12 miles northwest planning overhead left downwind entry to runway X shortly after a comanche reported south of the airport for runway Y. After our call comanche stated 'we will use runway X also so we will both be going the same direction.' after fully configuring on downwind and turning base leg to runway X the comanche states on CTAF 'I have you in sight and will be flying right underneath you.' I said to sic 'what??? Why would he be doing that.' sic looking out right window says 'I see him at 5 o'clock and that's exactly what he is going to do.' 'he will be coming out your side.' I looked out the window and see him at 8 o'clock directly behind left wing. I said 'does he think he is landing ahead of us?' sic asked him 'to pull off his approach and come around behind us.' he agreed to do that. When he approached our aircraft on right rear our TCAS indicated -200 ft. Based on my sighting of him I'd say that was accurate. Based on where he was versus where we were I can't figure out how he thought he would land on runway X before us. The position I saw him at would have been a right downwind for runway Y. Certainly not on final approach to either runway. I would expect to see him at 12 0'clock when I'm on base if he is on final landing before us. After he landed he taxied to hangars on the north side of the field; so appears to be based at this airport. I can't in any way make what this guy did make sense. Arrogant and stupid to think it's a good idea for a single engine airplane to fly directly under jet engines. I'm hoping you figure out who this was and can hopefully prevent this from happening again.

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Original NASA ASRS Text

Title: CE680 Captain reported a NMAC while on downwind for the active runway.

Narrative: Inbound to ZZZ airport cancelled IFR when Center switched us to CTAF. Was told there was another aircraft inbound to airport; and that we were much faster. Announced 12 miles NW planning overhead left downwind entry to Runway X shortly after a Comanche reported South of the airport for Runway Y. After our call Comanche stated 'we will use Runway X also so we will both be going the same direction.' After fully configuring on downwind and turning base leg to Runway X the Comanche states on CTAF 'I have you in sight and will be flying right underneath you.' I said to SIC 'WHAT??? Why would he be doing that.' SIC looking out right window says 'I see him at 5 o'clock and that's exactly what he is going to do.' 'He will be coming out your side.' I looked out the window and see him at 8 o'clock directly behind left wing. I said 'does he think he is landing ahead of us?' SIC asked him 'to pull off his approach and come around behind us.' He agreed to do that. When he approached our aircraft on right rear our TCAS indicated -200 ft. Based on my sighting of him I'd say that was accurate. Based on where he was versus where we were I can't figure out how he thought he would land on Runway X before us. The position I saw him at would have been a right downwind for Runway Y. Certainly not on final approach to either runway. I would expect to see him at 12 0'clock when I'm on base if he is on final landing before us. After he landed he taxied to hangars on the north side of the field; so appears to be based at this airport. I can't in any way make what this guy did make sense. Arrogant and stupid to think it's a good idea for a single engine airplane to fly directly under jet engines. I'm hoping you figure out who this was and can hopefully prevent this from happening again.

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.