Regional Jet 200 ER/LR (CRJ200)
Flight Crew Commercial
Conflict NMAC
Flight Crew Took Evasive Action
Airspace Structure; Human Factors
We were flying along at 4000 ft direct to MISHA (outer Marker). HDG was approximately 255-265 degrees. The Pilot Flying grabbed my attention with an OH SUGAR!! I started to scan for incoming hazards and things that could have been wrong such as; a wrong altimeter setting or the FMS acting funny. I then could see the pilot flying (PF) reaching for the controls to disconnect the autopilot at this time an aircraft began to come into my peripheral vision from left to right. I could see the belly; panels and detailed paintings on the aircraft as it crossed over the center pillar into my central vision. It looked to be a single engine warbird navy blue in color with decals on the wings. Milliseconds prior to this time period the PF began an abrupt aggressive maneuver in pitch up and shallow turn to the left. Autopilot warning was going off and stick shaker was activated I could see the warbirds right wing getting closer and thought for sure the planes would have come in contact with each other. It also looked as if the warbird was doing a climbing left hand turn; like a chandelle. Our aircraft had gained altitude and as I looked back behind us out of the right window I could see the warbird parallel to us with its propeller facing us at the same level shallowing out its turn. Our TCAS did not go off due to the warbird not having a transponder or in the on position. We were never notified of a primary target. We returned to assigned altitude and heading. The rest of the 6 minute flight was no eventful and I am glad to be alive. [I suggest] all Aircraft have working transponders.
After the event we proceeded to make a normal visual approach and landing in SBN.
CRJ200 flight crew reports a NMAC with a warbird at 4000 feet east of SBN. Evasive action is taken by both aircraft and a collision is narrowly avoided.