What happened
A Piedmont Airlines flight, designated as Flight PI255, was operating a Boeing 737 aircraft with the registration N749N. The aircraft was descending under instrument flight rules in visual meteorological conditions when it encountered a close proximity situation with another aircraft. Simultaneously, a twin-engine Comanche, identified as a Piper PA-30 with the registration N7399Y, was cruising westbound at an altitude of 8,500 feet.
The two aircraft converged and passed each other with a reported vertical separation of only 50 to 100 feet. At the time of the incident, Flight PI255 was descending toward an altitude of 7,000 feet. Air traffic control at Lansing Approach had previously advised the flight crew of visual traffic, specifically a Beechcraft E33 located at 6,500 feet. However, the controller was unaware of the presence of the Piper PA-30 because its radar return was blocked out by an overlap with the Beechcraft, which was positioned 2,000 feet below the Comanche.
The pilot of the Beechcraft E33 was receiving visual flight rules advisories from approach control. In contrast, the Piper PA-30 was not operating under a flight plan and was not in radio contact with air traffic control. Neither crew involved in the near collision reported seeing the other aircraft visually. All three aircraft involved were equipped with transponders capable of altitude reporting and were squawking either an assigned code or an appropriate code supplemented by altitude replies.