What happened
While operating in Class E airspace approximately 15 miles north of its destination airport, the crew of a McDonnell Douglas MD-81 experienced a near midair collision. During the initial approach, the aircraft was descending through 4,000 feet MSL under instrument meteorological conditions (IMC). Approach control cleared the pilot to descend to 3,000 feet MSL on a heading of 160 degrees, noting traffic approximately one mile to the southwest at an indicated altitude of 2,500 feet MSL.
As the MD-82 began to level off at 3,000 feet MSL and descended below the cloud layer, the captain observed a Piper Seneca climbing from 2,500 feet toward his aircraft. The pilot had minimal time to react before the twin-engine airplane passed to the left and below the MD-82 with a horizontal separation of approximately 500 feet and a vertical separation of 200 feet. Following the encounter, the pilot reported to air traffic control that the encounter was very close.
The investigation
A review of approach control records determined that the Piper Seneca was not in communication with approach control, which was not required for its operation within Class E airspace. Additionally, an investigation into the MD-82's equipment revealed that the aircraft's traffic alert and collision avoidance system (TCAS) was inoperative at the time of the incident, and no collision avoidance alert was provided to the crew during the encounter.