Small Transport; Low Wing; 2 Recip Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
ATC Issue All Types; Conflict NMAC
Flight Crew Requested ATC Assistance / Clarification
Airspace Structure; Procedure; Human Factors
During the ILS 18 approach at LEB we began our descent when we captured the glideslope. When emerging from the cloud base we saw a small twin flying northeast across the approach course. We verified that the airplane was not climbing or performing any maneuvers other than straight and level flight. The airplane crossed below us approximately 200 feet below our altitude. We notified the tower and they informed us that they were not talking to anyone. The tower checked with Boston Center and found out that the aircraft was no longer under their control and was last under radar contact well east of the field. We continued our approach and landed without further incident.
Air taxi pilot reported a near-mid-air-collision on approach to LEB when another aircraft passed below without communication.