What happened
On the evening of the accident, Flight 1808 departed from Boston's runway 04L at 21:30, bound for Auburn. The flight was initially cleared to an altitude of 7000 feet with instructions to communicate with Portland Approach Control. At 21:58, the controller provided clearance for a runway 04 ILS approach.
By 22:01, air traffic control observed that the aircraft had drifted east of the prescribed course and inquired if the crew was receiving the Lewiston localizer signal. The captain reported they were not receiving it, prompting instructions to execute a left turn to a heading of 340°. As the aircraft began its turn to 354°, it passed the Lewie Outer Marker at an altitude of 2,600 feet and a speed of 165 knots, which was noted as being 600 feet above and 30 knots faster than required.
At 22:02, while maintaining a heading of 354°, the aircraft drifted left of the localizer. The crew attempted to intercept the glide slope and re-enter the localizer course; however, by 22:04:08, the plane had descended through the bottom boundary of the glide slope. At approximately 22:04:16, the aircraft type struck trees roughly 4,007 feet before reaching the runway and 440 feet to the right of the extended centerline. The impact continued for another 737 feet before the plane hit level ground in a nearly inverted position. The accident resulted in one fatality among the passengers.
Findings
Investigations determined that the primary factor was the captain's decision to continue an unstabilized approach, which led to the aircraft descending below the glide slope. This instability was further exacerbated by the controller providing and the pilot accepting a non-standard radar vector, which caused an excessive intercept of the localizer.