Event Overview
On a night arrival, a developmental controller (DC), supervised by an instructor controller (IC), was handling the flight. The pilot was instructed to descend and cross the STILL Intersection at 3,000 ft. STILL Intersection was aligned with the localizer approach course, 10.1 nautical miles from the approach end of Runway 17. The final approach fix (FAF) was located 3.9 nautical miles from the runway.
Sequence of Events
Approximately 5 nautical miles before reaching STILL Intersection, while on course and level at 3,000 ft, the pilot was cleared for the Loc Rwy 17 approach. Radar data indicated the aircraft continued to STILL Intersection and then turned onto the localizer course toward the FAF. Shortly after departing STILL Intersection, while inbound on the localizer course, the aircraft began a descent.
Before the aircraft reached the FAF, the DC issued a frequency change to UNICOM. During this transmission, the IC noticed a low altitude alert on the radar display and issued a verbal low altitude alert, stating: "check altitude, you should be at 1,500 ft (should have said '1,800 ft' as that was the minimum crossing altitude at the FAF), altitude's indicating 1,200, low altitude alert." There was no response from the pilot. This occurred about 2 nautical miles before the FAF. The minimum descent altitude (MDA) for the approach was 720 ft. The aircraft struck tree tops at 750 ft, approximately half a nautical mile before the FAF.
Contributing Factors
The IC's remark "you should be at 1,500 ft" was based on an expired approach plate that listed a lower FAF minimum crossing altitude. The current minimum crossing altitude at the FAF was 1,800 ft. Approach control management had not made the current approach plate available to the controllers. The investigation could not determine whether a current approach plate would have prompted an earlier warning by the controllers.
Probable Cause
Failure of the pilot to follow the published instrument (IFR) approach procedure, by failing to maintain the minimum altitude for that segment of the approach.