Approach and Descent Instructions
During a nighttime 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. The 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 3.9 nautical miles from the runway.
Clearance and Descent
Approximately 5 nautical miles before reaching STILL Intersection, while on course and level at 3,000 ft, the pilot was cleared for the Localizer Runway 17 Approach. Radar data showed the aircraft continued to STILL Intersection, 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.
Low Altitude Alert
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 then 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.
Impact
The minimum descent altitude (MDA) for the approach was 720 ft. The aircraft struck tree tops at an altitude of 750 ft, about half a nautical mile before the FAF.
Expired Approach Plate
The IC's remark that the aircraft should be at 1,500 ft was based on an expired approach plate that had 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 plate available to the controllers. Investigation could not determine whether a current approach plate would have prompted an earlier warning by the controllers.