What happened
During the approach phase, the pilot obtained clearance for the SDF approach with instructions to remain at 4,000 feet until established on the procedure. However, radar tracking indicated that the aircraft began a descent prior to reaching the Initial Approach Fix. The aircraft passed the IAF at an altitude of 2,900 feet and entered a descending left-hand turn that led into unprotected airspace. During this maneuver, the aircraft completed a 180-degree turn and disappeared from radar at 1,900 feet. The flight path did not align with any of the four available instrument approaches at the airport. Ultimately, the aircraft struck a guy wire attached to a lighted communications antenna located 3.3 nautical miles southeast of the airport while flying on a heading of 360 degrees.
Findings
Investigations determined that there were no mechanical failures or system malfunctions present on the aircraft at the time of the accident. A critical factor was that the navigation signal for the approach was out of service and had been inactive for more than four years. An FAA flight check confirmed that no signal was being broadcast. The pilot failed to notify air traffic control regarding the absence of the signal and did not initiate a missed approach procedure. Additionally, the air traffic controller issued the approach clearance without verifying that the navigation aid was operational.