What happened
On August 6, 2023, at approximately 0107 CDT, a Bell 206 L-3 helicopter, registration N288AE, was involved in an accident near Carthage, Illinois. The aircraft, operated by Air Evac EMS, Inc. as a Part 135 air ambulance, was enroute to a remote area to retrieve a patient.
The flight crew, consisting of a pilot, a flight nurse, and a flight medic, was operating under night visual meteorological conditions. Prior to arrival, the crew received a landing zone (LZ) briefing describing an east-to-west running dirt and gravel road with a wire fence located on the south side of the road.
Upon approach, the pilot and flight nurse utilized night vision goggles (NVGs), while the pilot used an NVG-compatible searchlight to scan for obstacles. After performing high and low reconnaissance, the pilot maneuvered the helicopter into a hover approximately 10 feet above the road. The pilot then initiated a right-pedal turn to orient the cabin door toward the accident scene for patient loading.
As the helicopter touched down, the pilot heard a "pop" and felt feedback through the tail rotor pedals. The pilot immediately realized the tail rotor system had struck the wire fence, prompting an emergency shutdown. The aircraft remained upright, and all 3 crew members were not injured.
The investigation
The investigation determined that the helicopter sustained substantial damage to the tail rotor blades, tail rotor driveshaft, tail boom, and vertical stabilizer. There were no preimpact mechanical failures or engine malfunctions reported.
Analysis of the environmental conditions showed the moon was 25° above the horizon with 73% illumination. While the operator's risk assessment did not specifically track moon altitude, the data was available in the crew's daily briefing. The investigation noted that the operator's NVG training program included distance estimation and depth perception but lacked specific instruction regarding shadows, terrain density, and vegetation.
Findings
- The crew misjudged the distance of the wire fence due to reduced depth perception and distance estimation while using NVGs.
- The low angle of the moon contributed to the difficulty in viewing the fence by creating challenging light levels and shadows.