Bell 206 N288AE accident near Carthage, Illinois

Probable cause

A collision with a fence during landing due to the crew’s reduced distance estimation and depth perception while using night vision goggles. Contributing to the accident was the low moon angle, which made viewing the fence more challenging.

NTSB analysis

The air medical helicopter was dispatched to a remote area to pick up a patient in night visual meteorological conditions. While enroute, the aircrew received received a landing zone (LZ) brief that included information that a wire fence was located on the south side of the road.

Upon approach to the LZ, the pilot and the flight nurse transitioned to and used night vision goggles (NVGs). The pilot performed both high and low reconnaissance and the aircrew verified that there were no poles or wires present. The flight nurse reported that she observed the fence on the south side of the road during this time; however, she thought the fence was farther away than it was.

The pilot maneuvered the helicopter to a 10-ft hover over the road; the aircrew identified the wire fence on the south side of the road and noted that there were ditches on both sides of the road. The pilot initiated a right pedal turn and he planned to land so that the cabin door was oriented toward the accident scene. As the helicopter touched down, the pilot heard a “pop” noise and he felt feedback in the tail rotor pedals. The pilot realized the tail rotor system impacted the wire fence; he lowered the collective immediately and performed an emergency shutdown. The helicopter sustained substantial damage to the vertical stabilizer, the tail boom, the tail rotor driveshaft, and the two tail rotor blades.

At the time of the accident, the moon was bright with 73% of its disk illuminated and it was at a relative angle of 25° above the horizon. According to NVG training references, terrain interpretation is generally more difficult when the moon is low on the horizon due to the lower light level and the shadows caused by the low angle. is the pilot likely had difficulty viewing the fence while using NVGs due to the lower light level and the presence of shadows.

It is likely that, during the landing, the aircrew misjudged the distance of the wire fence due to the reduced distance estimation and depth perception while using NVGs.

Text from the NTSB. Open the NTSB docket for CEN23LA350 for the full record.