N288AEBell 206L-3 · 1989

On August 6, 2023, about 0107 central daylight time, a Bell 206 L-3 helicopter, N288AE, sustained substantial damage when it was involved in an accident near Carthage, Illinois. The pilot, the flight nurse, and the flight medic were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 air ambulance flight.

The crew (consisting of the pilot in the front right seat, the flight nurse in the rear left seat, and the flight medic in the rear right seat) were dispatched to pick up a patient from a remote area. The pilot completed the flight risk assessment just before the flight, with a low risk score, and accepted the flight. The operator’s operations control specialist also reviewed and acknowledged the completed flight risk assessment.

The helicopter departed from the Southeast Iowa Regional Medical Center, Fort Madison, Iowa, at 0051. While enroute, the aircrew received an LZ brief that included information that the intended LZ was an east-to-west running dirt and gravel road with no wires; a wire fence was located on the south side of the road and corn fields were on the north and south sides of the road. Night visual meteorological conditions prevailed at the LZ and marking lights were placed on it. All of the helicopter’s external light sources were on for the approach and the landing.

Upon approach to the LZ, the pilot and the flight nurse transitioned to and used NVGs. The flight medic was unaided and was not using NVGs. The pilot used the NVG-compatible searchlight to aid in scanning the LZ and the surrounding area for obstacles. The helicopter approached the LZ from the northwest and performed a high reconnaissance about 700 ft agl in a left orbit, confirming the LZ brief received from the fire department. The helicopter approached the LZ from north to south, perpendicular to the road. During the descent the aircrew performed a low reconnaissance, verified that there were no poles or wires present, and established visual contact with the fire department. The flight nurse reported that she observed the fence on the south side of the road during this time; however, she thought fence was farther away than it was.

The pilot maneuvered the helicopter to a hover about 10 ft 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 the cabin door was oriented toward the accident scene. This intended landing position would also provide visual contact with the fire department personnel entering and departing under the main rotor system and aid in patient loading.

The heading of the helicopter was 310° when the pilot initiated the descent toward the road. As the helicopter touched down, the pilot heard a “pop” and 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 came to rest upright on a heading of about 360°, perpendicular to the road and not the intended landing direction. The three aircrew members egressed the helicopter without further incident. The helicopter sustained substantial damage to the vertical stabilizer, tail boom, tail rotor driveshaft, and two tail rotor blades.

At the time of the accident the moon was at a relative angle of 25° above the horizon and at an azimuth of 106°, with 73% of its disk illuminated. The moon had risen at 2311 on the previous day and set at 1155 on the accident day. The operator’s risk assessment program did not include information about moon altitude and brightness; however, this data is captured and analyzed on the operator’s daily crew briefing form.

The operator reported there were no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.

The operator’s 14 CFR Part 135 NVG training program included information about moon angle, distance estimation, and depth perception. However, it did not include information about shadows, construction, terrain density, and vegetation. After the accident and at the suggestion of the NTSB investigator-in-charge, the operator updated their 14 CFR Part 135 NVG training program to include these additional areas regarding terrain interpretation.

While not a required training reference for civilian aircrews, the U.S. Army has published Training Circular 3-04.4 Fundamentals of Flight, released in July 2022. This document discusses various topics used in the U.S. Army NVG training program. This document discusses distance estimation and depth perception and states in part:

Distance estimation and depth perception are closely related. Distance estimation relates to determining distance to objects, while depth perception primarily refers to the relationship of objects to each other. The quality of both is affected by ambient light, type, and quality of NVG, degree of contrast in the field of view, and viewer's experience utilizing monocular cues. Objects tend to appear further away than they actually are.

Reduction in visual acuity negatively influences distance estimation because we expect objects that are less distinct in detail to be farther than ones that possess sharp detail.

This document further discusses moon angle when using NVGs and states in part:

Moon angle has the same effect as the angle of the sun regarding ambient light levels. The position of the moon should be considered during each phase of aviation operations. However, the angle of the moon would have the most impact of LZ selection and landing direction.

A low-angle moon may be beneficial in areas with little terrain relief. Terrain interpretation is generally more difficult when the moon is low on the horizon. This is due to the lower light level and the shadows caused by the low angle. However, when the moon is low on the horizon, terrain features or objects on the skyline may be more recognizable.

The Federal Aviation Administration does not provide reference material for NVG operations for the public regarding topics such as NVG operating characteristics, terrain interpretation, and operational considerations.