N473EMcdonnell Douglas Helicopter 369E · 1991

On July 13, 2024, about 1045 mountain daylight time, a McDonnell Douglas Helicopter 369E, N473E, sustained substantial damage when it was involved in an accident near Cottonwood, Arizona. The pilot and 3 passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 sightseeing flight.

The pilot reported that he departed from Cottonwood Airport (P52), Cottonwood, Arizona, with three passengers for a local area sightseeing flight over mountainous terrain. About 30 minutes into the flight, the pilot made a slow, high-powered, descending maneuver near an area with ruins about 15 nm northeast from P52, so that the passengers could take photos. As the pilot attempted to leave the area an uncommanded right yaw occurred. The pilot applied full left pedal control, but the helicopter continued its right yaw. Shortly after, the low rotor rpm audio warning sounded, and the temperature caution light flashed. The pilot then performed a low-level autorotation and nosed down the helicopter to gain airspeed. The helicopter's airspeed increased, and the right yaw stopped, but the helicopter landed hard on the ground and its left skid separated. The helicopter bounced several times and slid forward.

The pilot scanned the instruments; observing that all values were normal, he then opted to depart the mountainous area and, after he believed he achieved satisfactory flight control response during slow maneuvers, returned to P52. He briefed the passengers on his plan for their egress, executed an emergency landing with a single skid, and successfully disembarked his passengers with assistance from another pilot. Shortly after, the pilot relocated the helicopter to a grassy area within the airport and away from structures. Once the helicopter was settled on the ground with one skid, he pulled the fuel cutoff switch while he simultaneously lowered the collective and rolled off the throttle. Subsequently, the helicopter came to rest on its left side and the main rotor blades contacted the ground. The helicopter’s drivetrain sustained substantial damage.

Postaccident examination revealed no preimpact mechanical failures or malfunctions to the airframe. The pilot reported a possible tail control rigging issue; however, the 4-bladed tail rotor assembly was examined for rigging and revealed no anomalies. Flight control and drivetrain continuity was confirmed from the cockpit to all main and tail rotor flight control surfaces.

The pilot reported that during the maneuver near the accident site, the helicopter was flying about 100 ft above ground level at about 20 kts. The helicopter flight manual’s height/velocity performance chart showed that the helicopter was at a combination of height and velocity which pilots are recommended to avoid operating (assuming a smooth hard surface and calm winds), as it would leave the pilot with insufficient time, altitude, or kinetic energy to cushion the touchdown from an autorotation.

According to the operator’s calculation, using a pressure altitude of 5,100 ft (the reported tour site altitude) and an outside air temperature of 32°C, the helicopter flight manual’s hover ceiling performance chart showed that the recommended maximum operational weight for out-of-ground effect (OGE) hover was about 2,740 lbs with the air conditioning (AC) unit off, and about 2,640 lbs with the AC unit on. According to the operator, the helicopter’s gross weight at the time of the accident was about 2,826 lbs.

The manual noted that the 4-bladed tail rotor’s OGE directional controllability in winds greater than 3 kts had not been determined; the manufacturer added that the 4-bladed tail rotor system was susceptible to LTE while operating in high winds, with low airspeed (typically below translational speed) and high-power settings, and near or above maximum gross weight.

FAA AC 90-95 (“Unanticipated Right Yaw in Helicopters”) discusses the phenomenon of LTE. According to the AC, LTE is a critical, low-airspeed aerodynamic flight characteristic, which can result in an uncommanded rapid yaw rate. Conditions under which LTE may occur include maneuvers that require the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind. Other factors that significantly influence the severity of the onset of LTE include high gross weight, high density altitude, and low indicated airspeed. The advisory circular further states that a pilot should conduct low-level, low-airspeed maneuvers with minimum weight and, when maneuvering between hover and 30 kts, the pilot should avoid OGE hover and high-power demand situations. At the time of the accident, the pilot was operating at a density altitude of about 8,119 ft.