NYC95LA012

The pilot was attempting to land on the property of a friend when the aircraft entered an uncommanded right turn. According to the pilot, he made two passes to the north while searching for the landing site. He spotted the site, turned right to the southwest to set up for a landing to the north and into the wind. The pilot reported that after he turned southwest, at 150 to 200 feet AGL, the helicopter began spinning to the right at an increasing rate to 1 revolution per second. After about 8 revolutions the aircraft settled into trees. The examination of the aircraft did not reveal evidence of mechanical malfunction. The investigation revealed the wind conditions were favorable for the loss of tail rotor effectiveness.

On October 15, 1994, at 1630 eastern daylight time, N9238, an Enstrom F-28A helicopter, a personal flight, collided with trees during a landing attempt at a private residence in Shelburne, Vermont. The private pilot, and passenger received minor injuries. The helicopter was substantially damaged. The flight originated at Pittstown, New Jersey with a refueling stop at Sarasota, New York. The flight was operated under 14 CFR Part 91.

The pilot and passenger were on a trip to visit friends, whose property was to be the landing site. The site had been marked with a white sheet.

According to the pilot, they had difficulty locating the marked spot as they approached the area. The pilot made several passes by the estimated location before he spotted the sheet. "We saw the sheet in the field establishing the landing location, and I turned to the right (southwest) planning to set up for an approach in a northerly direction which would have been into the wind.

"My altitude was about 550 feet MSL (150-200 AGL). As I completed the turn, the tail of the helicopter started coming around to the left, at first slowly, but with increasing speed until the ship was spinning in an uncontrollable fashion. Anti- torque pedals were totally ineffective and I couldn't tell if cyclic controls were of any use as the ship was spinning at about one turn per second."

According to an eyewitness, "Just as they passed overhead they turned clockwise, I assume to in order to make a second pass to land. As they completed the turn it became evident the something had gone wrong, since the aircraft did not straighten out but continued to turn. Gradually losing altitude the helicopter continued to spin in a clockwise direction, clearly out of control. They spun 5-7 complete circles before coming down in the small trees at the edge of our property."

The 1550 hour surface weather observation for Burlington, Vermont reported winds as 310 degrees at 10 knots.

The helicopter was examined by the FAA and Enstrom Helicopter on October 19, 1995. The examination did not disclose evidence of mechanical malfunction.

FAA circulated information (see attachment for detailed information) based on a reprint from Bell Helicopter Textron, Inc. company newletter, Rotorbreeze, discusses the identification of the loss of tail rotor effective (unanticipated right yaw). According to the reprint, unanticipated right yaw is identified with the following four charactristics during low speed flight:

1. Weather cock stability (120 to 240 degrees) 2. Tail rotor vortex ring state (210 to 330 degrees) 3. Main rotor disc vortex interference (285 to 315 degrees) 4. Loss of translational lift (all azimuths)

According to the reprint, "The aircraft can be operated safely in the above relatively wind regions if proper attention is given to controlling the aircraft. However, if the pilot is inattentive for some reason and a right yaw rate is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable correction action is taken."