ATL02TA083

The pilot brought the helicopter to a 400-foot hover so that his flight observer could observe some recently installed signs. As he began his transition to forward flight, the helicopter began an uncommanded right yaw. The pilot applied left pedal to stop the yaw but the helicopter continued to yaw. In an effort to correct the yaw condition the pilot entered an autorotation. The helicopter collided with the ground and bounced and came t o to rest on the left side in about three feet of water. Examination of the helicopter failed to disclose a mechanical malfunction or component failure. Advisory Circular 90-95 states in-part that, " Loss of Tail Rotor Effectiveness (LTE) is a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control".

On April 14, 2002, at 1545 eastern daylight time, a Hughes OH-6A Helicopter, N56TP, registered to and operated by the City of Tampa Police Department as a Public Use flight, experienced a loss of tail rotor effectiveness (LTE) and ditched into Tampa Bay near MacDill Air Force Base, Tampa, Florida. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The helicopter sustained substantial damage, the pilot received minor injuries, and the passenger received serious injuries. The flight originated from Tampa International Airport at 1515.

According to the pilot, while on patrol around MacDill Air Force Base (AFB),Tampa, Florida, flying along the perimeter at the south end of the base, he brought the aircraft to a hover so that his flight observer, could observe some recently installed signs that were posted to mark the base perimeter. He said that they were at an altitude of about 400 feet . After reading the first sign from a hover, they were moving forward to read some additional signs that were further inside the perimeter. As he began his transition to forward flight, the helicopter entered an uncommanded right yaw. He applied left pedal to stop the yaw with no results. The pilot executed an autorotation, and stated that the helicopter hit very hard while still in a right spin with a high descent rate. The helicopter bounced airborne then came down again on the left side nose low. The rotor blades impacted the ground and began to disintegrate as it rolled over on its left side. The helicopter finally stopped on its left side in about three feet of water.

During the course of the investigation the two tail rotor blades, two tail rotor drive shaft fragments, tail rotor gearbox input pinion gear, tail rotor gearbox output pinion gear and the lower spline component of the main rotor drive shaft, were sent to Boeing--Mesa Engineering Laboratory for further examination. According to Boeing-Mesa's Materials, Processes and Standards Department the examination of the above parts revealed the following: One of the two tail rotor blades showed evidence of impact damage manifested by bending and corrugation of the trailing edge section. The other blade did not show any evidence of impact damage. The tail rotor drive shaft fractured as a result of torsional overload. The tail rotor input pinion gear teeth showed evidence of impact marks from the mating output pinion rear teeth. These marks are indicative of sudden stoppage, and non-destructive testing inspection did not reveal any evidence of cracking. The tail rotor output pinion gear teeth showed evidence of impact marks from the mating input gear teeth. The main rotor drive shaft lower spline fractured as a result of torsional overload.

Advisory Circular 90-95 states in part that, " LTE is a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control. LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less then 30 knots. Any maneuver which requires the pilot to operate in a high-power, low airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. If a yaw rate has been established in either direction, it will be accelerated in the same direction when the relative winds enter the 120 to 240 degree area unless corrective pedal action is made. If the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region, the yaw can accelerate rapidly."

The pilot reported winds out of the west at eight knots. The nearest weather reporting facility reported variable wind conditions. The winds 400 feet above the ground, at the time of the accident, were not determined.