Uncommanded Yaw Preceded Crash: NTSB
A phenomenon known as loss of tailrotor effectiveness may have had something to do with the crash of a Bell 206 in the East River off Manhattan 14 months ago. The NTSB factual report into the crash (PDF) says an uncommanded right yaw while the pilot was trying to land the aircraft preceded the crash, which killed two of five people on board. Pilot Paul Dudley told the NTSB the he thought he'd had an engine failure on takeoff and tried to return to the helipad when the uncommanded right yaw occurred. Dudley said he thought the sudden yaw was the result of tailrotor failure or loss of tailrotor effectiveness and tried to minimize the impact of the helicopter by manipulating the collective. No pre-crash mechanical faults were found with the aircraft. Although the factual report does not determine cause, the investigators included a detailed description of loss of tailrotor effectiveness.
The FAA issued an advisory circular in 1995 dealing with the phenomenon that appears to match the circumstances in this case. "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," it quotes the FAA as warning. Mainstream media has seized on the possibility the aircraft may have been slightly overweight. The NTSB calculates it might have been 28 pounds heavier than its legal gross of 3,200 pounds.