On May 14, 1996, at 0945 eastern daylight time (EDT), a Hiller UH-12E, N90467, lost directional control during an aerial application flight near Hellam, Pennsylvania. The helicopter sustained substantial damage. The certificated airline transport pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed and a flight plan was not filed. The local, aerial application flight was conducted under 14 CFR Part 137 and originated at 0835 EDT. According to the Chief Pilot, the pilot reported that he completed two or three aerial application passes successfully, and he was into the fourth application pass when he felt a high frequency, medium strength vibration in the tail rotor peddles. He stated that the vibration progressed rapidly resulting in the helicopter losing directional control and yawing control. The pilot told the Chief Pilot that he performed a forced landing with nothing but trees within gliding distance. He also stated that his airspeed decreased to zero and the helicopter spun straight down. The helicopter impacted terrain on a tree covered slope. The Chief Pilot stated that the oak tree canopy was dense, ranging from approximately thirty to eighty feet in height. A witness reported that he observed the helicopter heading north just above tree level. He said, "Approximately 1-3 minutes later I observed it heading south just above tree level. Once out of sight I heard the pitch of the propeller blades change and become higher and then the sound abruptly stopped." The helicopter was examined at the accident site by a Federal Aviation Administration (FAA) Aviation Safety Inspector. The examination revealed that the main rotor blade end caps separated from the two main rotor blades. The main rotor blade end caps were located approximately 200 yards from the main wreckage. The main rotor blade end caps and the tail rotor blades were sent to the NTSB Metallugical Laboratory in Washington, DC, for further examination. The examination revealed that the tail rotor blades separated from the outboard tips. Examination of both blades revealed a series of impact line damage adjacent to the separations. The damage extended chordwise from the leading edge towards the trailing edge of each blade and appear to be from a relatively solid metallic object. One main rotor blade end cap contained an impact mark at the leading edge and was compressed chordwise towards the trailing edge. Wooden chips were entrapped in the folds of the deformed metal. The cap had pulled through most of the attachment screws resulting in tears between the screw holes and the inboard edge of the cap. The other main rotor blade end cap contained impact damage at the leading edge. All of the holes for the screws that hold the cap to the main rotor blade were undamaged, consistent with the cap being disassembled from the blade. Examination with the aid of a low power binocular microscope did not reveal any indication of paint transfer in the damaged areas of the tail rotor blades and main rotor caps.