ERA12TA538

The pilot stated that the accident helicopter was configured for single-pilot operation (the copilot controls had been removed for the assigned mission). While in cruise flight between 110 and 120 knots and about 2,500 feet mean sea level, the helicopter began pitching up and yawing right, because the force trim was the "off" position, which rendered the altitude hold feature inoperative, and the pilot responded by pushing the cyclic forward and left. The helicopter then pitched "severely" nose down and entered a right spin. At that time, the pilot saw a portion of the tail boom, tail rotor, and tail rotor gearbox falling away separately from the helicopter. The pilot entered an autorotation and landed in wooded terrain. About 1 year after the accident, another pilot for the operator performed a maintenance test flight in a similarly configured make and model helicopter. While in cruise flight and with the force trim in the "off" position, the pilot released the cyclic momentarily, and the helicopter pitched up and rolled right. The flight scenario was duplicated several times with the copilot controls installed on the helicopter and, when the pilot released the cyclic, it maintained its same relative position, even with the force trim in the "off" position. Therefore, it appears that aerodynamic forces drove the cyclic aft in the single-pilot configured helicopter because the added weight and lever arm in a dual-pilot configured helicopter was not available to neutralize the forces. The manufacturer should have known about this characteristic of the helicopter and warned pilots of unanticipated and unequal aft cyclic pressure in the single-pilot configured helicopter compared to the dual-pilot configured helicopter. Detailed examination and testing of the helicopter and its components revealed no preaccident anomalies and no evidence of foreign object damage.

While performing a maintenance test flight, the chief pilot for the NYSP recounted an in-flight upset that he experienced in an NYSP Bell 407 during a maintenance flight approximately one year after the accident. The helicopter was configured for single-pilot operation, with the copilot cyclic and collective controls removed. According to the chief pilot, he was in cruise flight with the force trim switch in the "off" position. He released the cyclic control "for an instant," and the cyclic moved aft and the nose pitched up and yawed to the right. He said he was able to recover from the unusual attitude and transition back to cruise flight, but not before the helicopter had pitched up approximately 15 to 20 degrees. An NYSP mechanic accompanied him on the flight and gave a similar account. After the flight, the copilot cyclic and collective controls were reinstalled and the flight was repeated. In cruise flight with the force trim switch in the "off" position, the cyclic control was released by the pilot, and it maintained its relative position. On December 5, 2013, a test flight was conducted in an NYSP Bell 407 configured for single-pilot operation, with the copilot cyclic and collective controls removed. An NTSB investigator accompanied an NYSP maintenance test pilot on the flight. The helicopter was flown at various speeds in cruise flight, with the force trim switch in the "off" position when the pilot would release the cyclic control. Each time, the cyclic would transition aft on its own, and the nose of the helicopter would pitch up and roll right. The rate of pitch and roll would change relative to airspeed. The helicopter manufacturer duplicated the test flight with a company Bell 407 and observed similar results. According to the NYSP chief pilot, other NYSP 407 pilots, and a representative of the helicopter manufacturer, the autopilot heading (hdg) and altitude (alt) modes will only function with the force trim switch in the "on" position. It was also noted that the hdg and alt capsules will illuminate when selected regardless of the position of the force trim switch. According to FAA and maintenance records, the helicopter was manufactured in 2002. The helicopter was maintained under a Manufacturer's Inspection Program, and its most recent inspection was completed July 6, 2012, at 2,918.4 total aircraft hours. On the day of the accident, the helicopter was configured for single-pilot operation only, with the copilot's side cyclic and collective controls removed. On August 31, 2012, about 1045 eastern daylight time, a Bell 407 helicopter, N11SP, operated by the New York State Police (NYSP), was substantially damaged during a forced landing to wooded terrain near Bath, New York. The certificated commercial pilot sustained minor injuries. Visual meteorological conditions (VMC) prevailed, and a company VFR flight plan was filed for the public use aerial surveillance flight that last departed from a field near Hammondsport, New York,about 1020. The pilot stated that the helicopter was in cruise flight between 110 to 120 knots, at an altitude of about 2,500 feet mean sea level, with the heading (hdg) and altitude (alt) modes of the autopilot "on," when the helicopter encountered turbulence. He elected to climb the helicopter to 3,500 feet, and "reached down to disengage altitude hold," when the helicopter began un-commanded excursions in the pitch and yaw axes. The pilot responded by pushing the cyclic "forward and left" to compensate for the nose-up pitch and right yaw. The helicopter then pitched "severely" nose down, and entered a spin to the right. At that time, the pilot saw a portion of the tail boom, the tail rotor and tail rotor gearbox falling separately and away from the helicopter. The pilot entered autorotation, and completed a power-off landing to wooded terrain. After landing, he egressed the helicopter and called for assistance on his cellular telephone. The pilot was interviewed with regard to the flight multiple times, and his account was consistent throughout. He described both a right yaw and a right roll, and at times used the terms interchangeably, but the sequence of events and the content of the events that he described remained the same. Witnesses described the helicopter as it passed overhead in cruise flight when "something fell off it" and then it entered a spin. Others stated that the helicopter was spinning before they saw objects fall from it. The pilot said that at the time of the upset, "I reached out with my left hand to activate the switch, but I'm not sure if my right hand was on the cyclic or guarding the cyclic." The New York State Police Forensic Investigation Center, Albany, New York, performed the toxicological testing for the pilot. Neither drugs nor alcohol were detected in the blood samples tested. At 1153, the weather conditions reported at Elmira/Corning Regional Airport (ELM), 22 miles southeast of the accident site, included clear skies, 10 miles visibility; temperature 26 degrees C, dew point 14 degrees C, and an altimeter setting of 30.03 inches of mercury. The wind was from 240 degrees at 10 knots. The pilot held a commercial pilot certificate with ratings for rotorcraft/helicopter and instrument helicopter. His most recent FAA second-class medical certificate was issued on April 13, 2012. A review of his flight records revealed the pilot had accrued approximately 1,600 total hours of flight experience, of which 40 hours were in the accident helicopter make and model. The pilot, a NYSP Trooper, joined the aviation division June 7, 2012. On August 3, 2012, he completed the Bell 407 Initial Pilot and Ground Training at the Bell Helicopter Training Academy, Ft. Worth, Texas. On August 14, 2012, the pilot was designated pilot-in-command in the Bell 407 helicopter for day/night/night vision goggle missions.The accident flight was the pilot's second single-pilot mission for the unit. On September 11, 2012, the engine's electroniccontrol unit (ECU) was found undamaged and downloaded at the NYSP hangar. Examination of the data indicated no engine faults prior to the accident, and appropriate responses to power demands up until the time of ground contact. On September 20, 2012, swabs of organic material taken from flight control surfaces above the transmission cowling were tested at the Smithsonian Institution, Washington, DC. The samples contained matter from various insects, but revealed no evidence of a bird strike. On October 1, 2012, a sample of the helicopter's hydraulic fluid was tested at Aviation Laboratories in Kenner, Louisiana. In the remarks section it stated, "Viscosity and silicone appear high. All other sample test results appear normal." On December 10, 2012, the hydraulic servo actuators of the flight control system were tested at the manufacturer's facility in Fort Worth, Texas. The tests revealed that there were no system anomalies that would have precluded normal operation. On February 24-25, 2013, the autopilot control head and its associated system components were tested at the manufacturer's facility in Grand Prairie, Texas under the supervision of the FAA. The tests revealed that there were no system anomalies that would have precluded normal operation. On June 6, 2013, the vertical and directional gyros were tested at the manufacturer's facility in Olathe, Kansas under the supervision of the FAA. The tests revealed that there were no system anomalies that would have precluded normal operation. Examination of photographs revealed separation of the entire tail boom, and substantial damage to the helicopter's fuselage. The helicopter and its associated components were recovered from the site and moved to the NYSP Aviation Division, Albany, New York, for examination. The examination was conducted September 11, 2012.The helicopter was wrapped in shrink-wrap, and presented for examination on a flatbed trailer at the NYSP's hangar. The landing skids, main rotor blades, and tail boom had separated during the accident sequence, but were present on the trailer. The plastic wrap was removed and the wreckage examined. The fuselage exhibited substantial vertical impact damage. The landing skids had spread and fractured. All four main rotor blades had separated in overload near the blade grips; the exhaust duct had been crushed downward and the tail boom had separated due to main rotor blade contact. The pilot's side chin bubble was fractured and the anti-torque pedals were found in the full left position. The top of the forward cowling ("dog house") exhibited extensive main rotor blade rub marks. The tail rotor assembly exhibited no evidence of rotational damage.Examination of the fracture surfaces and separations in the powertrain, flight control, and airframe components revealed failure modes consistent with overstress.Examination of the cockpit revealed the force trim switch was in the "off" position. The NYSP aviation division chief pilot, who responded tothe accident site, stated that he found the force trim switch in the off position at the scene.