History of Flight
On June 24, 2014, at about 1235 central daylight time, a Robinson R44 II, registration N544BS, was substantially damaged during an autorotation near John C. Tune Airport (JWN) in Nashville, Tennessee. The commercial pilot and passenger were uninjured. Visual meteorological conditions prevailed, and no flight plan was filed. The personal flight originated from JWN and was destined for Decatur Airport (DEC) in Decatur, Illinois, under 14 CFR Part 91.
The pilot made an intermediate stop at JWN to obtain fuel during a flight from Florida to Wisconsin. After departure, he turned the helicopter north toward DEC, toward a group of broadcast towers. About 1.5 nautical miles from the airport and at about 400 feet above ground level, he initiated a climb to clear a hill covered by tall trees. The helicopter was traveling about 70 knots. The engine began running "very rough," and the pilot felt an uncommanded movement of the throttle twist grip, followed by an abrupt loss of altitude. He lowered the collective, the low rotor rpm horn activated, and the low rotor rpm light illuminated. The pilot turned west to initiate an autorotation. He glanced at the engine/rotor rpm gauge and saw the engine was producing about 70% rpm and the rotor rpm was in the green; however, the engine continued to run rough. The pilot initiated an autorotation to a nearby highway. The helicopter landed hard in a level attitude, and the main rotor contacted the tail boom, resulting in substantial damage. The pilot reported he "didn't have enough rotor energy left to put it down gently" and that he did not increase throttle at any time during the descent but did decrease engine throttle when initiating the autorotation.
A helicopter flight instructor who taught at JWN reported a similar uncommanded throttle movement about two months before the accident while flying a Robinson R44 helicopter east of the same group of broadcast towers. During that event, the low rotor rpm light activated, the instructor lowered the collective and entered an autorotation, but moments later he noticed he still had engine power, increased throttle, and returned safely to the airport. The instructor believed the event resulted from sporadic governor operation and decided to fly near the towers again to replicate the uncommanded throttle motion. Accompanied by another pilot, he repeatedly flew the same route. During each pass near the broadcast towers, the low rotor rpm light activated, and the governor-actuated twist grip throttle was felt moving in the throttle off direction. The pilots were able to override the governor each time by increasing throttle.
Personnel Information
The pilot held a commercial pilot certificate with ratings for helicopter, airplane multi-engine land, and instrument airplane. He had accumulated about 10,197 hours of total flight time, of which 1,678 hours were in the accident helicopter make and model. His most recent second-class FAA medical certificate was issued on January 23, 2014.
Aircraft Information
The four-seat, single main rotor, single-engine helicopter was constructed primarily of metal and manufactured in 2003. It was powered by a 205-horsepower Lycoming IO-540-AE1A5 series reciprocating engine. The collective control was equipped with a "twist grip" throttle, and a 24-volt, dual-wound, DC reversing motor electronic governor system was installed to maintain engine rpm by adjusting the throttle when the collective was moved. The Pilots Operating Handbook (POH) stated that the governor was "easily overridden by the pilot," was only active above 80% engine rpm, and could be switched on or off via a toggle switch on the end of the right-seat collective.
Review of maintenance records revealed that the helicopter had been operated for about 31.2 hours since its most recent 100-hour inspection, completed on April 1, 2014. At that time, the airframe and engine had accumulated about 1,658 hours of operation.
Meteorological Information
The 1253 automated weather observation at Nashville International Airport (BNA), about 11 nautical miles east of the accident site, included winds from 190 degrees at 13 knots with gusts to 21 knots, visibility 10 statute miles, few clouds at 2,900 feet, broken ceiling at 3,500 feet, temperature 29°C, dew point 21°C, and an altimeter setting of 30.16 inches of mercury.
Wreckage and Impact Information
According to an FAA inspector, the helicopter came to rest about 1.5 miles northeast of JWN near a high-power radio station. The helicopter remained intact except for the tailboom, which was fractured about 3 feet forward of the tail rotor. Control continuity was traced from the flight controls to the cyclic and collective pitch links. Tail rotor continuity was traced from the control pedals through the fracture to the tail rotor pitch change links, which remained attached. The pilot did not report any preimpact anomalies or malfunctions with the control system.
Postaccident examination of the engine at a recovery facility under FAA supervision revealed it ran smoothly and continuously during an initial test run. A compression check showed all cylinders operating in the normal range. The lower spark plugs displayed normal wear. Both magnetos produced spark at their terminal leads when rotated by hand. A cylinder borescope inspection showed normal cylinder wear and a uniform heat pattern on exhaust valve faces.
Additional Information
According to the helicopter manufacturer, the Robinson R44 Raven II governor controller determines engine speed from unpowered magneto points. With known engine speed, the governor calculates the appropriate throttle response and engages a motor tied to the throttle control. The governor controller output to the motor is a proportional controlled pulse width modulated signal operating at 300 Hz to 500 Hz.
The FAA Spectrum Engineering reviewed circuit diagrams provided by the manufacturer. According to the FAA's interpretation, the input signal for the governor picks up the engine rpm signal through a pair of dry contacts in one of the two magnetos. The signal is also applied to the tachometer circuit. The FAA's research revealed that the governor input circuitry was not designed to reject a high-power external radiated signal. The input wiring's grounded twisted pair includes an overall braid that provides some attenuation to RF but is not effective against certain electric fields. The assembly offers no protection from magnetic fields, making the wiring harness susceptible to transformer effect. Within the controller, a 1000pF feedthrough capacitor acts as an initial RF filter, but this does not protect against medium and strong high-frequency RF fields and is not effective at lower RF frequencies.
The circuit acts as an amplitude demodulator when RF is introduced into the governor controller wiring, producing a signal that artificially increases the signal frequency representing engine speed (adds pulses to the pulse train). Once the signal increases to more than 5% rpm from the centerpoint, the governor calculates the appropriate throttle response and engages a motor to decrease engine power.
At the time of the accident, there were four 100KW towers transmitting at a frequency greater than 3 MHz near JWN. A search for towers with emission A3E amplitude modulation, power greater than 5KW, and frequency between 2 MHz and 30 MHz revealed 49 high-power stations in 8 physical locations within the United States.
In April 1999, the manufacturer issued Safety Notice SN-35 to address previous reports of radio interference. The notice stated that flying near broadcast towers may result in "erratic operation of the governor," which may cause the governor to roll the throttle to the idle or open position. It also instructed pilots to keep one hand on the collective and throttle and to refrain from flying near broadcast towers. The pilot stated he was familiar with SN-35 but had not read it since purchasing the helicopter in 2007. His preflight research did not produce any relevant NOTAMs on the day of the accident.