N450MC

HISTORY OF FLIGHTOn January 9, 2020, about 2029 eastern standard time, a Robinson R66, N450MC, was destroyed after it was involved in an accident near Mechanicsburg, Pennsylvania. The private pilot and the passenger were fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to a chronological summary of flight communications prepared by the Federal Aviation Administration (FAA), about 1954, the pilot contacted ground control at Martin State Airport (MTN), Baltimore, Maryland, and requested visual flight rules (VFR) flight following to Buffalo Niagara International Airport (BUF), Buffalo, New York, at 3,000 ft mean sea level (msl). About 1957, the pilot contacted local control at MTN and advised that the flight was ready to depart, and was then approved to do so. About 1959, the pilot was instructed to contact Potomac Approach Control.

According to FAA automatic dependent surveillance-broadcast (ADS-B) data and a transcription of audio communications, the helicopter climbed to about 2,300 ft msl and proceeded on a north-northwesterly track. The helicopter remained at that altitude and on that track from about 2002 to 2023, during which time the pilot made initial contact with the Harrisburg approach sector. At 2023:15, the controller instructed the pilot to fly along a heading of 310°, and the helicopter turned to a northwest heading, consistent with the instruction. The helicopter’s altitude remained about the same until 2028:50, at which time the helicopter began descending. About 2029, radar contact with the helicopter was lost, and the controller attempted to contact the pilot but received no response. Also about 2029, the Harrisburg approach radio frequency recorded a “grunt-type sound” from an unidentified source. The last radar target, at 2029:17, showed that the helicopter was at an altitude of about 1,150 ft msl.

The helicopter was equipped with a Garmin GTN 750, a Garmin GDU 1060, and an engine monitoring unit (EMU), which recorded data at a rate of at least one sample per second. The synchronized downloaded data from the devices depicted that the final portion of the flight contained no pitch or roll excursions, nor any significant heading changes until about 2028:46 (which was 29 seconds before the end of the GDU 1060-recorded data and 32 seconds before the end of the EMU-recorded data). Immediately before that time, pitch was steady at -3°, roll was steady at near-zero values, and the engine’s torque was about 58%. From 2028:46 to 2029:05, the helicopter’s pitch began to oscillate slightly and eventually reached -12°, and the helicopter began to roll to the left with an increasing rate, reaching about 8 to 10° between 2029:03 and 2029:05. After 2029:05, the left roll continued to increase quickly (within about 5 to 7 seconds); during the rapid left roll, the engine torque spiked briefly and then decreased. At 2029:06, the normal acceleration parameter trended negatively during the next 3 seconds, reaching -1 G.

Between 2028:07 and 2029:07, the average indicated airspeed was about 107 knots. During the same timeframe, the average recorded G loading was -0.035, with the G loading decreasing afterward to -1 with the corresponding left roll. One second later, at 2029:08, the indicated airspeed was zero.

At 2029:10, the helicopter’s left roll was about 119°, pitch was -8°, and engine torque was about 30%; 1 second later, the helicopter was in a left roll of 179° and a -23° pitch, and the engine torque increased to 64%. This time corresponded with the grunt sound that was recorded by the Harrisburg air traffic control facility. Between 2029:11 and 2029:12, the helicopter’s pitch decreased to -46°, the torque decreased to about 35%, the power turbine speed (N2) increased from 98% to 101%, and the left roll continued past 180° (inverted) to 114° to the right.

At 2029:13, the helicopter’s pitch was -58°, roll was 126° right, N2 was about 99%, and torque was about 32%. One second later, the helicopter’s pitch was -36°, N2 was about 75%, and torque was at about 13%. At 2029:15, the helicopter’s N2 was about 128% (overspeed), and the torque was about 1%. The last ADS-B target about 2029:17 depicted the helicopter being at about 1,150 ft msl. At 2029:18, the compressor speed, N2, and torque values were 72%, 111%, and less than zero, respectively.

A witness, who was outside and located about 986 ft east of the accident site, reported hearing and seeing a low-flying helicopter that was “struggling to fly.” The witness reported hearing a high-pitched noise that sounded as if the rotor was having difficulty turning, and then she heard a loud “boom” and saw a flash of light. At that point, the helicopter disappeared from her view.

Another witness who was in his house located about 800 ft northeast of the accident site reported hearing a “thumping” sound that slowed. The witness then heard a loud “bang,” which shook his house. The witness also reported hearing a “percussion” sound before the sound of impact. His house had a doorbell video camera that also recorded audio; the video camera was provided to the National Transportation Safety Board (NTSB) for examination. (See the Video Study section of this report for further information.)

PERSONNEL INFORMATIONDuring 2001 and 2009, the pilot was diagnosed with obstructive sleep apnea and coronary artery disease, respectively. According to documentation provided to the FAA, cardiac catheterization had shown mild-to-moderate triple-vessel coronary artery disease that did not require stenting or surgery, and the pilot did not have any typical symptoms of related heart pain or congestive heart failure.

Medical records from the pilot’s most recent visit to his primary care provider on November 27, 2019, showed that the pilot was using a continuous positive airway pressure (CPAP) device to treat his sleep apnea with good adherence and symptom control. The records also showed that the pilot was not adhering to treatment for high blood pressure but that he had no cardiovascular symptoms and a blood pressure of that was slightly higher than his goal.

During the year preceding the accident, the pilot used his CPAP 85% of the time with an average use of 8 hours 6 minutes. During the 7 days preceding the accident, the pilot used his CPAP every night with an average use of 7 hours 38 minutes per night and 8 hours 6 minutes of use on the night before the accident.

According to records provided by Robinson Helicopter Company, the pilot attended a safety course in February 2019 for the R66. At that time, the pilot had not accumulated any hours in the R66. On the evaluation document, the instructor indicated that the pilot had “very good control” of the helicopter when performing normal and emergency procedures and that he was “very comfortable practicing the [autorotations].” The instructor suggested that the pilot obtain more training on emergency procedures. The pilot was rated “average” on all demonstrated maneuvers.

AIRCRAFT INFORMATIONThe helicopter was manufactured in January 2019 and was purchased shortly afterward by a company associated with the pilot. At the time of the accident, the hour meter indicated about 167 hours. The helicopter was equipped with a semirigid rotor system that rotated counterclockwise (when viewed from above). According to Robinson Safety Notice (SN) 11, Low-G Pushovers – Extremely Dangerous, pushing the cyclic forward following a pull-up or rapid climb, or even from level flight produces a low-G (weightless) flight condition. The notice goes on to state that if forward cyclic is applied, the main rotor torque reaction will then combine with tail rotor thrust to produce a powerful right rolling momement of the fuselage.

The helicopter was equipped with a Genesys Aerosystems HeliSAS two-axis autopilot system comprised of a flight control computer, HeliSAS control panel, and two servo-actuators (one each for the pitch and roll axes). The autopilot system was designed to record faults in the system. The downloaded data were unrecognizable by the manufacturer; therefore, no accident-related data were available from the HeliSAS.

According to the R66 Pilot’s Operating Handbook Supplement for the HeliSAS,

The primary autopilot mode is stability augmentation system (SAS) mode which maintains a steady helicopter attitude by applying corrective inputs to the cyclic. The autopilot does not provide any collective or pedal inputs. Additional modes providing heading hold, altitude hold, and navigation functionality are also selectable.

The SAS mode of the autopilot was engaged by pressing either the SAS button on the control panel or the trim button mounted on the cyclic for more than 1.25 seconds. With the SAS mode engaged, the pilot can “fly through” the SAS to achieve a desired attitude and can subsequently press and release the trim button to reset the baseline attitude for the system to the current attitude. Pushing the SAS button on the control panel or the “AP [autopilot] OFF” button on the cyclic disengages the SAS mode (for manual control), and four aural beeps would sound in the pilot’s headset. Additionally, the HeliSAS is designed to automatically disengage when one of several predefined faults or malfunctions is detected, which would also produce four aural beeps in the pilot’s headset. Disengagement of an autopilot mode, such as heading or navigation, not commanded by the pilot (caused by, for example, the loss of a valid navigation signal) would be accompanied by a single beep in the pilot’s headset. Any intentional disengagement of an autopilot mode other than SAS would not trigger a beep sound in the pilot’s headset (because SAS mode would remain engaged).

METEOROLOGICAL INFORMATIONAccording to the NTSB’s weather study, at the helicopter’s cruising altitude of about 2,300 ft, the wind was from about 190° at 27 knots. At that altitude, there was an 87% probabili...