History of Flight
On April 16, 2019, about 0706 mountain standard time, an experimental Bell 206B helicopter, registration N61PH, was destroyed when it was involved in an accident near Fort McDowell, Arizona. The commercial pilot and flight test engineer were fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 test flight.
The operator stated that the flight purpose was to evaluate developmental main rotor blades installed on the helicopter. The helicopter was fueled for two test flight legs. It departed at 0545 for the first leg and returned about 0620, parked on the ramp with the engine at idle, and 100 lbs of ballast was added. Radar data indicated that the helicopter departed at 0632 to continue the second leg. The test flight consisted of multiple autorotations at maximum gross weight following a simulated loss of engine power. The commercial pilot performed all maneuvers, while the flight test engineer held a 17-inch, 7.72-lb engineering laptop with his left hand above the cyclic control, simultaneously using and stowing a clipboard and pencil to manage data collection. A corded event marker connected to equipment in the aft cabin was draped over the laptop when not in use. The clipboard was normally stowed between the center console and the engineer’s seat. Flight cards listed the maneuvers to be performed. The accident flight was to be the last test flight of the main rotor blades before their certification process.
Radar data showed the accident flight duration was about 34 minutes. The pilot executed multiple turns and descent maneuvers near the accident site. The final radar-recorded altitude was 4,400 ft mean sea level (msl), about 3,000 ft above ground level (agl).
A witness walking to a bus stop about 1/3-mile northwest of the accident site heard a loud bang southeast of her position. She saw the helicopter falling from the sky and recorded video of the helicopter and several other objects descending before losing sight behind trees.
Wreckage and Impact Information
Examination of the accident site revealed postcrash fire and impact damage consistent with a right side-down, nose-level attitude during ground impact. The main rotor hub assembly, vertical fin stabilizer, tail rotor assembly, tail rotor driveshaft, and forward induction cowl fairing separated from the main wreckage and were found in the debris field. One main rotor blade was found furthest from the main wreckage; the other was near the main wreckage. The debris field was about 1 mile long and 1,000 ft wide, covering wooded desert terrain and flood-irrigated alfalfa fields.
Examination of the wreckage revealed thermal and impact damage. Both left and right seat cyclic controls were installed. Both main rotor blades separated approximately 4 ft outboard of the blade grips. Both blades exhibited witness marks consistent with tail boom or tail rotor contact. The main rotor mast showed damage consistent with a mast bumping event. Examination of the airframe and engine found no evidence of preaccident mechanical malfunctions or anomalies that would have precluded normal operation.
A tail rotor blade, main rotor blades, and main rotor hub were sent to the NTSB Materials Laboratory for further examination. The components exhibited features consistent with overstress fractures. There were no indications of pre-existing damage or failures.
Additional Information
The operator stated that the flight test engineer’s cockpit position cyclic control stick was typically removed during test flights.
The two rotor blades installed were developmental prototype blades used to collect data for certification requirements. The main rotor blade was an aftermarket replacement for the Bell model 206B helicopter, constructed using carbon fiber/epoxy pre-preg woven fabric and unidirectional tape. The blade interfaced with the existing Bell main rotor hub using two titanium grip plates bolted to the composite sub-assembly. The blade incorporated a NASA-developed laminar flow airfoil; the tip was tapered in both chord and thickness directions. No modifications were required to install the blades.
One blade was instrumented with strain gages to measure blade loads during the test flight: flap bending, chord bending, and torsion at several spanwise locations. Data were stored on test equipment in the helicopter’s passenger compartment and the engineer’s laptop.
The flight test engineer’s laptop remained largely intact, and files from the accident flight were obtained from the hard drive. Data from near the time of the accident were not recovered. The obtained data showed multiple maneuvers during the flight, during which the blade displayed consistent load behavior. Maneuvers not identified in the data were a simulated power failure involving a one-second delay before pilot reaction, and two autorotations with lateral reversals.
The simulated power failure maneuver procedure required the pilot to initiate from level flight at 100 kts, reduce engine power to flight idle by using the collective twist grip, allow a one-second delay, then lower the collective, apply right pedal to maintain directional control, and enter an autorotative descent. The purpose was to represent a realistic reaction to a sudden loss of engine power. It is possible to impart a low-G condition on the helicopter during execution if the collective is lowered rapidly.
The FAA Helicopter Flying Handbook (FAA-H-8083-21B) states that low-G conditions and mast bumping are critical for helicopters with two-bladed teetering rotors. Helicopters rely on positive G for control response. In a low-G condition, thrust and control authority are greatly reduced. For two-bladed teetering rotors, which rely entirely on thrust vector tilt for control, low-G conditions can be catastrophic.