History of Flight
On February 17, 2010, at 1450 Pacific standard time, a Robinson R22 Beta (registration N313DA) force-landed in a field near Parlier, California. The commercial pilot and one passenger were not injured. The helicopter sustained substantial damage to the main rotor assembly. The flight departed from Fresno Yosemite International Airport, Fresno, California, at 1430, and was on the first leg of a trip to North Las Vegas Airport, Las Vegas, Nevada, with the first planned stop at Meadows Field Airport, Bakersfield, California. The pilot operated the helicopter under visual meteorological conditions.
According to the pilot, while established in straight-and-level flight, he initiated a 10- to 20-degree banked turn to the right. The helicopter immediately rolled inverted. The pilot applied aft cyclic and lowered the collective, regaining control and performing a powered landing to a field. Damage included a fractured rotor blade spindle tusk, fractured pitch link, crushed elastomeric teeter stops, and a shallow, diagonal crease in the tail boom just forward of the tail rotor.
Pilot and Passenger Statements
The pilot reported that he and a passenger were ferrying the helicopter from Fresno to Las Vegas. Approaching Reedley, California, at 1,100 feet mean sea level (800 feet above ground level) and 85 knots, he initiated a shallow right turn while slightly increasing collective. The helicopter abruptly rolled to the right at a rate exceeding 100 degrees per second. Recognizing a low-G situation, the pilot applied aft cyclic and lowered the collective, resulting in a split-S maneuver, then entered a precautionary autorotation. At about 100 feet AGL and 70 knots, he arrested the descent and continued the autorotation to a field. He initiated a power recovery and flare, executed a normal set down, rolled off power, and disengaged the clutch. As the main rotor slowed, the droop stops broke and the blades began to sag. He applied the rotor brake, and the retreating blade impacted the tail boom at slow speed.
The passenger, seated in the left seat, noted to the pilot that the cyclic appeared shaky. She then observed the helicopter tilt to the right, front side, and become inverted. After a few seconds, the helicopter was upright and descending slowly. She described the landing as very smooth and gentle.
Aircraft Information
The helicopter had undergone a 2,200-hour overhaul and had operated 3.2 hours at the time of the accident. According to the test pilot, the helicopter performed normally during post-overhaul flight tests. The pilot stated that the helicopter's performance and handling characteristics before and after the overhaul were similar.
Meteorological Information
Light wind conditions were reported just above the surface, and light chop was reported near the coast at higher altitudes. There were no indications of turbulent activity at the helicopter's altitude or along its route. No mountain wave activity was reported.
Examination Findings
The NTSB Materials Laboratory examined a fractured pitch change link and the root of the red blade with the spindle. Optical examinations of the spindle tusk fracture revealed shearing overstress fracture features, with the shearing direction indicating the inboard end of the tusk moved downward relative to the main rotor blade. The pitch change link upper rod end fitting fracture exhibited cyclic overstress features, consistent with reverse bending a few times to failure. The orientation of the bending axis was measured at about 81 degrees relative to the fitting head axis.
Mating damage patterns on the fitting and adjacent areas of the pitch change arm positioned the link at nearly 45 degrees to the arm. According to the metallurgist, this angle would occur with the main rotor blade at a large droop angle, which could result from a fractured tusk on the spindle. A representative from Robinson Helicopter Company indicated that this angle could also occur during extreme teetering of the main rotor hub during a mast bumping event.
Additional Information
Section 4 of the R22 pilot's operating handbook states that mast bumping may occur with a teetering rotor system when excessive main rotor flapping results from low G (load factor below 1.0) or abrupt control input. A low-G flight condition can result from an abrupt cyclic pushover in forward flight. High forward airspeed, turbulence, and excessive sideslip can accentuate adverse effects. Excessive flapping can cause the main rotor hub assembly to strike the main rotor mast, potentially leading to main rotor separation.
Robinson Helicopter Company Safety Notice SN-11 warns that pushing the cyclic forward after a pull-up or rapid climb, or from level flight, produces a low-G (weightless) condition. If the helicopter is still pitching forward when the pilot applies aft cyclic to reload the rotor, the rotor disk may tilt aft relative to the fuselage, and main rotor torque reaction combined with tail rotor thrust can produce a powerful right rolling moment. With no lift from the rotor, lateral control is lost, and mast bumping can occur, often resulting in main rotor shaft separation or rotor blade contact with the fuselage.
Safety Notice SN-29 advises that airplane pilots transitioning to helicopters must develop safe helicopter reactions, as everything happens faster in a helicopter. It emphasizes practicing procedures repeatedly with a competent instructor.