Accident Summary
On April 23, 2024, about 1050 Pacific daylight time, a Bell 206 L-3 helicopter, registration N17592, sustained substantial damage during a forced landing near Tracy, California. The pilot and passenger were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as an aerial observation mission.
Flight Details
According to the pilot, the helicopter was inspecting a pipeline at approximately 280 feet above ground level (agl) on a northerly heading. The pilot initiated a left turn to the west to continue the inspections. During the turn, the pilot and passenger heard and felt a loud pop followed by a shudder. The helicopter then entered an uncontrolled descent that the pilot could not correct with collective control inputs. Engine power and rotor rpm were normal, and no warning annunciations or lights were observed. The pilot selected an orchard with small trees for the forced landing because most nearby landing areas were obstructed by wires. The helicopter descended over the trees, and the pilot used cyclic inputs to keep the helicopter level before it dropped to the ground from about 12 to 15 feet agl.
Damage and Postaccident Examination
The helicopter sustained substantial damage to the tailboom. Postaccident examination revealed that collective control authority was continuous from the collective stick to the collective servo through the collective servo input arms. The collective servo exhibited two fractures at the cylinder extension lower arm and upper arm (referred to as the output support upper and lower leg in the NTSB materials laboratory report). The cyclic control system was continuous from the cyclic stick through the servos to the swashplate assembly.
An NTSB materials laboratory examination of the collective servo showed that both fractures were relatively flat and oriented 90° relative to the longitudinal direction of each arm. Both cracks had propagated from the interior surfaces outward and exhibited fatigue striations consistent with fatigue crack propagation. Dimpled rupture features were observed outside the fatigue cracking regions, consistent with subsequent overstress fracture following fatigue crack propagation.
Lower Arm
On the lower arm, the crack had propagated through 80% of the arm before transitioning to overload. The initiation site contained a prominent corrosion pit about 1.1 mm wide by 0.34 mm deep. This and other corrosion pits served as individual initiation sites for multiple fatigue cracks, separated by small ratchet marks. The compounds in the corrosion pit included elements consistent with corrosion products in a marine environment, such as sodium, potassium, calcium, chlorine, and sulfur, with elevated chlorine in the deepest regions. The base material of the servo arm was consistent with 7075 aluminum alloy composition.
Upper Arm
The upper arm contained two thumbnail cracks with three major initiation regions. The smaller crack's initiation site contained a corrosion pit and fatigue striations. A wider corrosion pit (0.30 mm wide by 0.14 mm deep) was found within the larger thumbnail crack. Fatigue striations were present near the corrosion pit. The corrosion pits on the upper arm also exhibited a chemical composition consistent with aluminum oxide with elements associated with salt compounds.
A follow-up examination at Bell Engineering Laboratories indicated that collective inputs, especially during less-than-1G maneuvers, were initially driving the crack and creating fatigue band features. Towards the end of the fatigue crack, two-per-revolution main rotor loads contributed to crack growth. An attempt to correlate fatigue band striations with two vibrations per blade revolution to determine if crack initiation started after installation of Van Horn composite blades (installed 10 days and 59 flight hours before the accident) was inconclusive due to non-uniform fatigue band structure.
Maintenance History
Maintenance records showed that the collective and cyclic servos were removed from another Bell 206 (serial number 51036, registration N170AM) on September 29, 2021, at 1,897.2 hours since overhaul, with the reason for removal noted as "conv. [convenience]." N170AM had been involved in an accident on July 9, 2021, due to a dynamic rollover. The servos were installed in the accident helicopter (N17592) on October 27, 2021.
The pilot reported accumulating 159.2 flight hours in the accident helicopter between 2021 and the accident date. During that period, the helicopter was routinely operated in eastern California (Modesto, Fresno, Victorville), about 50 nautical miles from the Pacific Ocean.