History of Flight
On July 30, 2023, at approximately 1219 Pacific daylight time, a Bell OH-58A helicopter, registration N613SS, sustained substantial damage during an accident near Blanchard, Idaho. The pilot and three passengers reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
According to the pilot, the helicopter was en route to its destination at an altitude of about 1,000 feet above ground level. Approximately 15 minutes into the flight, while the helicopter was in a level attitude, the pilot felt a small but rapid yaw oscillation, which he characterized as about 5° in either direction. Shortly thereafter, the low-rotor alarm sounded. The pilot initiated an autorotation and began searching for a suitable landing site. He aligned the helicopter for touchdown in an open field and performed a run-on landing in tall grass.
As the helicopter’s skids slid across uneven terrain, they separated from the airframe, and the pilot lost control. The helicopter pivoted to the right and came to rest on its left side, with the tail boom severed from the fuselage.
Aircraft Information
The helicopter was powered by an Allison T63-A-720 gas turbine engine. The most recent documented engine inspection was a Condition Inspection on December 28, 2022, which occurred 6.9 operating hours before the accident.
The pilot stored Jet-A fuel for the helicopter in a 180-gallon aluminum tank. A filter was installed between the tank and a 20-foot-long, 1-inch diameter rubber fueling hose that was not rated for aviation use. The pilot stated that he did not filter fuel left in the hose between uses before transferring fuel into the helicopter’s tank.
Tests and Research
Engine Examination
Postaccident examination of the engine revealed all mounts were intact. Linkages from cockpit controls to the power turbine governor and fuel control unit were checked and no anomalies were found. The bleed valve was open, and all external air, fuel, and oil line connections were at least finger-tight. The inlet particle separator was dirty but not obstructed. The gas generator (N1) rotor turned freely and was continuous from the starter generator to the compressor and first-stage turbine wheel. Borescope inspection of the compressor and first-stage turbine wheel showed no anomalies. The power turbine (N2) rotor turned slightly in the drive direction, but rotation was impeded by output shaft damage from impact. The drive was continuous to the helicopter transmission. The N2 rotor freewheeled appropriately when turned opposite the direction of normal rotation. No foreign object damage was noted on the first-stage compressor blades, compressor inlet, or fourth-stage turbine wheel. The engine oil filter had no significant debris, and the oil appeared normal and clean. The fuel nozzle primary and secondary orifices were not obstructed, and the air shroud displayed normal carbon sooting.
The engine was installed in a correlated test stand where it started on the first attempt and operated successfully for about one hour. No anomalies or power perturbations were noted. The engine satisfactorily met all specified test conditions, including start time, governor droop, anti-ice, three decelerations/accelerations, and five power calibration points.
Fuel Examination
The helicopter’s mounted fuel filter was full of fuel containing debris on the input (pre-filter) side. The fuel retained from that filter was dark in appearance. The engine-mounted fuel filter bowl contained a residual amount of fuel that was also dark. Due to the damaged landing skids, fuel could not be drained from the main tank.
The Federal Aviation Administration (FAA) Aviation Fuels Research Laboratory analyzed samples from the helicopter and found contamination and degradation. Visual inspection of unfiltered fuel showed a dark cloudy appearance with suspended particulates. After filtering, the fuel remained visibly dark, and heavier gum deposits were found on the filters. These gums, identified as oxidation products of Jet-A fuel, were present at concentrations about 10 times higher than a control sample.
Further testing revealed that the fuels exhibited higher water content, density, dynamic viscosity, and kinematic viscosity than the control sample, indicating fuel degradation. Some particulate matter appeared to be inorganic crystalline particles and degraded gel substances. Testing also showed a higher presence of oxygen, nitrogen, and chlorine in the fuel residues, supporting evidence of advanced oxidation and contamination. Oxide particles were additionally found, which could originate from the fuel itself or result from the breakdown of tubing materials in the fuel system.
Based on energy-dispersive x-ray spectroscopy analyses of the concentrations of carbon, nitrogen, and oxygen in the fuel, there was no evidence of diesel exhaust fluid contamination.
The FAA report concluded that the fuel samples displayed very low oxidative stability, substantial gum formation, and particulate contamination, all of which would compromise safe engine performance.
The engine maintenance manual emphasizes the importance of maintaining the entire aircraft and engine fuel system to the highest standards of cleanliness. Rolls-Royce has conducted testing on apple jelly type contamination, noting that it behaves differently than a solid contaminant; it can pass through various airframe and engine fuel filters undetected without actuating the impending bypass indicator, and it can cause partial or complete blockage of the fuel nozzle screen, resulting in reduced engine performance or flameout. The manual states that if this gel-like material is detected at any point in the aircraft or engine fuel system, the entire system must be inspected. Additionally, an external low-pressure fuel filter must be used when refueling from remote fueling sites, such as drums, to prevent engine fuel system contamination that could cause engine flameout.