N613SS

HISTORY OF FLIGHTOn July 30, 2023, at 1219 Pacific daylight time, a Bell OH-58A helicopter, N613SS, was substantially damaged when it was involved in an accident near Blanchard, Idaho. The pilot and three passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that he was en route to his destination at an altitude of about 1,000 ft above ground level. About 15 minutes into the flight, with the helicopter configured in a level attitude, he felt a small but rapid yaw oscillation. He characterized the oscillation to be about 5° in either direction. The low-rotor alarm then sounded. In response, the pilot initiated an autorotation and began searching for a suitable landing site. The pilot aligned the helicopter to touch down in an open field and performed a run-on landing in tall grass.

As the helicopter’s skids slid on the uneven terrain, they separated from the airframe and the pilot lost control of the helicopter. The helicopter pivoted to the right and came to rest on its left side, with the tail boom severed from the fuselage. AIRCRAFT INFORMATIONThe helicopter was equipped with an Allison T63-A-720 gas turbine engine. The last documented engine inspection in the logbook was a Condition Inspection on December 28, 2022, 6.9 operating hours before the accident.

The pilot stored Jet-A fuel for the helicopter in a 180-gallon aluminum tank with a filter installed between the tank and a 20-ft-long, 1-inch diameter, rubber, non-aviation-grade fueling hose. The pilot stated that he did not filter fuel left in the hose between uses before transferring fuel into the helicopter’s tank. AIRPORT INFORMATIONThe helicopter was equipped with an Allison T63-A-720 gas turbine engine. The last documented engine inspection in the logbook was a Condition Inspection on December 28, 2022, 6.9 operating hours before the accident.

The pilot stored Jet-A fuel for the helicopter in a 180-gallon aluminum tank with a filter installed between the tank and a 20-ft-long, 1-inch diameter, rubber, non-aviation-grade fueling hose. 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 RESEARCHEngine

Postaccident examination revealed all of the engine mounts were intact. Linkages from the cockpit controls to both the power turbine governor and the fuel control unit were checked with no anomalies noted. The bleed valve was open and all external air, fuel, and oil line connections were at least finger-tight. The helicopter was equipped with an inlet particle separator, which 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. The compressor and first-stage turbine wheel were viewed via borescope with no anomalies noted. 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 was examined with no significant debris observed, and the oil was normal in appearance. The oil reservoir was full of oil that appeared clean. The fuel nozzle primary and secondary orifices did not appear 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. During the engine test, no anomalies or power perturbations were noted. The engine satisfactorily met all specified test conditions to include start time, governor droop, anti-ice, three decelerations/accelerations, and five power calibration points.

Fuel

The helicopter’s mounted fuel filter was full of fuel that contained 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 in appearance. Due to the damaged landing skids, fuel could not be drained from the main tank.

The FAA Aviation Fuels Research Laboratory analyzed samples from the helicopter, revealing contamination and degradation. The visual inspection of the unfiltered fuel showed a dark cloudy appearance with suspended particulates. After filtering, the fuel was still visibly dark, and heavier gum deposits were found on the filters. These gums, identified as the oxidation products of Jet-A fuel, were present in 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, which are indicative of fuel degradation. Some of the particulate matter appeared to be inorganic crystalline particles and degraded gel substances. Testing also showed a higher presence of oxygen, nitrogen, and chlorine in these fuel residues, supporting the evidence of advanced oxidation and contamination. Oxide particles were additionally found, which can 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.

According to the engine maintenance manual:

It is very important that the entire aircraft and engine fuel system be maintained to the highest standards of cleanliness.

Rolls-Royce has conducted testing of apple jelly type contamination which shows 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.

If this gel-like material is detected at any point in the aircraft or engine fuel system, the entire airframe and engine fuel system must be inspected.

Warning: To prevent engine fuel system contamination, which could cause engine flameout, an external low-pressure fuel filter must be used on any aircraft refueling from remote fueling sites (drums, etc.).