Accident Details
On September 17, 2021, about 1030 eastern daylight time, a Hughes 369HS helicopter, registration N420AT, was substantially damaged during an accident in Ash, North Carolina. The pilot was not injured. The helicopter was being operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.
Pilot Account
The pilot reported flying from his home base to a remote job site and landing on a flatbed support truck to load the chemical hopper. The helicopter had about 25 gallons of fuel on board upon arrival. After landing, the pilot reduced the throttle to flight idle while ground crew loaded 60 gallons of chemical into the hopper. Before liftoff, the pilot increased the throttle until engine and rotor rpm needles were in the green range on the tachometer. Immediately after liftoff, the pilot heard the low-rotor rpm warning horn, and the helicopter began to descend. He attempted to increase collective pitch, but the helicopter did not respond. The helicopter descended, impacted a field, and rolled onto its right side.
Postaccident Examination
Examination revealed substantial damage to the tailboom, which was fractured just forward of the horizontal stabilizer. Flight controls were continuous from cockpit to main and tail rotors. All main rotor blades were fragmented and fractured near their roots. The engine started and ran but would not achieve 100% N2 (power turbine) speed with no load applied, and engine temperature exceeded its limit (about 1,460°F), precluding further operational testing. A borescope examination of the first-stage gas-producer turbine blades showed several blades with impact damage to the outer one-third of their leading edges.
One split-line bolt in the compressor case halves was corroded and could not be removed with standard tooling; it had to be drilled out. The removed compressor case half exhibited significant damage to the third- and fourth-stage stator vanes: all third-stage vanes had separated from the case at their base and were missing; most fourth-stage vanes were in place but significantly damaged (fully rolled over, severely distorted, or twisted). Significant erosion of the black plastic case lining was noted at the base of the first- and second-stage stator vanes. Heavy impact damage was observed on compressor rotor blades from the trailing edge of the third stage to the outer one-third of the sixth stage. When the second compressor case half was removed, several split-line bolts were found rusted, requiring grinding for removal.
Metallurgical examination found erosion throughout the compressor case. The polymer coating was eroded, exposing areas of the first- and second-stage compressor vane bands. First- through third-stage compressor vanes exhibited erosion of the leading edge, braze joint fillet, and mid-chord thickness. Corrosion damage was present on several vanes. The leading-edge root of the third-stage compressor blades exhibited corrosion pitting.
A commercial service letter (CSL-1135), issued by the engine manufacturer in 1986, advised operators of corrosion/erosion risks when operating in corrosive/erosive environments and instructed a daily water-rinse procedure. The letter stated that engines subjected to salt water contamination or chemically laden atmospheres (industrial pollutants, pesticides, herbicides, etc.) should undergo water rinsing after shutdown following the last flight of the day, performed as soon as practical after flight but not before the engine cooled to near ambient temperature.
Maintenance Records
A review of available maintenance records found no entries related to the service letter or daily water rinsing. The manufacturer’s recommended maximum interval for inspection of the engine’s compressor section (for engines with coated compressor wheels, as installed in the accident engine) was 300 hours or 12 months, whichever came first. The engine maintenance manual noted that inspection frequency must be based on the nature of the erosive and/or corrosive environment, which could dictate a more frequent interval. The helicopter’s records indicated the compressor was installed on July 17, 2017, about 327 flight hours before the accident. Only one inspection of the compressor case was recorded, on July 23, 2018, after 58 flight hours since installation. Inspection of the compressor section in this model requires engine removal from the airframe.