N420AT

On September 17, 2021, about 1030 eastern daylight time, a Hughes 369HS helicopter, N420AT, was substantially damaged when it was involved in an accident in Ash, North Carolina. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

According to the pilot, he flew from his home base to the remote job site and landed on a flatbed support truck to load the chemical hopper. The helicopter had about 25 gallons of fuel on board when it arrived at the truck. After landing, the pilot reduced the throttle to flight idle while the ground crew loaded 60 gallons of chemical into the hopper. Before liftoff, the pilot increased the throttle until the engine and rotor rpm needles were in the green range of the tachometer. Immediately after liftoff, the pilot heard the low-rotor rpm warning horn, and the helicopter began to descend. He attempted to increase the collective pitch, but the helicopter did not respond. The helicopter descended, impacted a field, and rolled over onto its right side.

Postaccident examination of the helicopter revealed that the helicopter sustained substantial damage to the tailboom, which was fractured just forward of the horizontal stabilizer. The flight controls were continuous from the cockpit controls to the main and tail rotors. All main rotor blades were fragmented and had fractured near their roots.

Examination and operational testing of the engine revealed that it would start and run but would not achieve 100% N2 (power turbine) speed with no load applied, and the engine temperature exceeded its limit (about 1,460°F). These conditions precluded further operational testing of the engine. A subsequent borescope examination of the first-stage gas-producer turbine blades revealed several blades with impact damage to the outer one-third of their leading edges.

One of the split-line bolts in the compressor case halves was found corroded and could not be removed with standard tooling. The bolt had to be drilled out to separate the compressor case halves and remove of one of the halves. The removed compressor case half exhibited significant damage to the third- and fourth-stage stator vanes. All the third-stage vanes had separated from the case at their base and were missing. All the fourth-stage vanes were in place, but most were significantly damaged; the vanes were found 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. The case half is shown in figure 1.

Figure 1. Compressor case damage (Source: Rolls-Royce Engines).

Heavy impact damage was noted on the compressor rotor blades from the trailing edge of the third stage to the outer one-third of the sixth stage. The rotor is shown in figure 2.

Figure 2. Compressor rotor (Source: Rolls-Royce Engines).

When the second compressor case half was removed, several of its split-line bolts were found rusted, which required grinding to remove them.

The damage to the remnants of the fractured stator vanes precluded a metallurgical examination of the fracture features by the engine manufacturer. Metallurgical examination found that the compressor case had erosion throughout the case. The polymer coating was eroded, exposing areas of the first- and second-stage compressor vane bands. The first- through third-stage compressor vanes exhibited erosion of the leading edge, braze joint fillet, and mid-chord thickness of the airfoils. Corrosion damage was present on several of the vanes. The leading-edge root of the third-stage compressor blades exhibited corrosion pitting.

A commercial service letter (CSL-1135), which was originally issued by the engine manufacturer in 1986, advised operators of the risk of corrosion and/or erosion of engine components when operating the engine in a corrosive/and or erosive environment. The letter instructed operators to perform a water-rinse procedure of the engine each day if the engine were operated in these environments. The letter states in part the following:

Engines subjected to salt water contamination or other chemically laden atmosphere (industrial pollutants, sulfur laden atmosphere, pesticides, herbicides, etc.) shall undergo water rinsing after shutdown following the last flight of the day. Perform the rinse operation as soon as practical after flight, but not before the engine has cooled to near ambient temperature.

A review of the available maintenance records revealed no entries related to the service letter or daily water rinsing of the engine components. The manufacturer’s recommended maximum interval for inspection of the engine’s compressor section (for engines with coated compressor wheels, which were installed in the accident engine) was 300 hours or 12 months, whichever came first. A note in the engine maintenance manual cautioned that the, “INSPECTION FREQUENCY MUST BE BASED ON THE NATURE OF THE EROSIVE AND/OR CORROSIVE ENVIRONMENT. THE OPERATING ENVIRONMENT CAN DICTATE A MORE FREQUENT INSPECTION INTERVAL [emphasis in original].”

The helicopter’s maintenance records indicated that the compressor was installed on July 17, 2017, about 327 flight hours before the accident. The only inspection of the compressor case after installation that was recorded in the records occurred on July 23, 2018; the engine had accumulated 58 flight hours since the time of the compressor installation. In this model helicopter, inspection of the compressor section necessitates removal of the engine from the airframe.