NTSB Factual Report

On August 8, 2018, about 0920 Hawaii-Aleutian standard time, a Hughes 369D helicopter, N369MH, experienced a significant inflight vibration event, which resulted in the pilot executing an emergency landing to a school field in Honolulu, Hawaii. The commercial pilot and his 3 passengers were not injured. The helicopter was substantially damaged. The helicopter was registered to Schuman Carriage Company Ltd, and operated as a Title 14 Code of Federal Regulations Part 136 air tour flight, doing business as Magnum Helicopters. Visual meteorological conditions prevailed for the flight. The flight originated from the operator's facility at Honolulu International Airport (HNL), Honolulu, Hawaii about 0909.

The helicopter was the lead aircraft in a two-helicopter flight that was proceeding east/southeast from HNL, approximately along the shoreline. According to the pilot, the helicopter was in cruise at an altitude of about 1,800 ft. when he felt "severe" vibrations and then heard a "loud bang," after which the helicopter began to shake "violently." The pilot lowered his collective control and entered a power-on autorotation, with the intent of landing the helicopter in a grassy clearing in a residential neighborhood. He radioed his colleague in the trailing helicopter regarding his intentions, and then advised his passengers of the same. The pilot reported that the vibrations caused the transponder to work free of its panel mount. He also stated that even small tail rotor pedal inputs significantly worsened the vibrations.

The pilot made a partial run-on landing onto the targeted clearing. He reported that on first contact, the helicopter bounced about a foot into the air, and that the remaining slide on the dry and rocky grass field was rougher than he expected. The helicopter came to a stop upright, and the pilot shut down the engine. After the rotor blades stopped their rotation, the pilot had the passengers exit the helicopter. The landing field was part of a public school grounds, and the pilot released the passengers to the care of the school staff, while he examined the helicopter, and coordinated with his company.

The landing site was situated about 13 miles east of HNL, and the event occurred about 11 minutes after the helicopter departed HNL. Both the departure location and the landing site were situated at an elevation of approximately sea level.

Contrary to applicable regulations, the operator recovered the helicopter back to its facility, and began disassembly for repair without NTSB or Federal Aviation Administration (FAA) knowledge or approval. More than a day after the event, the NTSB became aware of the operator's maintenance activity, and instructed the operator to cease that activity, because evaluation indicated that the helicopter had been substantially damaged, and that the helicopter was therefore now the subject of an NTSB accident investigation.

Photographs and descriptions provided to the NTSB indicated that multiple tail rotor blade and gearbox components had failed during in-flight operation. The helicopter and removed components were preserved for subsequent detailed examination by personnel from the NTSB and other agencies.

The pilot held commercial and flight instructor certificates with helicopter ratings. He reported that he had about 7,300 hours total flight experience, including about 2,400 hours in the accident helicopter make and model. His most recent FAA second-class medical certificate was issued in January 2018, and his most recent flight review was completed in March, 2018.

FAA records indicated that the helicopter was manufactured in 1978, and was equipped with a Rolls-Royce 250-C20B series turboshaft engine. The helicopter was registered to the operator in September 2012. Prior to that, the helicopter had been registered in Panama, Florida, Texas, and California. The operator reported that the helicopter had a total time (TT) in service of about 14,328 hours, and that the engine had a TT of about 17,176 hours. The helicopter's most recent 100-hour inspection was completed in January 2018.