Robinson R44 accident near Hillsboro, Oregon, April 6, 2020

The NTSB's narrative final · quoted from the NTSB record

During takeoff, about 30 to 40 ft above ground level, the pilot heard a loud "bang," followed by the illumination of the low rotor RPM light and the sound of the low rotor horn. In response, he lowered the collective and performed an autorotation on the paved taxiway. As a result of the hard impact, the skids spread, and the tailboom received substantial damage. A postaccident examination of the engine revealed that the fuel servo induction air inlet duct was partially collapsed. The inner rubberized fabric liner of the duct had partially delaminated and separated from the outer rubberized fabric, obstructing the interior volume of the duct. Further examination of the duct revealed two different contaminants that prevented co-curing of the inner and outer layers. One contaminant was similar to a water-based mold release agent used by the manufacturer at the manufacturing plant. The other contaminant consisted of low-carbon steel, likely from the mandrel used as a form for assembling the duct Because there is no mechanical support structure for the inner layer inside the tube, the inner layer relies on the co-curing with the outer layer and the adhesive bond with the helical wire to resist the force it encounters when the duct is placed under negative pressure. Because the inner layer was not bonded to the outer layer, the inner layer could not withstand the negative pressure and the inner layer was pulled into the internal volume of the duct, causing it to collapse, starving the engine of air, and resulting in the loss of engine power. About a year prior to this accident, the helicopter manufacturer issued Service Bulletin (SB)-97 after a similar accident occurred. SB-97 required all inlet duct tubes installed on R44 II helicopters with serial numbers 14248 through 14286 (except 14269) to be inspected for indications of layer separation. Three months later, the Federal Aviation Administration (FAA) issued Airworthiness Directive (AD)-2019-12-18, mandated compliance with SB-97. Because SB-97 did not cover all inlet duct tubes in the R44 II fleet, as a follow up, the helicopter manufacturer issued SB-100, which expanded the inspection to essentially all R44 II helicopters up to aircraft serial number 14314. The inspection was to be completed in the next 100 flight hours or by August 31, 2019, whichever came first. At the time of the accident, neither SB-97 nor AD-2019-12-18 were applicable for the accident helicopter (serial number 14213). Conversely, SB-100 was applicable to the accident helicopter; however, there was no indication in the maintenance records for the helicopter indicating SB-100 had been performed. It could not be determined whether the aircraft owner did not comply with SB-100, or the visual examination and crinkle test were not adequate to detect the layer separation. As a result of this accident, the FAA issued AD-2020-18-08, which superseded AD-2019-12-18 and expanded the serial number range of R44 II helicopters and subject inlet duct tubes requiring inspection.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.