Accident Overview
On April 6, 2020, at about 1410 Pacific daylight time, a Robinson R44 Raven II helicopter, registration N725GT, was substantially damaged during a forced landing near Portland, Oregon. The pilot, who was the sole occupant, was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91.
Events During Takeoff
The pilot reported that during takeoff, at an altitude of approximately 30 to 40 feet above ground level, a loud "bang" was heard, followed immediately by illumination of the low rotor RPM light and activation of the low rotor horn. The pilot responded by lowering the collective and performing an autorotation onto a paved taxiway. The hard impact caused the skids to spread and resulted in substantial damage to the tailboom.
Engine and Duct Examination
Initial examination of the engine revealed that the fuel servo induction air inlet duct was partially collapsed. The inner rubberized fabric liner had partially delaminated and separated from the outer rubberized fabric, obstructing the interior volume of the duct. The duct was retained for further analysis. The outer layer appeared normal with no damage, while the inner layer exhibited multiple bubble-like areas measuring 0.5 to 0.75 inches wide and varying in length. When the duct was flexed 90 degrees, a distinct crinkle sound was heard.
Operational Test
An exemplar R44 helicopter was fitted with the removed duct, which was bent 90 degrees and secured to the airbox. The engine was started and power increased to 102 percent, with collective raised to 20 inches of manifold air pressure. The engine ran smoothly for 74 seconds, then manifold air pressure fluctuated slightly and increased as the engine stalled. A cough sound, described as similar to an after fire, was heard from the cockpit. Subsequent examination with the hose partially unclamped revealed that the inner layer's bubbles had significantly reduced the duct's inside diameter.
Materials Laboratory Findings
The air intake duct tube was sent to the NTSB Materials Laboratory for further examination. Dissection revealed that the inner and outer layers had few, if any, areas of co-curing. The outer layer appeared bonded primarily to the helical steel wire. In some areas, the silicone rubber separated cohesively, leaving remnants on the wire; in others, separation occurred at the rubber-wire interface, indicating weak or no bond. The internal surfaces were smooth, suggesting lack of co-curing between layers. Two unidentified substances were observed: discrete gray particulates (identified by SEM/EDS as iron and silicate minerals) and a friable translucent substance (similar to a water-based mold release agent used by the manufacturer, with spectral matches for silicone-based materials).
Related Service Bulletins and ADs
A similar accident occurred on April 4, 2019 (NTSB accident ERA19FA142). Following that event, Robinson Helicopter Company issued Service Bulletin SB-97 on April 11, 2019, requiring inspection of inlet duct tubes on R44 II helicopters with serial numbers 14248 through 14286 (except 14269). The FAA made compliance mandatory via AD-2019-12-18 on July 5, 2019. Subsequently, Robinson issued SB-100 on July 3, 2019, expanding the inspection to essentially all R44 II helicopters up to serial number 14314, with compliance required within 100 flight hours or by August 31, 2019. The accident helicopter (serial number 14213) was not subject to SB-97 or AD-2019-12-18 but was covered by SB-100. Examination of maintenance records found no mention of the SB-100 inspection.
Post-Accident Actions
Following this accident, Robinson Helicopter Company issued a Safety Alert on June 17, 2020, urging operators to ensure compliance with SB-100. Additionally, the FAA issued AD-2020-18-08 (effective August 27, 2020), which superseded AD-2019-12-18 and expanded the serial number range of R44 II helicopters and subject inlet duct tubes requiring inspection.