Robinson R44 II N4046J accident near Tampa, Florida

Probable cause

Contamination of and an inadequate bond between the two layers of fabric comprising the helicopter engine's air induction inlet duct, which resulted a partial collapse of the duct, obstruction of the airflow into the engine, and a total loss of engine power.

NTSB analysis

About 5 days before the accident, the helicopter's engine lost power and the owner performed an off-airport landing to a field. The helicopter was not damaged during the landing, and several days later, maintenance personnel examined the helicopter while still in the field. The mechanics examined the helicopter's fuel system, replaced a fuel servo, and after a successful test run of the engine, the commercial pilot departed with one of the mechanics on board for the accident flight in order to reposition the helicopter from the field to a maintenance facility. About 15 minutes into the flight, the engine lost power, and the pilot performed an autorotation to a roadway. After touchdown, the helicopter slid on the roadway, the main rotor contacted a telephone pole, and a 2.5-ft piece of a main rotor blade separated and impacted a truck driving on the roadway, fatally injuring the passenger and seriously injuring the driver of the truck.

Postaccident examination of the helicopter's engine revealed that the air induction 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. The wire stiffener between the two layers of fabric was displaced in two locations near the center of the duct length at a 90° bend. Further examination of the duct found a liquid residue in some areas of the duct that was consistent with castor oil, which was normally used in the assembly and curing of the duct during its manufacture. Given this information, it is likely that during the duct's manufacture, castor oil was inadvertently introduced between the duct's two fabric layers, interfering with the bond between the them. It is also likely that the duct then partially collapsed, which restricted airflow to the helicopter's engine and resulted in the losses of engine power that occurred both before and during the accident flight.

Text from the NTSB. Open the NTSB docket for ERA19FA142 for the full record.