Accident Overview
A sheriff department helicopter was beginning an evening patrol flight when the engine experienced a deceleration event during the transition from climb out to cruise. The helicopter subsequently crashed into a residential street during an attempted autorotation. First responders, including sheriff's air unit mechanics, found the engine running at idle and a fire in the engine compartment.
Flight and Maintenance History
The helicopter had just come out of a scheduled 100/300-hour inspection, and this was the first mission flight since that maintenance. During the inspection, the engine's fuel control Hydromechanical Unit (HMU) had been removed for compliance with a service bulletin. Prior to the accident flight, the helicopter had completed a 10-minute post-maintenance flight check. The pilot in command (PIC) conducted the preflight inspection. No problems were noted during the preflight, and the takeoff was normal.
Crew Information
The mission observer flight officer, who held a private pilot certificate with helicopter rating and was attempting to upgrade to a pilot position, had been given permission to fly the helicopter and installed the dual flight controls to the right side. About 500 feet above ground level during the transition from climb out to cruise, the pilot flying heard the LOW ROTOR voice warning (two times) followed by ENGINE OUT voice warning (two times). Without initiating an autorotation, he requested that the PIC take the flight controls. Simultaneously, the PIC had sensed a problem and took the flight controls. Prior to and during the departure up until the engine deceleration, the PIC performed the observer flight officer duties, which included radio communications with dispatch, and had not monitored the flight instrument readings or the progress of the departure.
Technical Findings
The LOW ROTOR voice warning activates when Nr falls below 95 percent. The voice warning system for ENGINE OUT activates when N1 falls below 55 percent or a high rate of decay in N1. No discrepancies were noted during the inspection of the airframe. A teardown of the engine disclosed no internal discrepancies; however, the fuel inlet line fitting to the engine HMU was found loose by two flats of the nut. Functional testing of the fire-damaged electronic control unit (ECU) found no discrepancies. The HMU was installed in a test bench and passed a functional check. The fuel line inlet fitting nut was then loosened incrementally one flat at a time with a functional test conducted each time. Significant fluctuations in metered fuel output flow were noted during one test with the nut three flats loose, and again at one complete turn loose. These results could not be reliably duplicated in subsequent tests.
Organizational Findings
The investigation found that the operator had not established guidance for crew resource management pertaining to crew responsibilities, instrument monitoring responsibilities, emergency procedures initiation, or flight control transfer procedures when flying in a dual pilot operation.