NTSB Factual Report

On May 08, 2013, about 1520 Hawaiian standard time, a Robinson R22 Beta helicopter, N5ZK, was substantially damaged when it collided with an unoccupied parked automobile following a total loss of engine power while maneuvering over Honolulu, Hawaii. HLM Aviation owned the helicopter, and Hawaii Pacific Aviation (doing business as Mauna Loa Helicopters) operated the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certified flight instructor (CFI) was not injured, and the passenger sustained minor injuries. The local aerial observation flight departed from Honolulu International Airport about 1455. Visual meteorological conditions prevailed and no flight plan had been filed.

The CFI stated that the purpose of the flight was for her to orbit the helicopter over an area known as Punchbowl located about five nautical miles south of the airport. She departed as planned, proceeded to Punchbowl, and completed two orbits before slowing the helicopter to about 35 knots in an effort to help facilitate the passenger in taking his aerial photographs. While maneuvering about 3,000 feet above ground level (agl), the engine suddenly experienced a total loss of power, followed immediately by activation of the low rpm light and horn.

The CFI reported that in response to the engine failure, she lowered the collective and entered an autorotation. She maintained a 60-knot airspeed, and maneuvered to land on a street. The pilot began the flare at an altitude of about 40 feet agl. The helicopter touched down on the road, and skidded into a parked automobile.

A Federal Aviation Administration (FAA) inspector performed a post accident examination of the helicopter. He stated that the mixture cable was fracture-separated from the mixture bellcrank control arm on the carburetor. The bellcrank was positioned near the idle cutoff position, rather than the full-rich position. The spring did not drive the arm to the full rich position per design. The inspector noted that it appeared that the spring was not correctly connected to the mixture arm. The helicopter's maintenance manual stated that during inspection the mechanic must "Verify spring moves arm to full rich position."

The mechanic who routinely maintains the helicopter stated that his company completed a 2,200 hour airframe overhaul of the helicopter on April 22, 2013, equating to about 16 hours of flight time prior to the accident. The mechanic recalled that at the completion of the overhaul, he inspected the helicopter which commenced with the tracking and balancing of the main rotor system, tail rotor system and engine.

The mechanic further stated that he had accompanied the FAA inspector during the post accident examination. They determined that the mixture arm assembly was improperly rigged. The mixture to bellcrank hardware stackup was incorrect and did not allow the mixture cable to move freely and the spring was installed so that the arm was driven to the idle cutoff position. The mechanic opined that the combination of the repeated stress on the mixture cable resulted in it breaking after a number of operation cycles. When the failure did occur, the bellcrank spring, which is designed to force the mixture to the full rich position, instead cut off the fuel by pulling the mixture to idle cutoff. The mechanic remarked that the rigging of the carburetor was accomplished by an apprentice who was under his supervision. The mechanic did not visually observe the misrigging, and reported that if he had performed a tactile inspection, he would have detected the error.

NTSB Probable Cause

The mechanic’s failure to properly inspect the helicopter following maintenance performed by an apprentice, which resulted in a total loss of engine power due to the improper rigging of the mixture control assembly.