Accident Overview
On March 17, 2022, at about 1430 central daylight time, a Hughes (MD Helicopters) 369D helicopter, registration N500MZ, was substantially damaged during an accident near Adair, Iowa. The commercial pilot sustained minor injury. The helicopter was operating under Title 14 Code of Federal Regulations Part 133 as an external load flight.
Preflight and Departure
The pilot performed a fuel sample check and preflight checks with no anomalies noted. The pilot was "hands on" during the preflight and checked components that had recent maintenance. The first engine start was aborted due to high turbine outlet temperature (TOT). The mechanic adjusted acceleration settings, and the second start was within TOT limitations. The pilot then departed for the landing zone.
Loss of Engine Power
During longline operations while flying to a structure, the helicopter experienced a partial loss of engine power. At about 90 feet above ground level and 30 knots airspeed, the pilot lowered the collective to attempt an autorotation. The pilot could not point the helicopter into the wind because a powerline was in that direction, resulting in a crosswind of about 20 knots from the north (left side). Insufficient altitude and airspeed prevented increasing main rotor rotation. The helicopter impacted a muddy grass hill and came to rest on its right side. After coming to rest, the pilot noticed the engine was still running at a slow speed below idle. He performed an emergency shutdown and egressed without further incident.
Damage and Postaccident Findings
The helicopter sustained substantial damage to the fuselage, main rotor system, and tail rotor system. During recovery, the operator observed that the compressor discharge pressure (Pc) line B-nut (tube coupling nut) at the fuel control unit (FCU) was not attached and was loose around the Pc line, appearing to have backed off the FCU fitting.
Maintenance History
On March 5, 2022, the current turboshaft engine was installed per the Rolls-Royce M250-C20 Series Operation and Maintenance Manual. The engine arrived with accessories already installed except for the starter generator. The helicopter had several abnormal starts after installation, and maintenance team continued troubleshooting. On March 16, 2022, the FCU from the previously removed engine was installed on the current engine for troubleshooting. A calibrated torque wrench tightened the various FCU B-nuts. The mechanic tightened and torqued each B-nut one at a time. Another mechanic inspected the work and found no issues. After torquing, pink torque stripe paint was applied. Following FCU installation, the fuel system was purged of air, and a leak check was performed with the helicopter running, with no issues noted.
Postaccident Examination
Postaccident examination of the airframe found no anomalies. The engine was removed and a test run performed with no anomalies after the Pc line B-nut was properly torqued. During torquing, a “squeeze out” of pink paint, consistent with torque stripe paint, was observed from remaining threads on the line. Additionally, previous torque stripes of green and orange paint were observed on the B-nut and tube threads. Maintenance records showed no uncorrected mechanical issues.
Mechanic's Statement
In a postaccident interview, the mechanic stated that the Pc line B-nut found loose is the hardest to work on in that area of the engine compartment. He believed that while torquing that B-nut, the torque wrench contacted the FCU, likely providing a false torque reading.
Operational Considerations
Review of the height velocity diagram from the MD Helicopters 500D rotorcraft flight manual showed that at the time of the accident, the helicopter was operating in a cross-hatched region. The manual states that flight in cross-hatched regions represents airspeed/altitude combinations from which a successful autorotation landing may be difficult. Operation within cross-hatched areas should be undertaken with caution. The helicopter was within weight limitations for the flight and height velocity diagram. Estimated density altitude at the accident site was 542 feet above mean sea level.
Related Warnings
The flight manual preflight inspection includes checking engine oil, air, and fuel lines, as well as fuel control and associated linkages. Rolls-Royce Customer Service Letter CSL-A-1166 discusses failures of tube assemblies due to various causes, including improper torque of coupling nuts. The Rolls-Royce maintenance manual warns that failure to properly install, align, and tighten fittings could result in engine failure. The accident helicopter had the required engine compartment placard with the same warning.
Additional References
The FAA Safety Team (FAASTeam) has published Notice 4301, which discusses B-nut maintenance, emphasizing the need for proper torque rather than just tightening, and the risk of distractions leading to forgotten torque application.