What happened
On March 17, 2022, at approximately 14:30 central daylight time, a Hughes (MD Helicopters) 369D helicopter, registration N500MZ, was performing external load operations near Adair, Iowa. While flying toward a structure during longline operations, the aircraft experienced a partial loss of engine power.
At the time of the power loss, the helicopter was flying at approximately 90 feet above ground level with an airspeed of about 30 knots. The pilot attempted an autorotation by lowering the collective; however, there was insufficient altitude and airspeed to increase main rotor rotation. Due to a powerline located in the direction of the wind, the pilot was unable to point the aircraft into the wind, resulting in a 20-knot crosswind from the left. The helicopter impacted a muddy grass hill and came to rest on its right side. The pilot, who was the only occupant, sustained one minor injury and was able to exit the aircraft without further incident.
The investigation
The investigation revealed that the helicopter sustained substantial damage to the fuselage, the main rotor system, and the tail rotor system. During the recovery of the aircraft from the field, the operator discovered that the compressor discharge pressure (Pc) line B-nut connection at the fuel control unit (FCU) was not attached and had loosened from the fitting.
Maintenance records showed that the engine's FCU had been installed on the engine the previous day. A mechanic used a calibrated torque wrench to tighten the B-nuts and applied pink torque stripe paint to the completed work. An inspection by a second mechanic had found no issues.
Post-accident examination of the engine showed that after the Pc line B-nut was properly torqued, the engine performed normally during a test run. Investigators observed pink paint "squeeze out" on the remaining threads of the line, consistent with the torque stripe paint used during the recent maintenance. The mechanic stated that the specific B-nut is difficult to access and suggested that the torque wrench may have contacted the FCU during the process, potentially providing a false torque reading.