Hughes (Md Helicopters) 369 N500MZ accident near Adair, Iowa

Probable cause

The mechanic’s failure to properly torque a fuel control unit B-nut, which resulted in a partial loss of engine power, an attempted autorotation, and an impact with terrain.

NTSB analysis

During longline operations, and while flying to a structure, the helicopter sustained a partial loss of engine power. The helicopter was at 90ft above ground level and about 30 kts airspeed, when the pilot lowered the collective to attempt an autorotation. There was insufficient altitude and airspeed to increase main rotor rotation, and the helicopter impacted a muddy grass hill. The helicopter came to rest on its right side. After the helicopter came to rest, the pilot noticed the engine was still, “at a slow run, less than idle.”

The helicopter sustained substantial damage to the fuselage, the main rotor system, and the tail rotor system. During the recovery of the helicopter from the field, it was noticed by the operator that the compressor discharge pressure (Pc) line B-nut connection at the fuel control unit (FCU) was not attached. It appeared the B-nut had backed off the FCU fitting.

A postaccident examination was performed on the airframe with no anomalies noted. The engine was removed from the airframe and an engine test run was performed with no anomalies noted after the Pc line B-nut was properly torqued. During the Pc line Bnut torquing process, a “squeeze out” of pink paint, consistent with torque stripe paint, was observed emanating from the remaining threads on the line.

On the day before the accident, the fuel control unit (FCU) was installed on the helicopter’s turboshaft engine. A calibrated torque wrench was used by the mechanic to tighten the various FCU B-nuts. The mechanic tightened and torqued each B-nut, one at a time, when completing the task. Another mechanic inspected the work after it was completed and found no issues. After the B-nuts were torqued, pink torque stripe paint was applied. Following the FCU installation, the fuel system was purged of air, and a leak check was performed with the helicopter running, an acceptable practice per the engine manufacturer, with no issues noted.

It is likely that the mechanic failed to properly torque the B-nut that secures the Pc line to the FCU, which resulted in a partial loss of engine power due to the Pc line separation. Based on the available evidence, it was undetermined as to why the mechanic failed to properly torque the B-nut.

Text from the NTSB. Open the NTSB docket for CEN22LA146 for the full record.