Accident Overview
On October 27, 2014, at 1604 mountain standard time, a Eurocopter AS350B2, registration N5204X, sustained a sudden loss of engine power while maneuvering near Bisbee, Arizona. The helicopter was operated by the Department of Homeland Security Customs and Border Protection as a public aircraft flight. The sole occupant, a certified flight instructor, was seriously injured, and the helicopter sustained substantial damage.
The flight departed from Sierra Vista Municipal Airport-Libby Army Airfield, Fort Huachuca, Arizona, around 1530 for a local area border patrol mission. Visual meteorological conditions prevailed, and no flight plan was filed.
According to the pilot's written statement, he was maneuvering the helicopter up a small valley to support Border Patrol agents on the ground. While completing a second pass at about 25 feet above ground level, he audibly detected the engine shutting down. He immediately selected a suitable landing site and initiated an autorotation. The helicopter touched down hard, and the tail impacted the ground, separating from the airframe. The helicopter came to rest inverted in a shallow canyon about 8 miles southeast of Bisbee.
Aircraft and Engine Examination
The helicopter was manufactured in 1998 and equipped with a Turbomeca Arriel 1D1 engine (serial number 9580). The operator reported that the most recent inspection was a 100-hour inspection completed on October 7, 2014, at which time the airframe had 5,781 hours and the engine had about 8,290 hours.
Under the auspices of a Federal Aviation Administration inspector, representatives from Airbus Helicopters and Turbomeca examined the airframe and engine at the accident site and later at a facility in Tucson, Arizona. Examination of the engine revealed that the B-nut fitting of the pneumatic control pipe (P2) from the engine to the fuel control unit (FCU) was disconnected at the compressor fitting. The union nut on that side showed no evidence of a torque stripe, whereas the FCU side had a torque stripe. Visual inspection of the B-nut, pipe, and union fastener found no anomaly that would preclude proper connection or torque.
The P2 pipe delivers P2 air pressure from the centrifugal compressor discharge to the FCU. Within the FCU, this pressure regulates the acceleration capsule, which adjusts the fuel metering needle via a lever mechanism. A failure of the P2 pipe allows ambient air pressure to enter, causing the FCU to command the engine to spool down to ground idle speed. Magnification of the P2 pipe revealed no cracks or malfunctions; threads and union fastener appeared normal.
Maintenance Observations
The last recorded removal of the FCU occurred in April 2014, about 300 flight hours before the accident, at which time an overhauled unit was installed, necessitating adjustment of the P2 pipe and fittings. Review of maintenance logbooks showed that three days before the accident (about six flight hours prior), a 25-hour engine wash was performed per Turbomeca Task #71-01-02-110-801-A01. The task manual contains a caution not to remove the FCU P2 air tapping pipe, as the engine wash procedure does not lead to pollution or water ingestion in the FCU P2 chamber. The mechanics who performed the wash were interviewed and correctly recited the procedure. However, according to the U.S. Customs and Border Protection Safety Officer, hangar surveillance video of the last engine wash showed no maintenance manual documentation in use by maintenance personnel.
Turbomeca specifies in Table 1 of the same task that a 6mm diameter pipe requires a tightening torque of 115.06 to 132.76 inch-pounds, with a note that insufficient torque can cause the union to work loose and excessive torque can risk leakage or failure. A painted torque stripe must be applied after torquing the union nut. Additionally, Turbomeca Task #75-29-00-900-802-B01 cautions that insufficient inspection or non-compliant assembly of the FCU P2 pipe may cause cracks or breaks and lead to engine power loss.
Turbomeca Service Letter No. 1807/98/ARRIEL1/40, released October 16, 2003, describes examples of incorrect pneumatic system pipe maintenance, including improper torque of air system unions, and their variable consequences. The service letter also references Service Bulletins recommending upgraded pipe wall thickness (0.8mm) and reinforced P2 pipe installation. The accident helicopter was equipped with the thicker pipe.