WPR15TA027

The commercial pilot was maneuvering the turbine engine-equipped helicopter about 25 ft above ground level when the engine suddenly experienced a total loss of power. The pilot performed an autorotation, and the helicopter touched down hard and rolled over, coming to rest inverted. Examination of the engine revealed that the pneumatic control pipe (P2), which delivers air pressure from the centrifugal compressor on the engine to the fuel control unit (FCU), was disconnected at the engine fitting. The loss of P2 pressure to the FCU resulted in the FCU commanding the engine to spool down to ground idle speed by limiting the fuel flow. There was no evidence of malfunctions or anomalies on the pipe, threads, and union fastener. There was no evidence of torque stripe residue on the disconnected fitting, consistent with the likely scenario that, during past maintenance, the B-nut was not properly torqued and gradually vibrated off its attach fitting while in flight. The engine manufacturer had several caution notifications within multiple maintenance task documents to prevent removal of the P2 pipe during washing and the risk of an engine failure due to insufficient torque.

The helicopter, manufactured in 1998, was equipped with a Turbomeca Arriel 1D1 engine (serial number 9580). The operator reported that the most recent inspection was a 100-hour inspection that was completed on October 7, 2014. At that time the airframe had accumulated a total of 5,781 hours and the engine accumulated about 8,290 hours. On October 27, 2014, at 1604 mountain standard time, a Eurocopter AS350B2, N5204X, experienced a sudden loss of engine power while maneuvering near Bisbee, Arizona. The Department of Homeland Security Customs and Border Protection (CBP) was operating the helicopter as a public aircraft flight. The certified flight instructor, the sole occupant, was seriously injured; the helicopter sustained substantial damage. The pilot departed from Sierra Vista Municipal Airport-Libby Army Airfield, Fort Huachuca, Arizona about 1530 for a local area border patrol flight. Visual meteorological conditions prevailed and no flight plan had been filed. In a written statement, the pilot reported that he was maneuvering the helicopter up a small valley in an effort to aid Border Patrol Agents on the ground. As he completed a second pass, with the helicopter maneuvering about 25 feet above ground level (agl), the pilot could audibly detect that the engine was shutting down. He immediately decided on the best suitable landing site and began an autorotation toward that location. 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. Under the auspice of a Federal Aviation Administration (FAA) inspector, representatives from Airbus Helicopters and Turbomeca performed an examination of the airframe and engine both at the accident location and then later at a facility in Tucson, Arizona. Engine Examination 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 contained no evidence of a torque stripe, whereas the union nut and fitting on the FCU side had a torque stripe. A visual inspection of the B-nut, pipe, and union fastener further revealed no evidence of an anomaly that would have precluded the ability of being connected or properly torqued. The P2 pipe is designed to deliver P2 air pressure from the discharge of the centrifugal compressor to the FCU. Within the FCU, P2 pressure regulates the acceleration capsule, which allows a lever mechanism to adjust the position of the fuel metering needle. If the P2 pipe fails, ambient air pressure will enter resulting in the FCU commanding the engine to spool down to ground idle speed. Magnification of the P2 pipe disclosed that there was no evidence of cracks or malfunctions. The threads and union fastener appeared normal. Proper alignment and installation was checked by attaching the P2 pipe and no anomalieswere noted. Maintenance Instructions The last recorded removal of the FCU occurred in April 2014, equating to about 300 flight hours prior to the accident, at which time an overhauled unit was installed, and the P2 pipe and fittings would have been adjusted. Review of the maintenance logbooks revealed that three days prior to the accident (equating to about six flight hours), a 25-hour engine wash, Turbomeca Task #71-01-02-110-801-A01, was recorded as having been accomplished. In the manual for conducting that work task there is a caution in part B. (2), which states: "CAUTION: DO NOT REMOVE THE F.C.U. P2 AIR TAPPING PIPE (OR ANY OTHER ENGINE P2 AIR TAPPING PIPE). IT HAS BEEN PROVED THAT THE ENGINE WASHING OPERATION, WHEN IT PERFORMED IN COMPLIANCE WITH THIS PROCEDURE, DOES NOT LEAD TO POLLUTION OR WATER INGESTION IN THE F.C.U. P2 CHAMBER." The mechanics that performed the engine wash were interviewed and both were aware of and correctly recited the correct procedure. According to a statement from the US Customs and Border Protection Safety Officer they reviewed hangar surveillance video during the last engine wash and noted that no maintenance manual documentation being used by maintenance personnel. According to Turbomeca in the same work task in Table 1, there is a directive that a 6mm diameter pipe would require a tightening torque of 115.06 to 132.76 inch-pounds. There is also a note that: "CAUTION: AN INSUFFICIENT TIGHTENING TORQUE CAN CAUSE THE UNION TO WORK LOOSE DURING OPERATION. AN EXCESSIVE TIGHTENING TORQUE CAN GENERATE A RISK OF LEAKAGE OR FAILURE OF THE UNION." It also stated that a painted torque stripe is required to be applied following the application of torque to the union nut. In addition, there is a caution in the Turbomeca Task #75-29-00-900-802-B01 part D. stating that: "IF THE INSPECTION BEFORE ASSEMBLY IS NOT SUFFICIENT OR IF THE ASSEMBLY OF THE F.C.U. P2 PIPE IS NOT COMPLIANT (INCORRECT TIGHTENING, STRESSING, DISTORTION, SHOCKS, ETC.), THIS MAY CAUSE CRACKS OR BREAKS AND THUS LEAD TO A POWER LOSS OF THE ENGINE." Turbomeca released Service Letter No. 1807/98/ARRIEL1/40, on October 16, 2003 which described examples of incorrect pneumatic system pipe maintenance, such as improper torque of air system unions, and the variable consequences. The service letter also addressed two Service Bulletins which recommended that the pipe wall thickness be upgraded to 0.8mm and details on installation for the reinforced P2 pipe. The accident helicopter did have the thicker pipe.