Accident Overview
On June 17, 2014, about 1330 Hawaii standard time, an Airbus AS 350 BA helicopter, registration N6094H, sustained substantial damage during a hard landing following a loss of engine power and an off-airport autorotation. The helicopter was registered to and operated by Sunshine Helicopters Inc., Puunene, Hawaii, as a visual flight rules (VFR) scenic tour flight under 14 Code of Federal Regulations (CFR) Part 135. Visual meteorological conditions prevailed. The pilot and the five passengers were not injured. The flight departed Kahului Airport (PHOG), Kahului, Hawaii, about 1300. Company flight following procedures were in effect.
Pilot Report
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on June 26, a representative for Sunshine Helicopters stated that the pilot reported he was 10 minutes out on his third flight of the day when, during cruise flight, the main rotor speed (RPM) started to decrease and the low rotor warning alarm sounded. He lowered the collective pitch control to increase rotor RPM, and the helicopter started to descend. He raised the collective pitch control, and the main rotor RPM started to decrease again; he entered an autorotation and landed the helicopter. The helicopter landed hard in tall grass, structurally damaging the fuselage and tailboom.
Engine Examination
After recovery, the helicopter's engine (Model LTS101-600A-3A, Serial # ADCC40239400315) was removed and shipped to the engine manufacturer's facility for examination under NTSB supervision. The engine had a total time since new of 8,917 hours, 547 hours since major overhaul, and 3 hours since the last 100-hour inspection. On August 7, 2014, an examination was conducted with representatives from the FAA, the operator, and the engine manufacturer in attendance.
The engine did not appear to have sustained physical damage during the autorotation and landing. The engine was installed in a test cell. Initially, it was hard to start but eventually started successfully. When advanced above flight idle, the engine would not achieve a speed above approximately 78% Ng, with fuel flow about 125-140 pph (required 300 pph). The engine appeared sluggish and slow to respond. During testing, vibration in the axial direction on the forward flange of the compressor housing exceeded the allowable limit of 0.7 in/sec, with a maximum of about 1.3 in/sec.
After shutdown, a leak test of the pneumatic controls was initiated. While removing the input line to the Pc filter housing, the fitting was found broken. This fitting sends compressor discharge air to the pneumatic system for engine control. The Pc filter assembly was replaced with a known good one, and the test was re-run successfully, achieving 300 pph fuel flow and appropriate response.
Fracture Analysis
On August 8, 2014, the fracture surface of the broken Pc filter housing inlet fitting was examined at Honeywell's materials analysis lab using a scanning electron microscope (SEM). Evidence of coarse fatigue striations was identified. The tube separation at one end of the Pc filter housing was a result of a high-strain fatigue fracture mode. The fatigue appeared to have initiated from multiple locations around the outer diameter of the tube. No defects or anomalies were observed on the fracture surface. The fatigue was observed around most of the fracture surface, and the final separation exhibited fracture features indicative of fatigue/overload mixed fracture modes. Damage was noted on the hexagonal end of the housing where tooling is used to tighten the fittings.
Manufacturer Actions
Following the examination, the manufacturer released a category 1 safety service bulletin (LT 101-73-10-0278) on December 8, 2015, affecting all LTS101 engines. The bulletin removed specific versions of the Pc filter with minimal wall thickness at the inlet fitting, making them more susceptible to overstress during removal and installation if not properly torqued or supported. It provided instructions for Pc filter fitting crack inspections, torqueing instructions requiring a backup wrench on attachment fittings while torqueing the B nut, and a pneumatic leak check. New Pc filter assemblies with increased wall thickness at the fitting attachment point were made available to provide a higher margin of safety.