History of Flight
On June 2, 2009, about 1110 eastern daylight time, a Hughes 369D (registration N8356F) operated by Aerial Solutions, Inc., executed a hard landing during an autorotative landing to a clearing near Greenville, Virginia, following a loss of engine power. The flight was conducted under 14 Code of Federal Regulations Part 133 for external load operations, specifically aerial tree trimming near power lines. Visual meteorological conditions prevailed, and no flight plan was filed. The helicopter had departed Greenville approximately 55 minutes prior. The pilot, the sole occupant, reported that after a fuel check and a power check with all instruments in the green, he heard a loud sound from the engine and the helicopter yawed left. The chief pilot estimated the helicopter was between 90 and 100 feet above ground level when the engine failure occurred. The pilot reduced collective, attempted to release the saw, and nosed the helicopter over to increase airspeed for a flare, but the helicopter impacted the ground. The pilot exited after releasing his seatbelt and shoulder harness.
Damage to Aircraft
The helicopter sustained substantial damage, including a deformed tailboom, a crease in the left side of the tailboom, separation of the left skid, compression wrinkles above and aft of the aft right skid tube, downward deformation of the pilot's seat, and extensive structural damage near the pilot's seat. Two main rotor blades exhibited bending damage; one main rotor blade was fractured near the blade root. One tail rotor blade was bent.
Personnel Information
The pilot, aged 41, held a commercial pilot certificate with a rotorcraft helicopter rating and a second-class medical certificate issued on January 20, 2009. His total flight time was 8,676 hours, with 7,780 hours in the accident make and model. He reported 105, 32, and 7 hours in the last 90 days, 30 days, and 24 hours, respectively. His last flight review was conducted on April 28, 2009, in the accident helicopter.
Aircraft Information
The helicopter, a 1976 Hughes 369D (serial number 1260063D), was powered by a Rolls-Royce Corporation 250-C20B engine rated at 420 shaft horsepower (375 usable). The engine (serial number CAE 823103) was installed on July 16, 2008, at a helicopter total time of approximately 9,597 hours. At the time of the accident, the helicopter total time was about 10,518 hours. The fourth stage power turbine wheel (part number 23055944, serial number X555507) was an Enhanced design installed during an overhaul in October 2006. Following a 1,750-hour inspection in May 2008, the wheel remained installed. The engine had accumulated about 2,690 hours since the new wheel was installed and about 921 hours since the last 1,750-hour inspection. A 100-hour inspection was completed on May 12, 2009.
Meteorological Information
A surface observation at Shenandoah Valley Regional Airport (SHD) at 1120, about 10 minutes before the accident, reported wind from 190 degrees at 7 knots, visibility 10 miles, clear skies, temperature 29°C, dew point 21°C, and an altimeter setting of 30.08 inches of mercury.
Wreckage and Impact Information
Post-recovery engine inspection revealed that neither the N1 nor N2 drive trains could be manually rotated. No visible damage was noted to the compressor module. Both exhaust stacks had small areas of peening on the inside. The exhaust collector was breached in the path of the No. 4 power turbine wheel, with an opening extending from the 2 o'clock to the 6 o'clock positions. The outer rim and knife seals of the No. 4 wheel exhibited separation of a section spanning approximately three airfoils; the middle airfoil was fractured near the hub. The engine was disassembled at the manufacturer's facility, confirming that one airfoil of the fourth stage turbine wheel was separated near the hub. Detailed examination by the NTSB Materials Laboratory revealed that the fracture of the airfoil near the hub root fillet was consistent with fatigue initiation at the trailing edge near the pressure side. The fatigue crack propagated about 0.50 inch from the initiation region, with no surface defects or fabrication anomalies noted. The chemical composition and hardness were within specifications. A concentration of subsurface carbides along the fillet radius was typical of cast precipitation-hardenable nickel-based alloy. Other turbine components exhibited secondary damage from the airfoil fracture.
Tests and Research
Bench testing of the power turbine governor and fuel control unit (both the installed unit and one removed the day before) revealed no discrepancies that would have caused an engine malfunction. Rolls-Royce Commercial Engine Bulletin CEB-A-1400 (Revision 3, January 19, 2009) recommended avoiding steady-state engine operation in the N2 speed avoidance range of 75 to 88 percent for operation above 85 shaft horsepower. Company personnel were briefed on the bulletin, but no steady-state operation in that range was reported or recorded in maintenance records between January 2006 and June 2009. Rolls-Royce later issued Bulletin CEB-A-1407 (Revision 1, February 7, 2011), calling for a one-time inspection of the third and fourth stage turbine wheels for cracks. A letter from Rolls-Royce to the NTSB dated August 12, 2010, indicated that engineering analysis suggested a higher stress state at the trailing edge root due to thermal differentials during engine start, which could produce residual stress leading to a fatigue crack; such a crack could propagate in high-cycle fatigue followed by overload failure if the engine was operated steady-state in the speed avoidance range.
Similar Incidents
The Safety Board investigated a similar fatigue failure of an Enhanced fourth stage turbine wheel airfoil in a McDonnell Douglas 369E helicopter (N556CP) on October 29, 2007. Rolls-Royce reported three other fatigue failures of the same component since 2005, with four of the five total cases involving variants of the McDonnell Douglas 369 helicopter and one involving a Bell helicopter.