Accident Overview
On August 18, 2015, at approximately 1330 central daylight time, a Bell 206B helicopter (registration N5743W) impacted terrain during an autorotation following a loss of engine power while maneuvering near Cresco, Iowa. The commercial pilot, the sole occupant, was not injured. The helicopter sustained substantial damage. The helicopter was registered to and operated by Fly Hangar 13, LLC, of Mico, Texas, as a 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed at the time, and no flight plan was filed. The local flight had departed from an off-airport location about 1320.
Pilot's Account
According to the pilot, the flight departed with 30 gallons of fuel and approximately 90 gallons of chemical for spraying a soybean field near Cresco. About five minutes into the application, while flying 3 to 5 feet above the soybeans, the pilot heard a loud screech lasting about two seconds and pulled up to gain altitude. The low rotor RPM horn then activated, and the main rotor RPM indicated 85 percent. The pilot initiated an autorotation to a waterway between two cornfields. Upon touchdown on uneven terrain, the main rotor blades contacted and severed the tailboom, and the helicopter came to rest upright. The pilot shut down and exited the helicopter. The helicopter was recovered for further examination.
Engine Examination
The helicopter was equipped with a Rolls-Royce M250-C20J turbo-shaft engine (serial number CAE-832989). Examination of the engine revealed that the exhaust collector support was fractured 360 degrees forward of the turbine mating flange, and the turbine section had shifted about 2 inches. The 3rd stage turbine wheel appeared to be missing four to five blades. No preimpact mechanical anomalies were noted with the airframe.
The engine was subsequently disassembled at Rolls-Royce Corporation under NTSB supervision. The 3rd stage turbine wheel (part number 23065818) was found to be missing five airfoils (blades), with several other airfoils damaged and portions of the shroud liberated. Rolls-Royce's Materials Laboratory examination determined that five airfoils on the third stage turbine wheel had cracked in fatigue, initiating from the trailing edge root area and progressing forward until final overload fracture. The cracks propagated in high cycle fatigue. Two additional cracks consistent with thermal fatigue were found on the trailing edge side of the wheel rim. Damage to other components was considered secondary to the liberation of the five airfoils.
Maintenance and Service History
The engine had a time since new (TSN) of 11,797 hours and 1,052 hours since major overhaul. The turbine assembly (serial number CAT-36021) had been overhauled on February 6, 2013, at 10,745 hours TSN and 11,210 cycles, with new post-Service Bulletin Commercial Engine Bulletin (CEB) 1365 1st through 4th stage turbine wheels installed. The most recent 100-hour inspection was completed on August 12, 2015, at 12,008 total airframe hours.
Rolls-Royce had issued CEB-1365 in April 1999 (revised April 2010) as a product improvement for the power turbine section, designed to increase power and fuel efficiency. The enhanced power turbine design became the only production and service-released hardware after earlier part numbers were discontinued.
Airworthiness Directives
On March 9, 2015, the FAA issued Airworthiness Directive (AD) 2015-02-22, superseding AD 2012-14-06. The AD mandated repetitive visual and fluorescent penetrant inspections on post-CEB-1365 3rd and 4th stage turbine wheels for cracks at the trailing edge of turbine blades, with initial compliance within 1,750 hours since last inspection, and re-inspection every 1,750 hours thereafter. Turbine wheels with detected cracks were not to be returned to service.
Root Cause Analysis
Rolls-Royce performed an evidence-based root cause analysis of the 3rd stage wheel but concluded that a single root cause could not be identified for the airfoil liberation. A new wheel design and process improvements were in process of being released to improve turbine wheel robustness and capability.