N5743WBell 206B · 1980

On August 18, 2015, about 1330 central daylight time, a Bell 206B helicopter, N5743W, impacted terrain during an autorotation following a loss of engine power while maneuvering near Cresco, Iowa. The commercial pilot, who was the sole occupant, was not injured. The helicopter sustained substantial damage. The helicopter was registered to and operated by Fly Hangar 13, LLC, Mico, Texas, as a 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions were reported by the pilot at the accident site about the time of the accident, and no flight plan was filed. The local flight originated from an off airport location about 1320.

According to the pilot, he departed with 30 gallons of fuel and about 90 gallons of chemical to spray a soybean field near Cresco. About 5 minutes into the application and 3 to 5 feet above the soybeans, the pilot heard a loud screech for about 2 seconds and pulled up to gain altitude. The pilot then heard the low rotor RPM horn and noticed the main rotor RPM at 85 percent. The pilot initiated an autorotation to a waterway between two cornfields. Upon touchdown on the uneven terrain, the main rotor blades contacted and severed the tailboom, and the helicopter came to rest upright. The pilot shutdown and exited the helicopter. The helicopter was recovered from the accident site for further examination.

The helicopter was equipped with a Rolls-Royce M250-C20J turbo-shaft engine, which features a 6 stage axial and 1 stage centrifugal compressor section that directs the diffused air via an external 180 degree compressor discharge tube system to the combustor. The hot gases from the combustor are then directed against a two-stage gas producer turbine and subsequently a two-stage power turbine before being exhausted. The RR M250-C20J produces 420 shaft horsepower.

The engine serial number (S/N) was CAE-832989, and the engine logbook revealed that the turbine assembly (S/N CAT-36021) was overhauled on February 6, 2013, at a time since new (TSN) of 10,745 hours, and cycles since new (CSN) of 11,210 cycles. At the time of the overhaul, new post-Service Bulletin (SB) Commercial Engine Bulletin (CEB) 1365 1st, 2nd, 3rd, and 4th stage turbine wheels were installed. According to the operator, at the time of the accident, the engine had 11,797 hours TSN, and 1,052 hours since major overhaul. The helicopter was maintained in accordance with the manufacturer's inspection program. The most recent 100-hour inspection was completed on August 12, 2015, at a total airframe time of 12,008 hours.

On August 26, 2015, the helicopter and engine were examined at the operator's facility under the supervision of Federal Aviation Administration (FAA) inspectors. Examination of the engine revealed the exhaust collector support was fractured 360 degrees forward of the turbine mating flange. The turbine section shifted about 2 inches. It appeared that the 3rd stage turbine wheel was missing about 4 to 5 blades. The engine was removed, crated, and shipped to Rolls-Royce for further examination. No preimpact mechanical anomalies were noted with the airframe.

On October 5, 2015, at the facilities of Rolls-Royce Corporation, Indianapolis, Indiana, the engine was examined and disassembled under the supervision of the National Transportation Safety Board investigator-in-charge. Disassembly of the engine revealed the 3rd stage turbine wheel (part number 23065818) was missing five airfoils (blades), several other airfoils were damaged, and portions of the shroud were also liberated.

Rolls-Royce Corporation Materials Laboratory completed an examination of the 3rd stage turbine wheel. The findings from the 3rd stage wheel were the following:

"Five airfoils on the third stage turbine wheel cracked in fatigue, initiating from the trailing edge root area and progressing forward until final fracture occurred in overload. All five cracks initiated and progressed in high cycle fatigue. Two additional cracks were found in the trailing edge side of the wheel rim, consistent with thermal fatigue cracking."

In addition to the 3rd stage wheel, several other components were examined due to their damage noted during the disassembly. Damage observed and documented on the other components was considered secondary to the liberation of the five airfoils on the third stage turbine wheel. See Rolls-Royce Metallurgical Investigation Report found in the NTSB accident docket for this investigation.

In April 1999 (revised in April 2010), Rolls-Royce issued Commercial Engine Bulletin (CEB) 1365. The 'enhanced' power turbine section was developed by Rolls-Royce as a product improvement, designed to increase both power and fuel efficiency. SB CEB-1365 hardware was a major re-design of the 3rd and 4th stage turbine assembly, with the main differences between the pre- and post-SB CEB-1365 being different airfoil size, shape, tilt, lean, flow, and quantity of airfoils per stage for both turbine nozzles and wheels.

The enhanced power turbine design was released for new production engines built after August 1999. It was then released as a customer option to field engines via Rolls-Royce SB CEB-1365 in November 1999. The modification applied to all M250- C20 series engines, with the exception of turbo-prop variants, and was to be complied with as a customer option. Release of enhanced power turbine to 250–B17F/2 turbo-prop variants occurred in August 2008, while release of all other turbo-prop applications was November 2009. The previous "non-enhanced" power turbine part numbers were discontinued from production in August 2009, and discontinued from Service/Spares orders in March 2013. Thus, the SB CEB-1365 enhanced power turbine is the only current production and service released hardware.

On March 9, 2015, the FAA issued airworthiness directive (AD) 2015-02-22, which was prompted by investigations that revealed that not all 3rd stage and 4th stage turbine wheel blade failures were identified by the one-time inspections required by AD 2012-14-06, dated July 10, 2012. AD 2015-02-22 superseded AD 2012-14-06. The FAA mandated a repetitive visual inspection and fluorescent penetrant inspection (FPI) on post-SB CEB-1365 3rd and 4th stage turbine wheels for cracks in the trailing edges of the turbine blades, and triggered by hours since last inspection (HSLI).

The AD compliance stated:

(1) Within 1,750 HSLI, remove the affected turbine wheels and perform a visual inspection and an FPI on the removed turbine wheels for cracks at the trailing edge of the turbine blades near the fillet at the rim.

(2) Any time the power turbine is disassembled, perform a visual inspection and an FPI on the affected turbine wheels for cracks at the trailing edge of the turbine blades, near the fillet at the rim.

(3) Thereafter, re-inspect every 1,750 HSLI.

(4) Do not return to service any turbine wheels that have cracks detected.

Rolls-Royce completed an evidence based root cause analysis (EbRCA) of the 3rd stage wheel. According to the final report, "At the conclusion of extensive engineering testing, a single root cause could not be identified for the liberation of the airfoils on the stage three turbine wheel." A new wheel design and process improvements are in process of being released to improve the robustness and capability of the turbine wheel.