History of Flight
On June 17, 2023, about 1059 central daylight time, a Gulfstream G-IV, registration N926TT, equipped with two Rolls-Royce Tay 611-8 turbofan engines and operated by Planet 9 Private Air, LLC, experienced a No. 2 (right) engine failure shortly after beginning a step climb at FL400. The flight crew secured the No. 2 engine, declared an emergency, and diverted to Sioux Gateway Airport (SUX), Sioux City, Iowa, where they made an uneventful single-engine landing. There were no injuries reported. The airplane was operating as a Title 14 Code of Federal Regulations Part 135 flight from Philadelphia International Airport (PHL), Pennsylvania, to Charles M. Schulz Sonoma County Airport (STS), California.
Damage to the Airplane
Multiple small impacts were found on the right wing, extending up to approximately 60 inches forward of the No. 2 engine inlet cowl leading edge lip. One impact on the right wing flap penetrated the wing skin. Additional small impact marks were observed on the fuselage inboard of the engine, on the aft right window, and on the leading edge of the right horizontal stabilizer.
No. 2 Engine Nacelle Examination
The No. 2 engine inlet cowl had multiple acoustic liner impact gouges into the honeycomb structure, 360 degrees around the circumference. The impact damage was most severe from the fan blade path leading edge to approximately 12 inches forward of the fan blades. There was a rounded slice at the 8 o'clock position with an arc length of about 15 inches. The inlet cowl had a penetration at the 1 o'clock position measuring approximately 12 inches circumferentially by 7 inches axially at the widest points, extending from the inlet cowl bulkhead to about 2 inches forward of the inlet cowl-to-fan case flange. The composite and metal material around the hole was petaled radially outward. Two additional small penetrations were at the 3 o'clock and 10 o'clock positions. The fan cowl metal and thermal blanket insulation around the 3 o'clock penetration were displaced radially outward. Light was barely visible through the 10 o'clock impact, with no evidence of material escape.
No. 2 Engine Examination
The engine was removed and visually examined. There was no evidence of undercowl fire or thermal damage. The fan spinner fairing had separated and was missing; three fairing attaching bolts had fragments of the fairing still secured under the bolt heads. All fan blades exhibited leading edge hard body impact damage, tearing, and material loss along the full blade span. One fan blade (position 7) was separated at the midspan; the released portion was contained within the fan case and recovered forward of the fan outlet guide vanes. The fan blade attrition lining had heavy 360-degree rub and impact damage. The ice-resistant liner aft of the fan blade plane of rotation also exhibited 360-degree impact damage and missing material at multiple locations. The visible intermediate pressure compressor blades had leading and trailing edge nicks, and blade tips had heavy rub/uneven wear consistent with an impact and operation with a significant out-of-balance. Additionally, an anti-ice servo air line b-nut connector was backed off at the 2 o'clock position, and below it, the air line was fractured at the boss weld into the main bleed air duct. The P3 limiter, attached to the fuel flow regulator at the 7 o'clock position, was separated at the mating flange; two connecting bolts were broken and one had backed off. A pressure fuel line off the acceleration reset solenoid was fractured at the 8 o'clock position on the bypass case, with no visual evidence of fuel streaking on the left side of the engine.
Materials Analysis
The recovered spinner fairing attaching hardware, all fan blades, and fan blade position 7 tip fragments were sent to Rolls-Royce Deutschland's materials laboratory for failure analysis. The laboratory identified fatigue cracks on two of the three bolt hole fixture pockets, initiating in the heat-affected zone of the tack welds holding the fairing spacer. The analysis could not determine if the fatigue mode was high cycle or low cycle fatigue, likely a combination of both. The tack welds on each of the three spinner fairing bolt assemblies were greater than 10 mm in length with visually good quality and no evidence of voids. Microstructural examination showed significant grain growth typical of normal welding. The spinner fairing material was consistent with Titanium 6/4 as specified. The fracture surfaces of fan blade position 7 (approximately 192-220 mm above the platform) were consistent with forced rupture from a leading edge impact.
Spinner Fairing Modal Analysis Testing
Modal impact testing on a spinner fairing identified that its natural frequency is close to a non-integral fan blade vibration mode excited at high altitude speeds. Variables affecting that mode include fan blade leading edge shape, fan pressure ratio, and fan speed. Manufacturing variability causes some fan blade sets to excite the spinner fairing 1st natural frequency while others do not. A fan blade mode crosses at 70% N1, mostly encountered transiently during acceleration to takeoff power or in crosswinds, with no evidence of dwell during normal operation. The gap between the spinner fairing and nose cone also affects vibratory response. Rolls-Royce released Notice to Operators (NTO) 38 in August 1997 to recommend nose cone replacement on engines with spinner fairing cracks; replacement has been effective at detuning vibration, with no further fairing cracking known.
Spinner Fairing Inspection
For the Gulfstream G-IV/Tay 611-8 in corporate operations, the spinner fairing inspection interval per the aircraft maintenance manual is every 2000 hours. For regional operations (e.g., Tay 610-15 on Fokker 100), the interval is every 600 cycles. According to the engine logbook, the No. 2 engine spinner fairing was inspected by Planet 9 Private Air, LLC on October 12, 2022, with approximately 542.2 hours accumulated since inspection. The applicable AMM procedure instructed technicians to inspect for cracks and indentations emanating from spinner fairing bolt holes but did not require removal of the spinner fairing from the nose cone. Rolls-Royce reported field findings of spinner fairing cracks that extended through the sheet metal until visible on the forward side; the origin of such cracks has been the heat-affected zone of the tack welds securing the spacer to the aft side, so early crack progression is only visible if the fairing is removed. It is unknown whether the incident engine's spinner fairing crack had grown to be visible on the forward side during the October 2022 inspection.
Corrective Actions
Rolls-Royce released NTO 155 on October 20, 2023, for all Tay engines, providing updated nose cone fairing inspection criteria to the engine maintenance manual. The updated procedure adds an instruction to remove the spinner fairing to allow inspection of the aft side, stating: "This facilitates earlier detection of cracks by allowing inspection of the welds, where cracks are known to initiate." A requirement was also added to inspect whenever the fairing is removed. The EMM was updated to recommend a one-time inspection of spinner fairings if an engine previously had a fairing replaced due to cracking after 150 hours (corporate) or 50 cycles (regional). Rolls-Royce has begun a redesign effort to replace the welded fairing spacer with a bonded spacer design. As of July 2024, the design concept has been approved, and Rolls-Royce is working to identify a manufacturer. The release date is yet to be determined; the redesign will be incorporated through attrition rather than a mandatory replacement campaign.