Investigation Summary
The failure originated from fan blade No. 11, which suffered a low cycle fatigue (LCF) fracture. The blade was a hollow-core Pratt & Whitney PW4000 series component. During its last overhaul in July 2015, the blade underwent Thermal Acoustic Imaging (TAI), a process P&W developed in 2005. Despite having inspected over 9,000 blades by 2018, P&W still classified TAI as a 'new and emerging technology.' This classification allowed them to bypass formal NDI (Non-Destructive Inspection) industry standards, such as certified training programs or requiring a Level 3 inspector. The inspector who examined the blade in 2015 noticed a thermal indication but dismissed it as a paint anomaly. Furthermore, the investigation found that the CFRP (Carbon Fiber Reinforced Plastic) aft bulkhead of the engine inlet was less capable of redistributing loads during a Fan Blade Out (FBO) event compared to the aluminum bulkhead used during original certification, contributing to the structural separation of the nacelle components.
Final Conclusions
The NTSB determined the probable cause was the fracture of a fan blade due to P&W's continued classification of the TAI inspection process as a new and emerging technology, which lacked formal training and certification programs. This led to an incorrect evaluation of a crack indication. A contributing factor was the lack of feedback from process engineers to inspectors regarding rejected blades.
Video Analysis
The Engine That Fell Apart Over The Ocean | United 1175 — Mentour Pilot examines the engine failure of United 1175 and the maintenance oversights involving Thermal Acoustic Imaging.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Thermal Acoustic Imaging (TAI)
A non-destructive inspection technique using ultrasonic excitation and infrared cameras to detect internal cracks in hollow fan blades.
Pratt & Whitney PW4077
A high-bypass turbofan engine featuring 112-inch diameter hollow-core titanium fan blades.
Engine Nacelle / Inlet
The aerodynamic structure surrounding the engine; the CFRP aft bulkhead failed to contain the displacement wave of the blade failure.
Interesting Facts
- 01The failed fan blade had been inspected via Thermal Acoustic Imaging (TAI) in 2010 and 2015, showing indications both times.
- 02The 2015 TAI inspector accepted the defect indication, annotating it as 'paint'.
- 03P&W inspectors were denied formal TAI training to prioritize clearing a backlog of blades.
- 04The aircraft involved was the 4th Boeing 777 ever built.
- 05Two small punctures were found in the fuselage below the window belt from blade fragments.
Safety Actions & Advisories
TAI Process Reform
Pratt & Whitney implemented improved procedures for TAI inspection and formal inspector certification programs.
Inlet Redesign
Boeing initiated a redesign of the 777 inlet duct to better withstand Fan Blade Out (FBO) loads.
Increased Inspection Frequency
Regulators mandated increased frequency of TAI inspections for PW4000 series 112-inch fan blades.