What happened
On the morning of 11 May 2018, a Bombardier CL-600-2C10, registration ZS-NBG, departed East London Airport (FAEL) for O.R. Tambo International Airport. The aircraft was operating a scheduled commercial flight with 52 passengers and four crew members on board.
Shortly after takeoff, the flight crew engaged the autopilot at 600 feet. However, as the aircraft climbed to approximately 800 feet, the crew experienced intense vibrations accompanied by a loud bang from the rear of the aircraft. The crew immediately identified a failure in the number 1 engine and initiated emergency memory items and Quick Reference Handbook (QRH) procedures. After stabilizing the aircraft at 3,000 feet, the captain declared a "PAN-PAN" urgency signal to air traffic control. The aircraft subsequently returned to East London and landed safely on Runway 2 and 29. There were no injuries to the passengers or crew.
The investigation
An investigation by the SACAA AIID involved a detailed teardown of the GE CF34-8C5B1 engine at a facility in Kansas, USA, with oversight from the NTSB. Investigators examined the engine's internal components to determine the source of the failure. The analysis focused on the high-pressure turbine (HPT) module and the physical characteristics of the fractured metal surfaces.
Findings
Technical examination of the engine revealed that the failure was caused by the separation of five blades within the stage-two high-pressure turbine assembly. The fractured surfaces showed evidence of a fatigue region that originated at the convex shank surface. This fatigue was identified as being corrosion-assisted, characterized by significant discoloration and a smooth topography on the metal.
While the manufacturer had previously issued Service Bulletin (SB) 72-0228 to introduce improved blades with enhanced corrosion protection, this specific engine had not yet been updated with the new part. The engine had reached 20,962 cycles, which was significantly higher than any previously recorded instance of such blade separation in this engine series.
Safety action
To prevent similar in-flight shutdowns, the investigation proposed that the installation of the improved stage-two turbine blades (P/N 4125T65G01) should be made mandatory at the next shop visit for all unmodified engines.