What happened
On March 29, 2008, an Air France Boeing 777-328 ER, registered F-GSQT, was operating an international passenger flight from Paris/Orly to Pointe-à-Pitre/Le Raizet. The aircraft was carrying 433 passengers and 15 crew members.
While cruising at FL3 320 within the New York FIR, the flight crew experienced an increase in N2 engine vibrations accompanied by several loud bangs and light yaw axis perturbations. The crew observed rapid fluctuations in the Exhaust Gas Temperature (EGT) for the #2 engine. Following the appropriate engine surge/stall checklist, the crew retarded the thrust lever to idle, which stabilized the engine indications. However, when the crew attempted to increase thrust, the engine became unstable again, exhibiting vibrations and loud bangs once the power reached approximately 60% N1. The commander subsequently decided to divert the aircraft to Lajes International Airport in the Azores, where the aircraft landed safely at 18:08 UTC with the #2 engine kept at idle.
The investigation
The GPIAAF investigation examined the engine's maintenance history and conducted both on-site and laboratory inspections. Prior to the incident, the engine had been flagged by an automated ACARS message regarding high N2 vibrations earlier in the flight.
An internal borescope inspection at Lajes revealed significant damage to the High Pressure Compressor (HPC). Specifically, the HPC4 rotor had one blade with a partially missing airfoil and other blades showing cracks, dents, and notches. Subsequent laboratory investigations by GE and SNECMA determined that the failure originated from the separation of an airfoil on the HPC Stage 4, blade #44. This separation was attributed to High Cycle Fatigue at the mid-chord on the pressure side. The detached fragment struck a Stage 4 vane segment, triggering a chain reaction of secondary impacts that damaged multiple downstream compressor components.
Findings
- The primary cause of the engine instability was the separation of the airfoil from the HPC Stage 4, blade #44.
- The separation was caused by High Cycle Fatigue.
- The resulting debris caused secondary impact damage to the Stage 4 vane and subsequent compressor blades, leading to a loss of compressor efficiency and an engine stall.
- The engine had previously shown signs of high N2 vibration earlier in the same flight.