On 13 November 2017, fumes on a GE90-powered Boeing 777-200 sufficient to require flight crew oxygen mask use occurred as it descended towards London Heathrow. The flight was completed without further event. Subsequent engineering assessments twice led to release to service followed by recurrence and after the fourth such release, a left engine overheat was annunciated. After flight, a hole in the engine combustor case was found and the engine was removed for repair. The Investigation attributed the delayed identification of the causal fault to inappropriate guidance in the aircraft manufacturers Fault Isolation Manual which was has since been amended.