What happened
On 20 January 2019, a Global Africa Aviation cargo flight, operating a McDonnell Douglas MD-11F with registration Z-GAA, departed Al Maktoum International Airport for Vietnam. The flight proceeded normally through the initial takeoff roll and climb. However, upon reaching approximately 2,100 feet above ground level, the crew reported hearing a loud noise.
Following this noise, the aircraft experienced a leftward yaw and a significant rise in the exhaust gas temperature of the No.1 engine. The pilot flying leveled the aircraft at 2,800 feet and initiated emergency procedures according to the quick reference handbook. The crew shut down the No.1 engine and discharged the fire extinguisher. After declaring a PAN PAN, the crew returned to the airport and landed safely after 17 minutes of flight time. There were no injuries during the event.
The investigation
Investigators from the GCAA AAIS examined the aircraft and engine components to determine the origin of the failure. An inspection of the No.1 engine revealed that while the external cowling and fan blades appeared intact, the low pressure turbine had sustained extensive damage. Debris from the turbine had exited the exhaust case, damaging the acoustic panels.
Further examination of the aircraft structure showed that engine debris had impacted the left wing's inboard aileron, lower wing panels, and the flap-hinge fairing. Ground services also recovered various pieces of engine debris from the end of runway 30. Analysis of the engine components, conducted in coordination with Pratt & Whitney, identified a fragment of an outer transition duct segment resting on the 3rd stage low pressure turbine vanes.
Findings
- The primary cause of the engine failure was the liberation of an outer transition duct segment located between the 2nd stage high pressure turbine and the 3rd stage low pressure turbine.
- The loss of this segment triggered secondary damage within the engine's turbine section.
- The operator had elected to defer the replacement of these specific duct segments until the next scheduled engine shop visit, which was not due until February 2020.
- The FAA Airworthiness Directive (AD 2012-2216) allowed for compliance at the "next piece-part exposure," which effectively permitted operators to delay replacement until the component reached its natural life limit.
Safety action
- The GCAA AAIS has issued a safety recommendation to the Federal Aviation Administration (FAA) to review the risk assessment process used when determining compliance timelines for Airworthiness Directives. The investigation suggests that allowing operators to defer the replacement of critical parts until a scheduled overhaul—when the failure could lead to uncontained engine damage—may be an inappropriate level of risk.