What happened
On 15 February 2001, an Airbus A330, registration C-FBUS, was operating an ETOPS charter flight from Medan, Indonesia, to Jeddah, Saudi Arabia. While climbing through flight level 350, the aircraft's No. 2 engine failed following a loud bang. The crew followed the ECAM checklist, discharging a fire bottle and shutting down the engine. The captain diverted the aircraft to Colombo, Sri and conducted a single-engine overweight landing approximately 90 minutes later.
This event followed a nearly identical failure of the aircraft's No. 1 engine just 13 engine cycles earlier, on 05 February 2001. During that initial incident, the crew noted engine vibration and odors, eventually leading to an engine shutdown and an emergency diversion to Medan, Indonesia.
The investigation
Investigators examined both Pratt & Whitney PW4168 engines at facilities in Connecticut. The investigation focused on why two engines on the same aircraft failed so closely together despite having undergone recent overhauls and receiving advanced corrosion protection coatings.
Analysis of the engines revealed that both had experienced significant corrosion in the under-platform cavity of the second-stage turbine blades. In the No. 2 engine, the corrosion was notably more severe than in the No. 1 engine. Investigators also found that the rear side plate seals on both engines were not providing an adequate seal. In the case of the No. 2 engine, the lack of witness marks indicated the seal was not contacting the blade face, creating a path for airflow.
Findings
- The primary cause of both engine failures was stress corrosion fractures of the second-stage turbine blades.
- The corrosion was driven by a chemical reaction between the blade's protective coating and anhydrite, which formed when sulfur from jet fuel combustion mixed with calcium from ingested dolomite dust.
- Ineffective rear side plate seals allowed hot gas path air to leak into the under-platform cavity, transporting the necessary contaminants to the blade roots.
- The rate of corrosion accelerated significantly after the engines' recent overhauls, as the repaired seals failed to restore the necessary airtightness.
Safety action
- Skyservice updated its training programs for pilots and dispatchers, specifically regarding ETOPS diversion decision-making and engine malfunction recognition.
- Airbus began developing updated FMGEC software logic to ensure auto thrust remains authorized following an engine shutdown via the fire pushbutton.
- Pratt & Whitney issued Alert Service Bulletins to implement new inspection procedures for high-pressure turbine blades and updated repair procedures for the side plate seals.