Incident Overview
On 15 February 2001, Skyservice Flight 6315, an Airbus A330 (registration C-FBUS), departed from Medan, Indonesia, at 1055 Coordinated Universal Time on a charter flight to Jeddah, Saudi Arabia, with 355 passengers and 11 crew members. About 3 hours 8 minutes after departure, while climbing from flight level (FL) 350 to FL390, a loud bang was heard and the No. 2 engine (Pratt & Whitney PW4168, serial number P733335) failed. The engine was shut down per the electronic centralized aircraft monitoring (ECAM) checklist. The aircraft was approximately 30 nautical miles west of Calicut, India. The captain initiated a descent to FL230 and diverted to Colombo, Sri Lanka. Ninety minutes after the failure, the crew conducted an uneventful single-engine overweight landing on Runway 22 with emergency services standing by.
This incident was the second engine failure on this aircraft. The first occurred on 05 February 2001, 13 engine cycles prior, involving the No. 1 engine.
First Engine Failure (5 February 2001)
On 05 February 2001, the No. 1 engine (Pratt & Whitney PW4168, serial number P733336) failed while the aircraft operated as Flight 6308, a ferry flight from Jeddah to Solo, Indonesia. One to two hours into the flight, the crew received an ECAM N2 vibration advisory for the No. 1 engine. The crew did not detect associated airframe vibration. An onboard maintenance engineer and the maintenance chief at Solo analyzed the vibration and concluded there was likely fan blade damage. The captain elected to continue. Approximately 4.5 hours later, engine odors were noted, followed by airframe vibration. The ECAM displayed Eng 1 STALL, and as shutdown procedures began, Eng 1 FAIL appeared. The engine was shut down. The crew declared an emergency, decelerated to single-engine speed, and descended to FL250. They proceeded to Medan at reduced speed with continuous N1 vibration and sporadic airframe vibration. The aircraft landed uneventfully after 2 hours 20 minutes of single-engine operation. The engine was removed and replaced; the aircraft returned to service on 10 February 2001.
Engine Examination and Stress Corrosion
Both engines were sent to Pratt & Whitney facilities in Cheshire, Connecticut, where they were dismantled and examined. In both cases, the failure was determined to be the result of a stress corrosion fracture of a second-stage turbine blade. The first engine (serial P733336) had 1306 cycles since its last shop visit (overhaul in March 1999), during which second-stage turbine blades were replaced with refurbished blades coated with PWA545 corrosion protection coating. The second engine (serial P733335) had 1020 cycles since its last shop visit (overhaul in July 1999), where second-stage turbine blades were replaced with new blades coated with PWA36395-1 (Platinum Aluminide).
Pratt & Whitney stated that the failure of the first engine was within statistically predictable time limits, but the second engine failure was premature and unexpected. Corrosion pitting on the uncoated portion of blade roots was more severe on the second engine, averaging 0.012 inch depth compared to 0.006 inch on the first engine, despite less service time. Dirt samples from both engines contained dolomite and calcium sulfate (anhydrite). Examination of rear side plate seals showed that the seal on the second engine was not contacting the turbine blades, providing a path for airflow into the under-platform cavity, allowing sulfur from combustion to mix with calcium from dolomite, forming calcium sulfate.
Auto Thrust Issue
In both events, the auto throttles could not be used after engine failure. The flight management, guidance, and envelope computer (FMGEC) receives signals from each engine's full authority digital engine control (FADEC). When the engine is shut down using the fire pushbutton, the electronic engine control reverts to N1 control mode. If the windmilling engine's N2 exceeds a threshold (3.8%-4.8%), the FMGEC interprets the signal as the engine still operating, preventing auto thrust engagement. Airbus was reviewing this software logic.