Switching Types Led To Coke Formation In A330's Turbines

In its latest round of recommendations for the Federal Aviation Administration, the National Transportation Safety Board on Thursday asked the FAA to amend its guidelines for engine oil use, and inspections when a new type of oil is used.

The recommendations come after an Edelweiss Air Airbus A330-200 experienced an uncontained turbine blade release from its No. 1 (left) Rolls-Royce Trent 772-60/16 turbofan engine, while climbing through FL230 after departing from Miami International Airport in October 2003. The flight crew declared an emergency and returned to Miami, where the pilots successfully landed the airplane. No injuries to the 12 crewmembers or 171 passengers were reported.

While the NTSB says the engine fire made it nearly impossible to examine the No. 1 engine, a borescope inspection of the No. 2 engine revealed the HP/IP turbine bearing chamber internal vent tube was obstructed with a black substance, later determined to be nodules of carbon deposits (also known as coke) left from the engine's lubricating oil. Such nodules aren't uncommon, but the engine showed a much greater amount of deposits than was considered normal.

Investigators then learned the airline had recently changed engine oil types... and the pieces started to fall into place. Based on its findings, the NTSB recommends the FAA:

• Revise Advisory Circular (AC) 20-24B to include guidance to ensure that each time a new engine/oil combination is introduced, procedures are developed and implemented to inspect, at appropriate intervals, those areas within the engine where testing or in-service experience has indicated porous carbon formation is likely and has the potential to result in hazardous oil system or engine behavior. AC 20-24B should also provide criteria for evaluating the results of these engine inspections to aid operators in determining whether continued operation is safe or whether the engine/oil combination should be discontinued. (A-06-85)
• Review the maintenance programs for all engine and oil combinations currently in service, with particular emphasis on the evaluation of airplanes approved for extended-range operation with two engines, to determine that operators have gathered and evaluated sufficient data, including operational experience and engine hardware disassembly inspection findings, to ensure that these combinations are not at risk of producing potentially hazardous porous-coke conditions. If such data do not exist or are insufficient, require operators to implement appropriate measures, including periodic inspections, to collect and evaluate the necessary data until the safety risk associated with the presence of porous coke is either ruled out or properly controlled. (A-06-86)
• Coordinate with international regulatory agencies to inform them of the circumstances of the event involving Edelweiss Air flight EDW 565 and to encourage them to develop and adopt comprehensive standards and procedures regarding the introduction of new engine/oil combinations, including the inspection at appropriate intervals of those areas within the engine where testing or in-service experience has indicated porous carbon formation is likely and has the potential to result in a hazardous engine condition. This coordination effort should also ensure that sufficient data, including operational experience and engine hardware disassembly inspection findings, have been gathered and analyzed, with particular emphasis on airplanes approved for extended-range operation with two engines, to support continued operation of engine/oil combinations. (A-06-87)

FMI: Read The Full NTSB Recommendation Letter