10 July 2008
During the last several years, there have been over twenty reported fatigue fractures of the Reduction Gearbox (RGB) 1st stage carrier bolts, P/N MS9490-34. All but one of these incidents occurred following overhaul. At least five of the bolt failures caused in-flight shutdowns (IFSD).
In one instance, an operator removed the power section of a PT6A-65AR engine (TSO 20 hours) due to an inability to start the engine. During start-up, the power section hung up, released and then hung up again. Subsequent investigation revealed that several 1st stage carrier bolts had fractured in fatigue. In another instance a hex bolt failure (TSO 3 hours) and ingestion by the 1st stage planetary gear system resulted in damage to the first stage sun gear. Due to excessive vibrations and engine overspeed, an in-flight shutdown was carried out.
As the primary purpose of the RGB is to reduce engine speed to a level suitable for driving a propeller or power shaft, any damage or malfunction will almost certainly jeopardize continued flight.
An engineering evaluation at Pratt & Whitney Canada (P&WC) has concluded that inadequate bolt lubrication at assembly, coupled with a normal bending moment induced by the carrier under operation, can result in stress levels in the bolt that can exceed the endurance limit of the AMS5731 material.
P&WC has recently updated the respective engine overhaul manuals to re-emphasize the requirement to incorporate new 1st stage carrier bolts at overhaul and to lubricate the bolts on assembly. Additionally, P&WC has issued an All Shop Message PT6-2008-01 advising all PT6 overhaul agencies of the carrier bolt lubrication and torque requirements.
Transport Canada Civil Aviation (TCCA) recommends that all overhaul agencies remain current with, and adhere to all instructions issued by the type certificate holder.
Similar defects or further occurrences of this nature, or malfunctions, defects and failures occurring on any aeronautical products should be reported to Transport Canada, Continuing Airworthiness, in accordance with CAR 591, mandatory Service Difficulty Reporting requirements.
Should you have any questions, please contact the nearest TCC Centre or Mr. Barry Caldwell at 613-952-4358 or email@example.com
For Director, National Aircraft Certification
Acting Chief, Continuing Airworthiness
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