Cracked Starter - Generator Spline

SDR # 20120502009

SDR submitted:

The starter-generator was removed for overhaul with 1786 hours total time airframe. During overhaul, a magnetic particle inspection detected a crack radially and longitudinally in the drive spline that engages with the engine. 

The starter-generator was manufactured in March 2006. The cracked shaft was replaced with a new one.

Transport Canada Comments:

Considering the date of manufacture and the total number of hours on the airframe, it is possible that there may have been a vibration or installation issue with this unit. It is fortunate that the inspection cycle for this component detected the fault before total failure occurred. 

On-condition Engines

SDR # 20120301010

SDR submitted:

During the pre-flight ground run, the pilot noticed an engine vibration. The aeroplane was snagged and inspected by maintenance. Six out of eight studs on the #3 cylinder were sheared off. All the studs were replaced. All 4 cylinders base nuts were checked for proper torque. The engine was on condition at the time. The cylinder has never been removed since overhaul. Two other cylinders were removed for the engine on condition implementation at 2065 hours. The engine received on condition inspections each 50 hours thereafter. The aeroplane has received two inspections since the occurrence, and no further defects have been found.

Transport Canada Comments:

Operating an engine ‘on condition’ can be economically advantageous but it does come with certain risks. In this instance, the engine was able to be repaired in accordance with the engine’s maintenance manual. An issue worth noting is, if the through studs that also hold down the opposite cylinder #2 loosened off, then the crankcase probably lost the preload of Main Bearing Journal. This could lead to the possibility of fretting damage during high cyclic friction between vibrating mating surfaces and material loss on the crank-case halves. Should the through studs be subsequently re-torqued, there is a possibility of excessive clamping pressure on the main bearing.

Operators are reminded that the requirements for ‘on-condition’ maintenance are clearly spelled out in airworthiness notice B041. 

Oil Transmitter Interference Fit With Core Cowl

SDR # 20120502007

SDR submitted:

During a scheduled #1 engine change, the upper core cowl was found with a 2.54 cm (1 inch) puncture in it. An investigation revealed that the core cowl was in contact with the top of the oil tank level sensor on the removed engine. A new core cowl was installed on the new engine.

Transport Canada Comments:

Maintainers are reminded to remain vigilant while installing or inspecting components and pay close attention to fits and clearances. 

Cracked Diffuser Tubes

SDR # 20120629003

SDR submitted:

After receiving a PT6A-28 engine back from overhaul, a trend request was opened stating that the temperature had increased by 20 degrees. Several attempts were made to correct the condition targeting calibrations, temp probes, etc. The problem was unable to be rectified through those attempts. Maintenance decided to do a hot section inspection at 1616 hours since overhaul to inspect in greater detail. Maintenance found three badly cracked diffuser tubes. Three field repair kits (part number 3102843-01) were installed in the engine as per manufacture recommendations and function checks found that the engine was once again serviceable.

Transport Canada Comments:

Great job done by maintenance to go ahead with the hot section inspection and discover the source of the problem. While a 20 degree increase in operating temperature may not seem like much, it is a good indication of engine distress. Trend analysis is a very useful tool when dealing with complex aircraft systems.

Intermediate Case Corrosion

SDR # 20120905002

SDR submitted:

The engine was returned to Rolls Royce Canada for midlife inspection. While dissembling the engine, during the outlet guide vane ring removal, it was noted that the external intermediate casing wall had excessive corrosion that had created a hole through the casing wall and generated a 25.4 cm (10 inch) crack. The engine had also excessive visible corrosion to other areas of the case and inside the low pressure compressor.

Transport Canada Comments:

Rolls Royce has issued Service Bulletin 72-1399 that deals with corrosion concerns on the TAY engine as well as a safety alert on this event. Operators are encouraged to contact Rolls Royce through their web site or field service representatives for additional information.

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