All PT6A engines number 1 bearing electrical discharge damage - Service Difficulty Advisory

No. AV-2007-05

All PT6A ENGINES (#1 BEARING) ELECTRICAL DISCHARGE DAMAGE (EDD) Electrical discharge damage (EDD) occurs when high electrical current inadvertently passes through the engine accessory gear train creating spark (arcing) damage to the gears, bearing(s) and bearing elements. EDD can lead to in-flight shutdowns, a reduced margin of safety and replacement of expensive components.

In the PT6A engine, EDD occurrences have caused failure of the No.1 bearing. Electrical current travels from the defective starter generator (SG) spline shaft, thru the engine accessory drive train to the engine No.1 bearing causing mild to severe bearing damage such as pitting, grooves or craters. The extent of bearing damage and therefore the time before failure, is dependant on various factors such as the electrical current, exposure time, bearing load and rotational speed.

The most common and preventable cause of EDD, is from SG armature leakage occurring as a result of an accumulation of brush dust. This dust can provide an electrical discharge path between the commutator and the shaft. Secondly, a breakdown of the SG commutator insulation and/or the lamination slots can cause an electrical short. Periodic field cleaning and resistance checks may provide an indication of armature insulation breakdown. Close visual inspection of the starter Generator spline shaft for arc damage anytime the SG is removed is an excellent indicator of possible EDD.

Transport Canada Civil Aviation (TCCA) recommends that owners, operators, approved maintenance facilities and other interested persons familiarize themselves with P&WC Service Information Letter (SIL) No. Gen. PT6-024 titled “No. 1 Bearing Electrical Discharge Damage”.

TCCA also recommend compliance with engine Maintenance Manual criteria for unscheduled inspections in the event of SG replacement. Please note that the SG is procured and then installed on the engine by the Aircraft Manufacturer and not the Engine Manufacturer.

TCCA, in conjunction with P&WC and the various airframe manufacturers will continue to closely monitor this situation in service and ensure any additional improvements warranted are made available.

Malfunctions, defects and failures occurring on aeronautical products should be reported to TCCA, Continuing Airworthiness in accordance with Canadian Aviation Regulations (CARs) 591 reporting requirements.

For further information, please contact a Transport Canada Centre (TCC) or Mr. Barry Caldwell at (613)952-4358, facsimile 613-996-9178 or e-mail

For Director, Aircraft Certification

B. GoyaniukChief,
Chief, Continuing Airworthiness

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