Why You NEED to Know EDD – Electrostatic Discharge Damage in the PT6A

Last month we disassembled and investigated a 3000 hour time since new PT6A-34AG engine with a #1 bearing failure that appears to be attributable to a phenomenon called: EDD! The damage from the failure combined with damage from the propeller sudden stoppage following the aircraft’s off airport landing has resulted in a BER; an engine that is “Beyond Economic Repair”!

The following information was adapted from FAA Special Airworthiness Bulletin number CE-0324: This S.A.I.B. alerts you, an owner or operator, of Pratt & Whitney Canada PT6A-60A, -65B, -65R, -65AR, -67B or -67R engines, to the possibility of engine failure due to deterioration of the number one engine bearing. This deterioration is apparently caused by electrical (or electrostatic) discharge damage (EDD).

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Also in the bulletin: The Australian Transport Safety Bureau investigated the matter in connection with five such failures which one of their operators experienced with Shorts 360 aircraft between November 1995 and August 2000. They found that there had been a total of 17 failures apparently caused by electrical discharge damage of the number one bearing in the PT6A worldwide fleet. Evidence suggests that an electric current from the starter-generator gear shaft passes through the accessory gear train and the compressor spline coupling. It appears that the electrical current initiates spalling damage to the engine bearing(s).

“The SAIB focuses on Goodrich (formerly TRW Aeronautical Systems, Lucas Aerospace) starter-generator models 23078 and 23085, but similar design starter-generators from other manufacturers would be expected to perform in a like manner”.

Air Tractor introduced Service Letter #212 on August 23, 2001 addressing the problem as it relates to all Air Tractor Models with Pratt & Whitney Canada PT6A Engines Installed!  In that bulletin Air Tractor states: “It is believed that as the brushes in the starter-generator wear, dust from these brushes collects on the internal components of the unit. This dust is believed to provide an electrical path for current to flow through the starter-generator shaft and into the accessory gearbox. Burns at arc locations on the gears and bearing in the accessory gearbox have caused “electrostatic discharge damage” which has led to the eventual failure of these components”. The Air Tractor bulletin focuses on the Lucas Aerospace starter-generators and references their recommendations for a 1000 hour TBO as well as complying with all Lucas Aerospace instructions for maintenance.

The 3000 hour TSN engine we investigated had come off a Thrush! Apparently it doesn’t matter what make of starter-generator / engine combination you use, they’re all affected by the EDD phenomenon! All service bulletins, Special Airworthiness Bulletins and similar publications strongly recommend maintaining starter-generators in accordance with the manufacturer’s instructions. I want to emphasize the need to adhere to the part of those instructions placing a TBO (Time Before Overhaul) on those units. I have personally worked on a PT6A-60AG, a 65AG and the -34AG identified above because of failures of the #1 bearing caused by Electrostatic Discharge Damage. Two of those had been damaged by starter-generators failing after having been operated substantially over their recommended TBO’s!   Call us up if you have questions about EDD!

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