Ten to 12 Pratt & Whitney Canada turbofan engine events on corporate jets have occurred over the past two years involving power rollbacks with the inability to regain power. In one instance, the corporate jet lost power in both engines. The aircraft that experienced the complete loss of power was able to glide to a runway and land safely. Upon investigating the incidents, the FAA attributed the cause of the engine problems to contaminated fuel.
Somehow diesel exhaust fluid (DEF), a liquid used to reduce the amount of air pollution created by a diesel engine, ended up in jet fuel at several airports and got dispensed into the aircraft. Although the majority of PT6A ag engines are unlikely to be affected by this problem, it is worth being aware of and speaks to the importance of fuel integrity.
These refuelings happened at several different fixed-base operations (FBOs) around the country. Shortly after departure from those airports, the DEF in the jet fuel reacted and started to plug orifices in the engines’ fuel control units (FCUs). Investigations of each fuel control unit revealed large amounts of urea partially or completely plugging fuel-related orifices in the fuel control. (DEF is an aqueous urea solution made with 32.5% urea and 67.5% deionized water.)
These aircraft all were serviced with Jet A fuel, and somehow, diesel exhaust fluid (DEF). These refuelings happened at several different fixed-base operations (FBOs) around the country. Within 20 to 30 minutes after takeoff, each jet had an impending fuel bypass caution light illuminate, and each aircraft had an engine experience an uncommanded rollback to idle thrust. One aircraft experienced a dual engine rollback. None of the engines that had a rollback were able to recover power with a throttle push. All aircraft were able to glide to a runway and land safely.
In discussions between Fletcher Sharp, the director of propulsion on NAAA’s board, and vendors that specialize in repair/overhaul of fuel control units, the general sentiment was that PT6 engines fitted with Bendix/Honeywell FCUs (PT6A-20 through and including PT6A-45 series) most likely would not experience any issues if DEF contaminated the fuel.
The larger PT6A-140AG, -60AG, -65AG and -67AG engines all have Woodward FCUs. Those might be prone to problems, mainly during winter storage, because unlike the smaller Bendix/Honeywell FCUs, the Woodward fuel control unit is always full of fuel. Fuel contaminated by DEF in the fuel control unit and left alone without any ground runs during the offseason could show signs of sticking control valves, corrosion or both. Urea is extremely corrosive if left alone, and corrosion is not covered under Pratt & Whitney Canada’s warranty.
DEF only goes with diesel fuel. It is not for use in any other fuel product. However, most diesel fuels are very similar to jet fuels. Operators should closely monitor fuel product at delivery to their fuel farms to ensure DEF is not mixed with their fuel.
P&WC Service Bulletin 13244 for the -60AG and -65AG and Service Bulletin 14504 for the -67AG and -67F engines address approved fuels and additives. The -140AG fuel and additive information is in the -140AG engine maintenance manual. If any engine issues arise that could be related to use of unapproved fuels or additives, Pratt & Whitney Canada’s warranty would most likely be null and void.