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Flight Crews: Preventing DEF From Contaminating Jet Fuel

In response to the increased presence of diesel exhaust fluid (DEF) at airports, business aircraft operators and FBOs are urged to implement best practices for the safe handling of DEF, which has been linked to some serious jet-fuel contamination incidents.

From 2017-2019, three separate DEF contamination events affected 14 business aircraft. Three of those incidents led to inflight engine failures. All the affected aircraft landed safely, thanks to the quick reactions of well-trained flight crews, but the danger proved significant enough to prompt an NTSB safety alert.

“Airport personnel mistakenly added DEF into the fuel system icing inhibitor (FSII) reservoir on refueling trucks,” said Steven Zabarnick, Ph.D., head of the University of Dayton Research Institute’s Fuels and Combustion Division.

“When that happens, the DEF is injected into the jet fuel instead of the FSII, where it rapidly reacts with the normal alkanes in the jet fuel,” continued Zabarnick. “This creates a clunky white deposit, which can foul the aircraft fuel tank and engine fuel system, and block filters, valves and nozzles.”

“Store DEF separately from FSII in clearly labeled containers, keep them locked up, and have a trained person hold the key to prevent someone from mistakenly grabbing the wrong chemical.”

Alex Gertsen Director of Airports and Ground Infrastructure, NBAA

According to Mark Larsen, CAM, NBAA’s senior manager of safety and flight operations, a factor in the rise of DEF contamination incidents is the increased use of selective catalytic reduction systems to help airport ground vehicles meet EPA emission standards. Larsen is part of an industry task force working with the FAA and EPA to push for an emissions exemption to reduce the presence of DEF at airports before more contamination incidents occur.

“When you consider the number of refueling events each day, the frequency of these contamination events is low. But the reality is that a DEF fuel contamination outcome could very well be catastrophic,” said Larsen.

Alex Gertsen, C.M. ACE, NBAA’s director of airports and ground infrastructure, said it’s critical to not only train ground handlers on the risk of DEF contamination during aircraft fueling operations, but also to create conditions where it’s nearly impossible to mix up the two substances.

“Store DEF separately from FSII in clearly labeled containers, keep them locked up, and have a trained person hold the key to prevent someone from mistakenly grabbing the wrong chemical,” suggests Gertsen. “This will require relying on an individual with proper training and awareness to gain access.”

Another best practice to prevent DEF contamination is to not order FSII and DEF in the same size drums, as visual differentiation between storage containers is also a strong preventative measure. (Both FSII and DEF come in 55-gallon drums and are colorless.)

Business aircraft operators can help mitigate the DEF contamination hazard by having their flight crews remain engaged during the aircraft fueling process and by learning the warning signs of DEF-related fuel contamination.

“If pilots suspect fuel-flow issues, they should react immediately and not delay returning to the airport or proceeding to the closest location where the aircraft can land safely,” said Gertsen.

Review NBAA’s fuel contamination resources at

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