The urea-based solution known as Diesel Exhaust Fluid (DEF) is not permitted for use in aviation fuel systems. When DEF is inadvertently added to jet fuel, it reacts chemically with the fuel’s constituents to produce hazardous crystalline deposits.

These crystals have the ability to spread throughout the aircraft’s fuel system, where they could pollute gasoline nozzles, clog filters, obstruct fuel metering systems, and damage other vital parts. Sometimes the deposits settle inside the gasoline tanks and may eventually come loose, creating blockages downstream and seriously endangering the safe functioning of the aircraft.