Diesel Exhaust Fluid (DEF) as a Fuel Contaminant

Diesel Exhaust Fluid (DEF) as a Fuel Contaminant

Diesel Emission Fluid

Aqueous Urea Solution




Diesel Exhaust Fluid (DEF), in some locations referred to as "Aqueous Urea Solution" and at others "AdBlue", is a clear, colourless, non-toxic, and virtually odourless, liquid that is injected into the post-combustion exhaust steam of a diesel engine to reduce emissions. If inadvertently mixed with jet fuel, it can cause clogging of fuel filters, fouling of fuel injectors, loss of engine power, RPM rollback and engine flameout. In the worst case scenario, these events could ultimately lead to the loss of the aircraft.


Diesel Exhaust Fluid is a blend of pure urea and deionised water in an approximate one third / two thirds ratio. It is the consumable component of a Selective Catalytic Reduction (SCR) exhaust system, a system which has been mandatory in many jurisdictions, including Europe, the United States and Canada, from around 2010 onwards. The DEF is introduced to the post-combustion exhaust stream where a complex chain of chemical reactions result in the combination of the nitrogen oxides in the exhaust with the urea in the DEF to form water and carbon dioxide. The process eliminates virtually all of the environmentally harmful nitrogen emissions.

Many of the ground handling services provided on the apron of any aerodrome rely on diesel powered vehicles such as fuel bowsers, aircraft tugs, external power units, air conditioning carts, and loading equipment. By necessity a supply of DEF must be kept on hand to service these vehicles.

Fuel System Icing Inhibitor (FSII), often referred to as "Prist", is also a fluid that is commonly found on most aerodromes. Like DEF, FSII is clear, colourless and virtually odourless. However, unlike DEF, FSII is intended to be mixed with jet fuel to help prevent the formation of ice crystals in the fuel whilst in flight. Most refuelling tenders have an integral tank for FSII so that the icing inhibitor can be selectively added during the aircraft refuelling process.

Major problems can occur when Diesel Exhaust Fluid is inadvertently substituted for Fuel System Icing Inhibitor.

The Problem

Due to their physical similarities (colour, clarity, odour), it is hard to distinguish between DEF and FSII. If their containers similar in size and shape, are not clearly labeled, or if the two products are not stored separately, there is a risk that DEF might be inadvertently be identified as FSII and added to the icing inhibitor tank on a fuelling tender and, from there, be introduced to the fuel system of an aircraft.

When mixed with jet fuel, DEF will react with certain chemical components of the fuel to form crystalline deposits within the fuel system. These crystalline deposits can then accumulate in filters, engine fuel nozzles, and fuel metering components resulting in a loss of engine power and potential flameout.

The Solution

Currently, there is no means to test for DEF contamination in jet fuel. Therefore, the solution to the potential contamination problem is one of prevention. To achieve this, the U.S. National Transportation Safety Board (NTSB) offers the following guidance:

  • Do not store or temporarily place chemicals into unlabeled containers; always use containers and labels that meet OSHA (Occupational Safety and Health Administration) requirements.
  • Ensure that all containers (including bulk storage tanks and larger cube tanks) are clearly marked with 4-inch or larger stenciled letters visible from all sides. Use “DIESEL EMISSION FLUID (DEF)” for all DEF fluid and “JET FUEL SYSTEM ICIING INHIBITOR” for all FSII storage containers.
  • Add a label to all DEF containers that says, “NOT FOR AVIATION USE.”
  • Even when the containers are clearly marked, do not store DEF and FSII close to each other since it is hard to differentiate the clear, colorless liquids.
  • All staff should be trained on the storage locations of DEF and FSII, the packaging and labeling of both chemicals, and the hazards associated with DEF fuel contamination.
  • Fueling agents or operators should remove from aircraft and discard jet fuel or FSII suspected of being contaminated with DEF. Do not attempt to repurpose DEF contaminated fuel or FSII to other aircraft or vehicles.

Accidents and Incidents

  • C550, en-route, north of Savannah GA USA, 2019 - On 9 May 2019, a Cessna 550 level at FL 350 experienced an unexplained left engine rundown to idle and the crew began descent and a diversion to Savannah. When the right engine also began to run down passing 8000 feet, an emergency was declared and the already-planned straight-in approach was successfully accomplished without any engine thrust. The ongoing Investigation has already established that the likely cause was fuel contamination resulting from the inadvertent mixing of a required fuel additive with an unapproved substance known to form deposits which impede fuel flow when they accumulate on critical fuel system components.

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