The Alarming Threat of Diesel Exhaust Fluid Fuel Contamination

Jan. 3, 2020

In the past 20 months, there have been at least three incidents in which aircraft have been fueled up, taken off and experienced multiple-inflight aircraft engine failures. That number includes two Cessna Citation 550 twinjets doing air ambulance flights. Thankfully, and luckily, they all landed safely and no one was injured.

The cause of these near-tragedies was the introduction of diesel exhaust fluid (DEF) into the fuel supply. The threat of DEF contamination of a fuel supply is a harrowing one, but, whether it’s by cautious loading of an aircraft tug or an update to ground crew training, with some basic precautions even near-tragedies can be averted.

What is DEF?

DEF is a non-hazardous fluid that, to meet federal environmental regulations, is added to the exhaust systems of diesel vehicles to break polluting emissions into water and nitrogen. While it’s good for the environment, the water and urea mixture can result in the formation of crystals that can plug up fuel filters, damage aircraft engines, and result in engine failure. DEF should never be used as an additive for any fuel, diesel or aircraft, and is sprayed directly into exhaust systems.

The Real-Life Risks

Unfortunately, it’s not all that hard for ground crew to accidentally contaminate the fuel supply with DEF. Investigations into the recent engine failure incidents have found that the most common cause of DEF contamination is crew members confusing the DEF with the additive Fuel System Icing Inhibitor (FSII). Both liquids are clear and colorless, they can be stored in similar-looking containers, and are often stored next to each other. Particularly when the work gets rushed, it’s easy to make mistakes.


Among the most direct and effective solutions for avoiding any dangerous DEF contamination events is choosing “NO-DEF” ground support equipment (GSE), which simply removes DEF from the equation. For example, technological advances have allowed GSE manufacturers and vendors to focus on producing and providing a cleaner, lower-emission breed of aircraft tug, pushback tractor, and aircraft ground power unit (GPU), among other GSE offerings.

Consider GSE provider AERO Specialties’ revolutionary AERO JetGo 99 28V Diesel-Hybrid GPU that offers the highest continuous amperage available in the class. Also, with the engine’s hybrid-efficiency, emissions are reduced sufficiently enough that DEF is unnecessary.

Storage, Labeling, and Ground Crew Training

In addition to NO-DEF GSE, there are basic preventive steps that can avoid confusion and contamination. A big one is simply storing DEF away from any fuel additives, particularly FSII. DEF control and recording steps include ensuring that DEF is kept behind lock and key as well as logging and recording any use or transfer of it. Labeling that is clear, legible, easy to read, and unobscured will further reduce the risk of contamination. An additional safety measure could include ordering and storing DEF in noticeably different containers than the FSII and other additives. Along with these steps, ensuring that all members of the ground crew are well-trained on DEF use and handling is crucial.