Diesel exhaust fluid (DEF) is a urea-based chemical that is added to ground vehicle emissions systems

to reduce nitrogen oxide (NOx) emissions. DEF is not designed, nor approved, for use in jet fuel.

If it is inadvertently added to jet fuel, as has happened in several incidents over the last 2 years, DEF will

react with certain chemical components to form crystalline deposits in the fuel system. The crystalline

deposits can then accumulate on filters, engine fuel nozzles, and fuel metering components and result

in a loss of engine power.

In January 2010, the Environmental Protection Agency began requiring reduced NOx emissions from

on-road medium and heavy-duty diesel vehicles. In 2014, all-new off-road vehicles, including airport

refueling trucks, were required to meet these standards. As a result, all diesel vehicle manufacturers

now use a selective catalytic reduction system and DEF to reduce harmful emissions.

Because the use of DEF is now required for all new off-road vehicles, its presence on airport property is

becoming more prevalent as refueling trucks are replaced.

DEF can be mistaken for other clear, colorless liquids, such as fuel system icing inhibitors (FSII). Both

products can be purchased in bulk and transferred to smaller containers for ease of use. Both are

usually stored in milky white containers. Also, airport refueling trucks are serviced with both products.

In a May 2019 incident that the NTSB is investigating, a Cessna C550

experienced a total loss of engine power to both engines while on an air

medical flight. The crew diverted to a nearby airport and safely landed

the airplane. The two airline transport pilots, two medical crew, and

three passengers were not injured. An airport lineman later reported

that the day before the incident, he had combined two partially filled

containers; one contained FSII and the other contained DEF, which he

had mistaken for FSII. He then added the combined fluid to the truck’s

FSII reservoir. The following day, the incident airplane was fueled with

480 gallons of Jet A fuel with FSII additive mixed at the time of fueling.

Analysis of fuel samples, fuel system filters, and fuel screens from the

airplane indicated the presence of urea, the primary chemical found in

DEF. (ERA19IA178)

Although not investigated by the NTSB, two other incidents have

occurred that resulted from the inadvertent introduction of DEF into

aircraft fuel tanks by way of a refueling truck FSII reservoir.

• In November 2017, at Eppley Airfield, Omaha, Nebraska, seven aircraft

had DEF directly injected and six aircraft were refueled with equipment

that was exposed to DEF.

• In August 2018, at Miami-Opa Locka Executive Airport, Opa-Locka,

Florida, five aircraft had DEF directly injected and nine aircraft were

refueled with equipment exposed to DEF.

At both airports, all 12 aircraft that were directly exposed to DEF

experienced service difficulties and unplanned diversions resulting

from clogged fuel filters and fuel nozzle deposits (see the special

airworthiness information bulletins [SAIBs] listed below for more


What can fuel providers do?

Because bulk fluids and chemicals are generally stored in the same

areas, use the following tips to prevent confusion:

Do not store or temporarily place chemicals into unlabeled containers;

always use containers and labels that meet OSHA 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 say, “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


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.

Review and consider implementing the recommendations contained within

the June 11, 2019, Aircraft Diesel Exhaust Fluid Contamination Working

Group Report.

The Federal Aviation Administration (FAA)

released SAIBs HQ-18-08R1 and HQ-18-28,

which cite the DEF contamination events from

2017 and 2018 discussed above and provide

more information on the hazards of DEF fuel


The National Air Transportation Association’s

Safety 1st DEF Contamination Prevention Training

for line personnel, fixed-based operator general

managers, and customer service representatives

reviews storage, handling, and personnel training

procedures to prevent fuel contamination.

The FAA released Safety Alert for Operators

18015 in November 2018, which discusses the

safety concerns for aircraft serviced with jet fuel

contaminated with DEF and highlights its hazards

The NTSB is an independent federal agency charged by Congress with investigating every civil aviation

accident in the United States and significant accidents in other modes of transportation—highway, marine,

railroad, and pipeline. The NTSB determines the probable cause of the accidents and issues of safety

recommendations aimed at preventing future accidents. For more information, visit www.ntsb.gov.

*re-published from NTSB safety alert 79