Cessna 550 N744AT incident near Savannah, Georgia

Probable cause

The lineman's inadvertent introduction of diesel exhaust fluid (DEF) to the fuel truck fuel system icing inhibitor reservoir, which resulted in fuel contamination and a total loss of engine power. Contributing to the accident was the fixed-base operator's failure to ensure that DEF was stored off-airfield in accordance with their own policy.

NTSB analysis

The crew of the air medical flight reported that, while in cruise flight at 35,000 ft mean sea level, the left engine began losing power. The crew requested a lower altitude from air traffic control, shut down and secured the left engine, and diverted. While descending, the right engine lost all power, and the crew performed an uneventful forced landing to the diversion airport.

Several days after the incident, the lineman who worked at the fixed-base operator (FBO) where the airplane was most recently fueled realized that he had inadvertently combined the contents of an unmarked, partially empty container of fuel system icing inhibitor (FSII) with a partially-empty container of diesel exhaust fluid (DEF), assuming that both containers contained FSII. The container of DEF was co-located with the container of FSII, which was contrary to the FBO's policy of isolating DEF off-airfield. The lineman then serviced the fuel truck FSII reservoir with the DEF/FSII combination. The following day, the incident airplane was fueled with Jet A fuel from the truck, and FSII additive from the truck was mixed at the time of fueling. Postincident testing revealed the presence of urea, one of the primary components of DEF, in the incident airplane's fuel system samples.

When combined with hydrocarbon jet fuels, urea forms a solid precipitate, or clathrate, that is known to interfere with fuel filtering and fuel system operation. The presence of DEF in the incident airplane's fuel system resulted in a blockage, which led to the total loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA19IA178 for the full record.