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C550, en-route, north of Savannah GA USA, 2019

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Summary
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.
Event Details
When May 2019
Actual or Potential
Event Type
Airworthiness, Ground Operations, Loss of Control
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft CESSNA 550 Citation 2
Operator Air Ambulance by Air Trek
Domicile
Type of Flight Public Transport (Non Revenue)
Origin Naples Airport (Florida)
Intended Destination Niagara Falls International Airport
Actual Destination Savannah-Hilton Head International Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Cruise
ENR
Location En-Route
Origin Naples Airport (Florida)
Destination Niagara Falls International Airport
General
Tag(s) En-route Diversion
GND
Tag(s) Fuel Loading
LOC
Tag(s) Loss of Engine Power
AW
System(s) Fuel,
Engine Fuel and Control
Contributor(s) Maintenance Error (valid guidance available)
Outcome
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Technical
Investigation Type
Type Independent

Description

On 9 May 2019, the crew of a Cessna 550 Citation 2, (N744NT) being operated by Air Ambulance By Air Trek on a non-scheduled aeromedical flight from Naples FL to Niagara Falls NY and in the cruise at FL 350 in day VMC began to observe a loss of thrust from the left engine and decided to turn back to Savannah GA which they had recently passed. During what became a single engine descent, the right engine also began to run down to idle. An emergency was declared and the already-planned straight-in approach was successfully accomplished without engine thrust and the aircraft was then towed off the runway.

Investigation

An Investigation is being carried out by the National Transportation Safety Board (NTSB). It was noted that prior to takeoff from Naples with a two pilot crew, two other crew members and three passengers on board, the aircraft had been positioned from its base at Punta Gorda FL where 480 gallons of fuel had been uplifted. The 47 nm sector from Punta Gorda to Naples was accomplished without event as was the climb to cruising altitude and the initial cruise at FL350 on the intended flight to Niagara Falls.

The pilots stated that approximately 1 hour and 20 minutes into the flight and at FL350, the Captain adjusted the N1 to “around 103%” but that also immediately after doing so, it had decreased. After a few repetitions of this behaviour but with all the engine instruments still indicating normally, the left engine had begun to "spool down very slowly”. After being unsuccessful in their attempt to recover the required thrust, the crew requested and were given a descent to a lower level and began this with the left engine at idle. The Captain then reported having noticed that the left engine oil pressure was zero and therefore shut it down.

After a 180° turn back towards the nearest suitable airport, Savannah, preparations were made for a single engine approach there as the descent was continued with 65% N1 set on the remaining right engine. However, at about 8,000 feet QNH, the right engine was reported to have become unresponsive to attempts to change its setting and had also begun to run down. At this point, the Captain declared an emergency and the crew then completed a successful straight-in glide approach to runway 19 at Savannah just 8 minutes after the first signs of an engine malfunction and the aircraft was then towed off the runway. The Co-pilot subsequently stated that during the period of engine malfunctions, the fuel filter bypass light for the right engine had illuminated but that for the left engine had not.

Several days after the event, a ramp worker at the Punta Gorda FBO remembered that the evening before the flight under investigation, he had noticed that the level of Fuel System Icing Inhibitor (FSII) on one of the fuel bowsers was low and had decided to top it up. On going to the shed where the inhibitor was stored, he found a partially filled FSII bottle and another partially filled bottle next to it and had combined the contents of the two bottles and then used this to top up the fuel bowser FSII reservoir. He realised that he had combined the contents of a 5 gallon FSII container and a 2½ gallon Diesel Exhaust Fluid (DEF) container. The Investigation then took fuel samples, the fuel system filters and the fuel screens from the aircraft and sent them for laboratory testing. This testing found that the contaminant urea, the primary chemical found in DEF, was present in the fuel system samples and that there was corresponding evidence of it in the removed fuel system components.

NB: As a result of this event and awareness of other 2018 instances where DEF contamination of bowser-supplied jet fuel had been found including an air turn back by a Dassault Falcon 900EX after multiple clogged filter warnings had occurred for one of its engines, a Working Group to assess the risk of aircraft fuel contamination by DEF was formed by four concerned US membership organisations, The Aircraft Owners and Pilots Association (AOPA), General Aviation Manufacturers Association (GAMA), National Air Transportation Association (NATA) and the National Business Aviation Association (NBAA). This Group published their Report on 11 June 2019 and provided a comprehensive list of both short and long term safety recommendations noting that “the risk of another inadvertent DEF contamination event is too great not to take a concerted, aggressive and multi-pronged, coordinated approach to prevent another occurrence".
On 23 July 2019, an NTSB Safety Alert was published addressing the risk of fuel contamination with DEF noting that diesel exhaust fluid (DEF) is a urea-based chemical that is added to ground vehicle emissions systems to reduce Nitrogen Dioxide (NOx) emissions and is not designed, or approved, for use in jet fuel. It also noted that “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 (and these) crystalline deposits can then accumulate on filters, engine fuel nozzles and fuel metering components and result in a loss of engine power”.

A Preliminary Report of the Investigation so far on which this summary is based was released on 6 June 2019.

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