# NTSB WPR23LA167: Gates Learjet 35A, N135SH, Page, AZ, April 24, 2023 - Type: NTSB occurrence investigation - Classification: Occurrence · No injuries · Class 3 · Limited · Final report - Date: April 23, 2023, 17:29 local (Apr 24, 2023, 00:29 UTC) - Place: Page, AZ - Coordinates: 36.926069, -111.44835 - Nearby events: https://openairdocs.org/map.md?lat=36.9261&lon=-111.4484&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute-cruise: Fuel contamination](https://openairdocs.org/ntsb-events/193) - Highest injury: No injuries - Injuries: 4 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Bombardier, Honeywell, Federal Aviation Administration - NTSB case key (mKey): 107149 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR23LA167 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/107149/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=107149 - Page: https://openairdocs.org/ntsb/WPR23LA167 ## Probable cause The fueler’s inadvertent addition of diesel exhaust fluid (DEF) to the FSII reservoir on the fuel truck, and its subsequent mixture with the JET A that was used to fuel the airplane, resulting in a total loss of power to the right engine and a partial loss of power to the left engine. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Fluids: fuel](https://openairdocs.org/ntsb-findings/01071010): Aircraft › Fluids/misc hardware · Incorrect use/operation (probable cause); cases with it: [Gates Learjet 35A, 1 case](https://openairdocs.org/ntsb-findings/01071010/cases.md?model=5170602), [All aircraft, 1,695 cases](https://openairdocs.org/ntsb-findings/01071010/cases.md) - [Fluids: fuel additive](https://openairdocs.org/ntsb-findings/01071040): Aircraft › Fluids/misc hardware · Incorrect use/operation (probable cause); cases with it: [Gates Learjet 35A, 1 case](https://openairdocs.org/ntsb-findings/01071040/cases.md?model=5170602), [All aircraft, 4 cases](https://openairdocs.org/ntsb-findings/01071040/cases.md) - [Fluids: fuel](https://openairdocs.org/ntsb-findings/01071010): Aircraft › Fluids/misc hardware · Fluid condition (probable cause); cases with it: [Gates Learjet 35A, 1 case](https://openairdocs.org/ntsb-findings/01071010/cases.md?model=5170602), [All aircraft, 1,695 cases](https://openairdocs.org/ntsb-findings/01071010/cases.md) - [Use of equip/info: use of equip/system](https://openairdocs.org/ntsb-findings/02063015): Personnel issues › Task performance · Ground crew (probable cause); cases with it: [Gates Learjet 35A, 2 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md?model=5170602), [All aircraft, 1,391 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute-cruise: Fuel contamination](https://openairdocs.org/ntsb-events/193) (defining event); cases with it: [Gates Learjet 35A, 1 case](https://openairdocs.org/ntsb-events/193/sequence.md?model=5170602), [All aircraft, 225 cases](https://openairdocs.org/ntsb-events/193/sequence.md) ## Analysis The flight crew requested the airplane be topped off with fuel that included a fuel system icing inhibitor additive (FSII/PRIST). The fixed-base operator serviced the airplane with 674 gallons of Jet A and FSII that had inadvertently been mixed with diesel exhaust fluid (DEF). The airplane then departed on a repositioning flight. About 30 minutes after departure, while level at flight level (FL) 350, the captain observed erratic temperature indications on the left engine inlet turbine temperature (ITT) gauge. The right engine indications were normal at the time. Shortly thereafter, he observed noticeable variations in fuel flow accompanied by small variations in the right engine N1 (fan rotational speed) and N2 (turbine rotational speed). The captain reported that the right engine variations became significant and resulted in “yaw issues”; according to the first officer, the right engine was “pulsing.” The flight crew informed air traffic control (ATC) that they wanted to return to the departure airport. During this time the right engine power decreased to idle. The captain stated that about this time the right engine fuel flow indicator indicated 0 lbs/hr, indicating the engine flamed out. They declared an emergency and informed ATC they were unable to maintain altitude and would have to descend. The controller gave them an immediate clearance to descend. The captain stated they did not try to restart the right engine at this time because their altitude was above the restart envelope. He stated that as they descended the variations in parameters for the left engine were increasing and, after descending through FL300, they unsuccessfully attempted to restart the right engine. The variations in the left engine parameters continued to increase along with a decay in the max N1 and the engine was unresponsive to thrust lever movements. The crew then informed ATC that they needed to land at the nearest airport. The flight crew completed a successful emergency landing to a nearby airport, which resulted in no damage to the airplane and no injuries to the crew. The investigation found that the refueler had erroneously deposited diesel exhaust fluid into the FSII tank of his fuel truck, which he subsequently used to refuel 5 airplanes, including the incident airplane. Crystalline deposits were discovered in the fuel tank. Postaccident examination of the airframe and engines did not reveal contamination of the airframe filter elements; however, crystalline deposits began to appear on the elements about 5 months later. The reason for the latency was not determined. The rest of the fuel systems exhibited various quantities of crystalline deposits, with extensive formation at the fuel nozzles of both engines. The contamination at the nozzles would have interfered with the spray pattern of the affected nozzles, inhibiting their ability to atomize the fuel and resulting in the loss of power to both engines. ## Factual narrative On April 23, 2023, about 1729 Pacific daylight time, a Learjet 35A, N135SH, sustained an uncommanded decrease in power on both engines near Page, Arizona. The captain, first officer, and two medical crewmembers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. According to the captain, the flight was a non-patient MEDEVAC flight destined for Chicago, Illinois. The captain placed a fuel order before the flight with Signature Aviation at Harry Reid International Airport (LAS), Las Vegas, Nevada, that included PRIST, a FSII; he observed the attendant write “T/O +” on the paperwork that pertained to his fuel order. The airplane was subsequently refueled with 674 gallons of JET A fuel. The airplane departed LAS at 1646 with the first officer flying and the captain monitoring the flight. They climbed to FL350 and accelerated to their cruise speed, which occurred about 19 minutes after takeoff. About 11 minutes later (30 minutes after departure), the captain observed erratic temperature indications between 0° and 750° from the left engine’s ITT gauge. The other engine indications were normal at the time. About 2 minutes later he observed small variations in the left engine’s ITT, followed by noticeable variations in fuel flow, as well as small variations in the right engine’s N1 (fan rotational speed) and N2 (turbine rotational speed). The captain reported that the right-engine variations became significant and resulted in “yaw issues”; according to the first officer, the right engine was “pulsing.” The flight crew informed ATC that they wanted to return to LAS and the controller gave them instructions to make an immediate left turn. During this time the variations in the right engine’s parameters increased and right-engine power decreased to idle. A review of digital electronic engine control (DEEC) data showed that the right engine reduced to idle power at 1719 and subsequently began to show fluctuations in N1 and N2 without corresponding movement of the power lever beginning at 1720. The right engine returned to idle thrust at 1721 without a consistent movement of the power lever. The captain stated that about this time the right engine’s fuel flow indicator indicated 0 lbs/hr. They declared an emergency and informed ATC they were unable to maintain altitude and would have to descend. The controller gave them an immediate clearance to descend. The DEEC data for the left engine showed fluctuations in N1 and N2 without a corresponding movement of the power lever beginning at 1717. The flight crew reported that the variations in the left engine’s parameters became more erratic as the airplane passed through FL300 at 1725. The captain observed a slow decay in the left-engine N1 at this time and noted that the engine was unresponsive to thrust lever movements. He also reported that an attempt to restart the right engine was unsuccessful. According to the first officer, the left engine alsoreduced to idle power at this time. At 1729, as the airplane passed through FL270, he advised the controller that they would make an emergency landing in Page, Arizona. The captain dumped fuel to reduce the airplane’s landing weight, and the first officer started a circling descent. After they passed below 12,000 ft mean sea level, ATC cleared them for the visual approach to runway 15 at their destination airport. According to the first officer, after the captain secured the fuel jettison valves, the captain took over the flying duties. After the airplane was established on short final the captain lowered the landing gear at 220 KIAS. The captain noted that he was intentionally high and fast at this point, in order to perform a dead stick landing. At 1744, they touched down successfully at a slightly higher than normal speed and slightly long. The airplane came to rest about 650 ft from the runway end. The first officer noted that the left engine was still at idle and the right engine was off. DEEC data showed the left-engine N2 between 50% and 60% for about 4 minutes, 23 seconds; about half of this time was before landing and the other half was after landing. In addition, after landing the ITT value dropped after the engine was shut down, when all the engine parameters decreased. According to the data, after the reduction in thrust to the right engine at 1719 until the end of the flight, the engine remained at an idle, sub-idle or non-operational condition and did not respond to pilot commands. According to Safety Alert for Operators No. 18015, published November 13, 2018, by the Federal Aviation Administration: “DEF forms crystalline deposits that are not soluble in fuel, so they cannot be removed by flushing the aircraft fuel systems with jet fuel. Although the deposits are soluble in water and other polar solvents, use of these chemicals may have adverse consequences on aircraft and engine fuel system materials. Operators should contact Original Equipment Manufacturers (OEM) to develop inspection techniques and maintenance actions appropriate for each specific aircraft model type and its level of exposure.” Fueler Investigation According to the company that fueled the airplane, the fueling truck was quarantined immediately after the company was notified of the emergency landing. An immediate inspection of the quality records and analysis of JET A samples revealed that the fuel from the truck’s tank was clean and met industry standards. A field test of the fluid within the truck’s FSII reservoir showed a 30% concentration of DEF within the sample. According to the company, a laboratory test of a sample taken from the nozzle of the FSII tank showed a nitrogen content that exceeded 1.0, which suggests the presence of DEF. The fueler reported that a line service technician informed them that he was trying to be productive during a busy day and decided to make sure his truck had DEF. He located a container and inadvertently poured about 1 gallonof DEF into the FSII reservoir, which contained 1 gallon of FSII already inside the reservoir. The fueler stated that he was unfamiliar with DEF servicing as he had never serviced a truck with DEF before. The company’s investigation also discovered that the fueler had not seen the company’s internal bulletin on DEF contamination prevention and that he did not notice the label that showed the contents of the FSII tank. The fueler also noted that the position of the location of the FSII and DEF tanks on this truck were in opposite positions on another truck. A total of five airplanes were refueled with FSII (PRIST) from the contaminated fuel truck; however, none of the other affected airplanes were inspected as the refueler did not disclose their contact information. The incident airplane was the only airplane that was supplied with the contaminated fuel that reported a loss of power. Three of the operators confirmed to the refueler that they drained their fuel tanks and engine filters to remove the fuel. One operator removed approximately 200 gallons of the contaminated fuel and discovered crystalline contaminates within the filter vessels. Fuel System and Component Examinations The airplane was equipped with two tip tanks (357 gallons total), two wing tanks (374 gallons total), and a 200-gallon fuselage tank. Fuel was driven to the engine fuel nozzles via a low-pressure airframe fuel pump that directed fuel through a fuel filter (the low-pressure filter), a high-pressure fuel pump, a fuel control unit, and then a flow divider. Fuel Filters, Fuel Control Units, and Flow Dividers The left and right airframe and engine fuel filter elements were void of any debris. The airframe and engine filters were subsequently shipped to a secure facility where they were stored for a period of about 5 months, during which time there was extensive formation of crystalline deposits on all of the filters. An examination of the left engine’s fuel control unit (FCU), the left-engine fuel pump, and right- and left-engine flow dividers was completed 10 months after the incident. The left engine’s FCU plate valve inlet screen associated with the main fuel flow metering valve was mostly obstructed. The inlet screen associated with the main fuel flow metering valve and the fuel flow divider screen exhibited trace amounts of a substance that was hard, brittle, and mixed with jet fuel, giving it a gelatinous appearance. The right engine’s fuel flow divider inlet screen was mostly obstructed by the same substance. A bench test of the right engine’s FCU and fuel pump assembly showed that it was able to produce a fuel flow from 135 pph to 1985 pph. According to the engine manufacturer, max cruise flight requires a fuel flow of 615 pph. Fuel Nozzles According to the engine manufacturer, within each fuel nozzle there are two separate paths of fuel flow. The secondary fuel flows through the center of the fuel nozzle body while the primary fuel flows through a concentric pathsurrounding the center of the secondary fuel flow path. Both fuel flow paths contain a “last chance screen” for fuel to pass through before being introduced into the combustion chamber. During engine operation the primary and secondary fuel flows are directed to the 12 fuel nozzles. When the engine is started, fuel is directed through the primary fuel path, after which the secondary flow path is energized for engine acceleration. The primary and secondary flow paths, together, are used during all other times of engine operation. Both the left- and right-engine fuel nozzle assemblies were flow tested under 100 psi with the primary and secondary flow paths tested separately. A flow test and inspection of the right engine fuel nozzle assembly revealed that, during the primary flow test, nozzles 1, 3, 4 and 12 did not exhibit spray from either of the nozzle heads. Nozzles 5 and 6 dripped when tested. The secondary flow test revealed that nozzles 1, 3, and 11 did not spray, nozzles 2 and 12 exhibited an intermittent spray, nozzle 9 dripped, and nozzle 10 displayed as a stream. Four of the fuel nozzles were chosen for disassembly and further examination. All four fuel nozzles showed varying degrees of crystalline contaminates. According to the engine manufacturer, “functional flow testing of the [right-engine] fuel manifolds showed limited to no flow in all the primary and secondary fuel nozzles. Functional flow testing of the fuel manifolds showed limited to no flow in all the primary and secondary fuel nozzles. The fuel flow blockages observed would have prevented normal engine operation. Disassembly of 4 of the 10 fuel nozzles displayed crystalline contaminates in the primary and secondary fuel circuits.” The primary flow test of the left engine fuel nozzles showed that nozzles 1, 2, 4, 5, 6, and 12 exhibited no spray. Nozzles 3 and 10 displayed a “fair” spray quality and nozzle 9 was “dripping.” The secondary flow test revealed a “fair” spray quality from nozzles 2, 3, 4, 5, 9. Nozzles 1, 10, 11, and 12 exhibited “poor” spray quality. Only one of the 12 fuel nozzles met the specification requirements during testing. Four of the fuel nozzles were chosen for disassembly and further examination. All four fuel nozzles displayed crystalline contaminants in the primary and secondary fuel circuits. In their evaluation of the left engine’s fuel nozzles, the engine manufacturer noted, “Functional flow testing of the fuel manifolds from the left engine showed limited to no flow in all the primary and secondary fuel nozzles. The measured fuel flow and spray patterns observed would have prevented normal engine operation. Disassembly of 4 of the 10 fuel nozzles displayed crystalline contaminates in the primary and secondary fuel circuits.” ## Aircraft - Registration: [N135SH](https://openairdocs.org/aircraft/N135SH), registered now, S/N 335, built 1981 (OpenAirDocs) - Reported make and model: GATES LEARJET CORP. 35A - Registry model: [Gates Learjet 35A](https://openairdocs.org/model/gates-learjet-35a) (registry code 5170602) (OpenAirDocs) - Registry engine: [Garrett TFE731 series](https://openairdocs.org/engine/garrett-tfe731-series) (registry code 29001, turbo-fan) (OpenAirDocs) - Category: Airplane - Serial number: 335 - Year built: 1981 - Airframe hours: 14243.9 - Engines: Garrett TFE731 3500 lbs thrust turbo fan; Garrett TFE731 3500 lbs thrust turbo fan - Damage: No damage - Flight: Positioning, Part 91 - Operator: Medway Air Ambulance - Registered owner: Med Air LLC - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, December 15, 2022, 63 hours before - Departed: LAS, Las Vegas, NV - Destination: MDW, Chicago, IL - Flight plan: IFR - Airport: PGA, Page Muni, runway 15/33, 1,590 × 150 ft, asphalt - Equipment: Angle of attack indicator, ADS-B, Stall warning system, Onboard weather, Electronic multifunction display, Electronic flight bag or handheld device ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 68; injuries: none; airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, with waivers/limitations; last flight review February 4, 2023; seat: left; restraint: 3-point; flight hours: 5,606 total, 3,975 in this make and model, 3,705 as pilot in command, 174 in the last 90 days, 53 in the last 30 days, 6 in the last 24 hours - Co-pilot; age 34; injuries: none; airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, without waivers/limitations; last flight review January 23, 2023; seat: right; restraint: 3-point; flight hours: 1,920 total, 190 in this make and model, 1,400 as pilot in command, 155 in the last 90 days, 61 in the last 30 days, 6 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 330° at 11 knots, gusting 17 - Visibility: 10 statute miles - Lowest ceiling: 12,000 ft broken - Temperature: 72 °F, dew point 27 °F - Altimeter: 29.82 inHg - Observed at: KPGA - METAR: METAR KPGA 240053Z AUTO 33011G17KT 10SM BKN120 22/M03 A2982 RMK AO2 SLP056 T02221028= ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 20230423010559I](https://openairdocs.org/incident/20230423010559I), Apr 23, 2023: Gates Learjet Corp. 35A, Page, AZ, No damage. On 04/23/2023 at 18:24 MDT [privacy data omitted], Learjet 35A, s/n 335 departed klas las vegas, NV for kmdw midway, IL on a part 91 repositioning flight for… ## Docket (12 files) The files NTSB released with the investigation on November 21, 2024, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=107149. 27.8 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 9 pages, 1.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17361503&FileExtension=pdf&FileName=6120_Redacted-Rel.pdf - Accident photographs (item 2, PDF, 4 pages, 1.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17874947&FileExtension=pdf&FileName=Accident%20Photographs-Rel.pdf - N135SH captain's statement (item 3, PDF, 2 pages, 94 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362871&FileExtension=pdf&FileName=N135SH%20-%20Captain%27s%20report-Rel.pdf - N135SH - sic's statement (item 4, PDF, 1 page, 64 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362344&FileExtension=pdf&FileName=N135SH%20-%20SIC%20Report-Rel.pdf - Record of conversation (item 5, PDF, 2 pages, 88 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17734825&FileExtension=pdf&FileName=record%20of%20convo-Rel.pdf - Fuel testing by medflight (item 6, PDF, 1 page, 211 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362387&FileExtension=pdf&FileName=medflight%20intertek%20fuel%20testing_Redacted-Rel.pdf - FAA inspection of signature fuel truck (item 7, PDF, 2 pages, 159 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362397&FileExtension=pdf&FileName=FAA%20inspection%20signature%20fuel%20truck_Redacted-Rel.pdf - FAA inspection statement of N135SH (item 8, PDF, 2 pages, 952 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362682&FileExtension=pdf&FileName=N135SH%20Ramp%20inspection_Redacted-Rel.pdf - Honeywell right fcu test and evaluation (item 9, PDF, 27 pages, 3.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17874736&FileExtension=pdf&FileName=right%20fcu_Redacted-Rel.pdf - Honeywell field notes, April 2023 exam (item 10, PDF, 14 pages, 3.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17362692&FileExtension=pdf&FileName=Honeywell%20exam%20field%20notes_Redacted-Rel.pdf - Left fuel control unit, left flow divider, and left fuel pump exam (item 11, PDF, 52 pages, 16.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17872136&FileExtension=pdf&FileName=PEI%20exam%20and%20teardown_Redacted-Rel.pdf - Release of aircraft wreckage, NTSB form 6120.15 (item 12, PDF, 1 page, 127 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17502404&FileExtension=pdf&FileName=Release%20of%20Wreckage%20-%20N135SH_Redacted-Rel.pdf From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.