WPR23FA185

The pilots’ loss of airplane control following a catastrophic fire that started on the left side of the aft equipment bay (tail cone). The fire likely initiated from a leak from the left fuel motive flow line due to a b-nut that loosened for reasons that could not be determined based on available evidence.

The accident airplane took off as the lead airplane in formation with a second Lear Jet airplane flying as wingman in a close formation position. The purpose of the flight was to participate in an exercise with the United States Navy in an over-water training area. Shortly after entering the training area at 15,000 ft mean sea level (msl) the wingman positioned on the right side of the accident airplane, observed the flaps on the accident airplane were partially extended. They notified the pilot of the accident airplane who acknowledged the radio call. The wingman then observed the flaps retract and observed white or gray colored “smoke or gas” coming from the left aft side of the airplane. The pilot in the accident airplane then radioed that they detected an odor in the cabin. Seconds later, the wingman observed red fluid on the underside of the tail cone followed by flames coming from around the aft equipment bay (tail cone) access door. They informed the accident pilots that their airplane was on fire and the accident pilot declared an emergency along with their intentions to land at a nearby airfield on the island. The wingman took over leading the formation and maneuvered in front of the accident airplane. The wingman last observed and heard radio transmissions from the accident airplane a short time later as they descended through about 7000 ft msl. The flight was above an overcast cloud layer that obstructed the view of the island at that time. Recorded ADS-B data showed that the accident airplane subsequently made a series of descending turns before the data ended. The airplane wreckage was located underwater about 4 miles northwest of the last ADS-B data point. The wreckage was highly fragmented, and the debris field extended several hundred feet along the ocean floor. Salvage operations were able to recover about 40 percent of the airplane wreckage. Examination of the wreckage showed areas of smoke and fire damage in portions of the airplane from the center wing fuselage outboard through the left- and right-wing roots and aft throughout the empennage. The aft equipment bay forward bulkhead, which also served as the fuselage fuel tank aft bulkhead, exhibited damage on the upper left side consistent with exposure to a focused heat source such as a fire from a leak in a pressurized fuel or hydraulic line. Additional evidence of focused fire damage was identified in the left engine pylon, which was located outboard and adjacent to the aft equipment bay. The effected area of the pylon contained engine fire detection circuits. Examination of the hydraulic shutoff valves attached to the hydraulic reservoir found that the left hydraulic shutoff valve was closed and the right valve was in the open position. The position of the left hydraulic shutoff valve indicates the aircrew likely shut down the left engine due to a fire indication (A hydraulic valve will close if the FIRE PULL tee-handle switch is activated by the flight crew in the event an engine fire is detected). Investigators were unable to determine if the witnessed flap extension and retraction was initiated by activation of the flap selector switch or induced by fire damage. The aft equipment bay houses electrical equipment capable of providing an ignition source for a fire, and hydraulic and fuel system components capable of providing fuel for a fire in the event of a leak. The left fuel motive flow line was intact from the fuel pump up to the fuselage fitting, where the line passes through the fuselage skin into the aft equipment bay near the aft left engine mount. The fuselage fitting had the stainless-steel fuel line and b-nut attached on the outboard, engine side. The inboard, aft equipment bay side of the fitting did not have an aluminum b-nut attached or the line that connects the fitting to the motive flow valve. The left engine hydraulic pressure line and PT2 line, which pass through the fuselage into the aft equipment bay adjacent to the motive fuel line, each had aluminum b-nuts present on the interior side of their respective fuselage fittings and the interior lines for each was sheared at the fitting. The left fuel motive flow fuselage fitting and the hydraulic fluid return fitting were examined. No thermal damage was observed on the outboard nuts and attached portions of tubing. Examination of the threads on the inboard side of the motive flow fitting did not exhibit evidence of thread stripping and comparison between the two fittings did not reveal any physical differences to explain why one nut remained attached and the other did not. The reason the motive flow nut was missing from the fitting could not be determined. Observed fire and heat damage patterns indicate the fire likely started in proximity to the where the pressurized fuel motive flow line connected to the firewall fitting where the missing b-nut was located. A leak from the pressurized motive flow line would have sustained and allowed an uncontrollable fire to develop. The sustained fire likely affected controllability of the airplane and resulted in the pilots’ loss of control of the airplane.

Autopsies of the crewmembers were performed by the County of Los Angeles Department of the Medical Examiner, Los Angeles, California. The cause of death for both pilots was determined to be blunt force trauma. Toxicological testing was not performed. The airplane wreckage was located in the Pacific Ocean about 13 miles south of NUC and about 1.5 miles west of San Clemente Island at a depth of about 300 ft. The identified wreckage debris field was surveyed using sonar equipment and remotely operated vehicles (ROV). The debris field extended several hundred feet along the ocean floor in a northwest direction. A portion of the airplane was able to be recovered in October, 2023. Investigators examined the wreckage two different times. The wreckage was fragmented and exhibited corrosion consistent with saltwater immersion. It was estimated that about 40% of the airplane was recovered (figure 2). The identifiable wreckage included portions of the nose cone, cockpit instrument panel, cabin floorboard, portions of both wings, both engines and nacelles, some components from the aft equipment bay, and the vertical stabilizer and left horizontal stabilizer. Impact damage and a lack of complete wreckage prevented verification of flight control continuity. Figure 2 –Portions of the Airplane Recovered (Highlighted in Green) Main hydraulic system components consisting of the hydraulic reservoir, accumulator, pressure filter, return filter, and auxiliary hydraulic pump were identified and examined. The input ports of both left and right hydraulic shut off valves that were attached to the hydraulic reservoir were probed and found that the left valve was in the closed position and the right valve was in the open position. The flap control valve was not located. Both engines were separated from the airplane structure when examined. Both engines’ fan blades and first stage compressor blades exhibited bending opposite of rotation. Neither engine was able to rotate to verify internal continuity and both engines exhibited corrosion degradation. No mechanical anomalies were noted with either engine during examination. The left fuel motive flow line was intact from the fuel pump up to the fuselage fitting, where the line normally passes through the fuselage skin into the aft equipment bay near the aft left engine mount. The fuselage fitting on the aft equipment bay side did not have a b-nut attached nor the line that should connect the fitting to the motive flow valve. The left engine hydraulic pressure line and inlet total pressure (PT2) line, which pass through the fuselage into the aft equipment bay adjacent to the motive fuel line, each had aluminum b-nuts present on the interior side of their respective fuselage fittings and the interior lines for each was sheared at the fitting (figure 3). Figure 3 – Left Fuselage Fittings and B-Nuts The left fuel motive flow fuselage fitting and the hydraulic fluid return fitting were examined. Both were steel bulkhead type fittings, and both fittings were corroded. No thermal damage was observed on the outboard nuts and attached portions of tubing. On the inboard side, the motive flow fitting was missing the aluminum nut and tubing. Examination of the threads on the inboard side of the motive flow fitting did not exhibit evidence of thread stripping. The hydraulic fluid return fitting still had the aluminum nut attached on the inboard side and the aluminum tubing had sheared at the nut face. According to the operator, there was no record of maintenance being performed on the motive flow fitting and b-nut since the airplane had been assembled. Portions of the wreckage exhibited exposure to heat and fire. See the Fire section of this report for a detailed description of those findings. On May 10, 2023, at 0749 Pacific daylight time, a Gates Lear Jet 36A, N56PA, was destroyed when it was involved in an accident near San Clemente Island, California. Both pilots and the additional crewmember were fatally injured. The airplane was operating as a Title 14 Code of Federal Regulations Part 91, other work use flight. Witness interviews, ADS-B data, Air Traffic Control Transcripts, and flight data from the accident airplane’s left engine Digital Electronic Engine Control (DEEC) Incident Recorder were compared and correlated to establish a sequence of events and an approximate timeline (figure 1). The airplane departed Point Mugu Naval Air Station (NTD), Oxnard, California about 0723 as the lead airplane in formation with a second Lear Jet utilizing the callsigns FENIX01 (N56PA) and FENIX02 (N544PA). The purpose of the flight was to participate in training exercises with the United States Navy in Warning Area 291 (W291). The flight proceeded south towards San Clemente Island with FENIX02 flying in formation with FENIX01. About 0740, after entering W291 at 15,000 ft msl, FENIX02 (positioned on the right side of FENIX01) observed FENIX01’s flaps partially deploy. They asked the pilots of FENIX01 if they had deployed flaps, and they replied that they didn’t think so. FENIX02 then observed the flaps retract and seconds later observed white or gray colored smoke coming from the left, aft side of the airplane. FENIX02 notified FENIX01 of the smoke and FENIX01 responded that they detected an odor in the cabin. At this point the aircrew of FENIX02 believed FENIX01 was no longer flying on autopilot because the airplane was “lightly rocking and pitching.” FENIX02 then observed pink fluid coming from the tail section of the airplane and informed FENIX01 that they had a hydraulic leak. FENIX02 then observed flames coming from around the aft equipment bay access door and told FENIX01 that they were on fire. At 0740:30 FENIX01 started a left, descending turn and declared an emergency with the Navy area controller at 0741:00. They stated their intentions to land at San Clemente Island Naval Auxiliary Landing Field (NUC), which was about 32 miles north of their position. Shortly thereafter FENIX02 observed the drag chute, mounted on the aft equipment bay access door, depart from FENIX01. FENIX02 then took over as the lead airplane for the formation and maneuvered in front of FENIX01. FENIX01 was last observed passing underneath FENIX02 from the left to right side as they descended between 9,000 and 7,000 feet msl. No additional radio transmissions were heard from FENIX01 from that point. According to the captain of FENIX02 there was an overcast marine layer of clouds below them from about 2,000 ft to 4,000 ft msl that obscured most of San Clemente Island and NUC. FENIX01 subsequently made a series of right-hand descending turns, during which the DEEC recorded several additional faults between about 0745:43 and when the DEEC data ended about 0746:32 following indications of a loss of power to the DEEC. The last recorded ABS-B data point at 0749:11 showed the airplane at 1,338 ft and heading northwest about ½ mile off the southwest side of San Clemente Island (figure 1) and was about 4 miles southeast of where the airplane’s wreckage was later located. Figure 1 – Flight Path and Sequence of Events Fire and heat damage was concentrated in the vicinity of the aft equipment bay (tail cone), aft portion of the fuselage fuel tank, both wheel-well areas and the inboard section of both flaps (figure 4). The right side of the aft equipment bay fuselage structure had heat/fire damage. The left side of the equipment bay and the entire fuselage structure aft of the avionics bay was not recovered. Figure 4. Diagram showing areas of fire/heat damage. (Diagram source: Learjet 20/30 Series Structural Repair Manual) The upper left side of the fuselage fuel tank aft bulkhead exhibited a region of damage that was different than what was observed throughout the rest of the bulkhead (figures 5 and 6). This region of damage was mostly devoid of any remaining charred primer paint and the aluminum substrate was eroded. In some ar