Accident Sequence
The accident airplane, a Lear Jet, took off as the lead aircraft in a formation with a second Lear Jet serving as the wingman. The flight was destined for an over-water training area to participate in a United States Navy exercise. Shortly after entering the training area at 15,000 feet mean sea level, the wingman, positioned on the right side of the accident airplane, observed that the flaps on the lead aircraft were partially extended. The wingman notified the accident pilot, who acknowledged this radio call. Subsequently, the wingman saw the flaps retract and observed white or gray-colored smoke or gas emanating from the left aft side of the lead airplane. The accident pilot then radioed that an odor was detected in the cabin. Moments later, the wingman spotted red fluid on the underside of the tail cone, followed by flames emerging from around the aft equipment bay (tail cone) access door. The wingman informed the accident pilots that their airplane was on fire, and the accident pilot declared an emergency, indicating intentions to land at a nearby airfield on an island. The wingman assumed lead of the formation and maneuvered in front of the accident airplane. The wingman's last visual and radio contact with the accident airplane occurred a short time later as it descended through approximately 7,000 feet mean sea level. The flight was above an overcast cloud layer that obscured the view of the island at that time. Recorded Automatic Dependent Surveillance–Broadcast (ADS-B) data showed that the accident airplane subsequently made a series of descending turns before data transmission ceased.
Wreckage Recovery and Examination
The wreckage was located underwater about 4 miles northwest of the last ADS-B data point. The debris field extended several hundred feet along the ocean floor, and salvage operations recovered approximately 40 percent of the aircraft structure. Examination of the wreckage revealed 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, displayed 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, located outboard and adjacent to the aft equipment bay. This area contained engine fire detection circuits. Examination of the hydraulic shutoff valves attached to the hydraulic reservoir revealed that the left hydraulic shutoff valve was closed while the right valve was open. The position of the left hydraulic shutoff valve indicates that the aircrew likely shut down the left engine due to a fire indication, as the hydraulic valve closes when the FIRE PULL tee-handle switch is activated in the event of an engine fire. Investigators were unable to determine whether the observed flap extension and retraction was caused by activation of the flap selector switch or induced by fire damage.
Fire Origin Investigation
The aft equipment bay houses electrical equipment capable of providing an ignition source for a fire, as well as hydraulic and fuel system components that could supply fuel 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 a b-nut attached on the outboard (engine) side. However, on the inboard (aft equipment bay) side of the fitting, the aluminum b-nut and the line connecting the fitting to the motive flow valve were missing. The left engine hydraulic pressure line and PT2 line, which also 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, but the interior lines for each were 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 tubing portions. Examination of the threads on the inboard side of the motive flow fitting showed no 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 for the missing motive flow nut could not be determined. Observed fire and heat damage patterns indicate that the fire likely started in proximity to 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 an uncontrollable fire. The sustained fire likely affected controllability of the airplane and resulted in the pilots' loss of control.