History of Flight
On December 6, 2015, about 1407 Pacific standard time, an Aero Vodochody L-39C, registration N39AY, was destroyed when it impacted terrain near Apple Valley, California. The airline transport pilot and the pilot-rated passenger were fatally injured. The airplane was registered to and operated by Jettran LLC as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which departed Apple Valley Airport (APV) about 1406.
A friend of the pilot-rated passenger reported that the passenger was considering purchasing a partial ownership in another L-39 and was a fan of the accident pilot's performances in the Red Bull Air Races. After the friend introduced the passenger to the pilot, they arranged to fly together.
Witnesses reported that after refueling, the pilot delivered a 1-hour safety briefing to the passenger inside the airport terminal. They then boarded, and the pilot taxied to the run-up area. After a brief pause, the pilot taxied to runway 18 and began a ground roll. One witness reported dark grey-colored exhaust from the exhaust nozzle as the airplane began its takeoff roll.
When the airplane was about 3,400 ft down the runway, witnesses heard a "pop, pop, pop" sound followed by intermittent bright orange flashes from the exhaust nozzle. The airplane was about 100 ft above ground level; multiple witnesses reported that during previous takeoffs it reached about 500 ft by that point. A witness reported that the airplane appeared to slow down after the popping sound but did not lose altitude. As it reached the departure end of the runway, it rolled into a left-wing-low attitude. Two witnesses reported the airplane rolled 90° to the left. The airplane then descended rapidly, and the left wing impacted the ground followed by the nose. The wreckage slid and came to rest between two taxiways, and a fire ensued.
Personnel Information
The pilot accumulated a total of 23,200.3 flight hours, including 170 hours in the accident airplane make and model. He had flown 12.2 hours in the accident airplane between August 25, 2015, and November 20, 2015. His last recorded flight in the accident airplane was on October 27, 2015. The pilot was a competitive air racer since 1990 and participated in the Red Bull Air Races from 2004 to 2009, winning the World Series Championship in 2005 and 2007.
A friend reported that the pilot worked at Racing Jets, Inc. and that it was customary for him to conduct a 1-hour safety briefing with passengers. The pilot's wife stated that it was not uncommon for him to deliver such briefings and that he might let a new passenger fly but would follow on the controls.
The pilot was a first officer for a major airline. According to his wife, he left home on December 3, 2015, and drove to Los Angeles International Airport (LAX) where he boarded a commercial flight to Sao Paulo, Brazil as first officer. The return flight arrived at LAX at 0900 on the day of the accident. Fellow crewmembers reported that the crew queried each other about rest; one crewmember recalled that during the return flight the pilot had two breaks, resting in a business class seat on the first and using the crew bunk for a 3-hour period on the second.
The pilot-rated passenger, age 42, held a private pilot certificate with an airplane single-engine land rating. He was issued a third-class medical certificate on May 20, 2014, reporting 377 hours total flight experience. His logbook indicated 409 hours total, with no time in turbine engine aircraft. According to a friend, the passenger was a general aviation pilot who flew reciprocating engine airplanes and was not intending to fly on the day of the accident.
Aircraft Information
According to FAA records, the single-engine, two-seat tandem airplane was manufactured in 1993; according to Aero Vodochody, it was manufactured in 1983. It was issued an FAA experimental special airworthiness certificate in the exhibition category on February 23, 2003, and registered to Jettran LLC on December 16, 2014.
The airplane was originally a military jet trainer, powered by an AI-25 TL engine manufactured by Ivchenko (now Motor Sich). It was owned by the Ukrainian military from 1991 to 2002, then decommissioned and imported to the United States. A logbook entry dated February 20, 2003, showed 1,908 total airframe hours and 532 engine hours. Records prior to US service could not be located.
A maintenance entry from February 20, 2003, stated that ejection seat rocket motors and charges had been removed, and the ejection system was inoperative. The most recent condition inspection was completed on November 23, 2015, at 2,062 total flight hours. The inspection included right- and left-wing flight controls, aft flight controls, and lubrication of elevators, rudder, wing flaps, and ailerons.
The AI-25TL engine was designed with a time between overhaul (TBO) of 750 hours and a life service limit of 8 calendar years. At the time of the accident, the engine had 686 hours and had been in service about 26 years. Recent service bulletins extended the life limit to 15 calendar years, but only for engines maintained per manufacturer's instructions. The engine was installed in 1991 as a replacement. When imported in 2003, it passed an FAA-approved inspection. The engine was never overhauled.
During the November 23, 2015, inspection, 1st and 2nd stage turbine blades and wheel assemblies were inspected by borescope, and no anomalies were observed. Maintenance personnel performed a critical tip clearance inspection of the 2nd stage blades and did not detect any blade curl. Two engine performance check ground run-ups were accomplished, and maximum fuel delivery pressure did not exceed published limits. No discrepancies were observed.
Airport fueling records indicated the airplane was serviced with about 131 gallons of JET-A fuel before the accident flight. A fuel sample from the facility was clear and free of contaminants.
The owner's maintenance program was approved by the FAA on July 11, 2002, with a revision on December 17, 2014. It listed the engine under "Life Limited Items" and required condition inspections at 1,500 and 3,000 hours and an engine overhaul at 4,000 hours. The condition inspection was standard factory-prescribed performance test and inspection at 100-hour intervals. The program also required a borescope inspection through the ignitor holes.
Meteorological Information
The 1415 recorded weather observation at Southern California Logistics Airport (VCV), Victorville, California, about 9.8 nautical miles west of APV, included wind calm, visibility 10 statute miles, scattered clouds at 12,000 ft, temperature 15°C, dew point -10°C, and altimeter setting 30.27 inches of mercury.
Airport Information
APV was publicly owned and operated by the County of San Bernardino and was not equipped with an operating control tower. Runway 18/36 was 6,498 ft long and 150 ft wide with displaced thresholds at both ends. Airport elevation was about 3,062 ft above mean sea level. Terrain beyond the airport perimeter along the extended centerline of runway 18 was mostly flat desert with sparse brush.
Wreckage and Impact Information
The airplane impacted flat terrain between two taxiways about 800 ft beyond the departure end of runway 18, and about 150 ft to the right of the runway centerline. The debris path measured 263 ft long by 100 ft wide, oriented on a 193° magnetic heading. The path consisted of airframe fragments and a black substance resembling oil residue. The initial impact point (IIP) was identified by a V-shaped ground scar. Red position light fragments from the left wingtip navigation light were observed around the left side of the debris path about 20 ft from the IIP, with small plexiglass pieces beyond.
All major structural components were accounted for. The main wreckage came to rest on a 080° magnetic heading and comprised the fuselage, empennage, and portions of both wings. The fuselage and engine compartment were destroyed by fire, but the engine and empennage remained intact. Visual examination of the engine case and exhaust shaft revealed no evidence of catastrophic engine failure. Inspection of runway 18/36 revealed no evidence of foreign object debris.
Primary flight controls (aileron, elevator, rudder) were activated by push-pull tubes and levers attached to two interconnected control sticks. Only two control tubes from the elevator and rudder assembly were recovered; one tube had separated and fractured into five pieces consistent with overload separation. The remaining control tubes were destroyed by fire.
Most of the wings were destroyed by fire. The left aileron control turnbuckles and main aileron bellcrank were intact but separated from their control tubes, which were destroyed.
The rudder control system had two pairs of interconnected adjustable rudder pedals. The front seat occupant's rudder pedal assembly remained intact and adjusted forward. Both control tubes were destroyed. The rear seat occupant's rudder assembly was intact with tubes attached and straight pedal mounting tubes.
The airplane had two slotted fowler flaps interconnected by a single actuating cylinder. One of three flap settings was achieved by depressing a control button. The inboard and outboard right-wing flap tracks separated, but portions of flaps remained attached, indicating a 25° deployed (takeoff) position.
The nose landing gear was extended and the nose gear door closed. Both main landing gear were collocated with the main fuselage, but their positions could not be determined due to fire.
The airplane had trim tabs for longitudinal and lateral axes. Longitudinal trim was provided by tabs on left and right elevators. The left trim tab deflected automatically when flaps deployed to landing. The right trim tab was controlled by the trim switch. Continuity of elevator and flap trim linkages was confirmed. Both elevator and flap trim were in neutral positions. The manufacturer indicated takeoff can be accomplished with elevator trim in neutral or takeoff positions. Elevator bearings appeared lubricated, and hinges showed no deformation.
The fuel system consisted of five interconnected fuselage tanks, two tip tanks, and one inverted-flight reservoir. All fuel tanks were destroyed by fire except the left-wing tip tank, which had significant vertical crush damage. The front seat passenger fuel cutoff remained attached with the lever in full aft (closed) position; the protective gate remained attached. Both lever and gate were bent about 10° by impact. The low-pressure fuel boost pump was thermally damaged with screen partially separated but no debris.
Engine Examination
The engine displayed no evidence of un-containment, such as case breaches or failed flanges. The compressor section was intact and generally undamaged. The combustor housing, turbine housing, and exhaust pipe were externally intact. Four exhaust gas thermocouples were present; three were deformed.
The engine was torn down. The fuel nozzle manifold was intact, as were the 12 individual fuel nozzle assemblies. The combustor was intact but displayed carbon deposits in the fuel nozzle head assembly cooling and anti-carbon build-up holes. All fuel nozzles were removed with ease; most were coated with black carbon or coke deposits.
Examination of the turbine (hot) section revealed evidence of thermal distress. The 1st stage turbine hub was intact, and all blades were present. Many blades showed rippling near the outer span at trailing edges, consistent with material becoming plastic at elevated temperatures.
The 2nd stage turbine hub was intact, and all blades were present. About half of the blades were fractured sequentially near the tip shroud blend radius; those with intact tip shrouds had fractures near trailing edges with missing particles. All fractures showed overload failure; no fatigue was observed. All blades exhibited a rippled pattern on mid-span consistent with melted plastic flow. Most vanes of the 2nd stage turbine nozzle showed a rippling pattern on airfoil surfaces along the inner half of the vane span, consistent with melted plastic flow.
The 3rd stage turbine hub was intact, and all blades were present. Two non-consecutive blades were bent. Blade tip shrouds from the 2nd stage turbine and other metal debris were found between blades and at the bottom of the housing. The 3rd stage turbine nozzle vanes displayed circumferentially-varying heat distress, most prominent between 9:00 and 12:30 o'clock and between 2:00 and 4:00 o'clock.
The FCU, fuel nozzles, and thermocouples were sent to a facility in Prague, Czech Republic for testing.
The AI-25TL engine measures metered fuel pressure between FCU and nozzles. Typical maximum metered fuel delivery pressure for maximum approved thrust is 48-50 atm. The absolute maximum fuel pressure for takeoff is 65 atm. According to specialists, any pressure over 55 atm warrants investigation. The maximum metered fuel delivery pressure measured for the accident FCU was 85 atm. Screw number 28 had to be adjusted counterclockwise by several turns to bring pressure back to 50 atm, indicating the FCU was intentionally adjusted above the maximum allowable.
Inspection of the fuel nozzles revealed black sooty deposits. After cleaning, they were tested in factory apparatus. All fuel nozzles failed the factory functional test procedure and would have required overhaul. They were all below allowable limits for flow rate and spray pattern. Fuel nozzle No. 3 had the highest spray pattern variance (97%), matching the location of thermal distress between 9:00 and 12:30 on the 3rd stage turbine nozzle vanes. Fuel nozzles Nos. 10 and 11 had the narrowest spray cone angles (70° and 73°), matching thermal distress between 2:00 and 4:00 o'clock.
The damage noted on the 2nd and 3rd stage nozzles and turbines was consistent with an over-temperature condition caused by a disturbed combustion process in the combustor due to poor fuel atomization by the fuel nozzles.
All four thermocouples were tested; only two of the thermocouples responded correctly. The other two showed no response, indicating failure.