WPR16FA153

An inflight fire in the floor area near the main bus tie circuit breaker panel that resulted from chafing between an electrical wire and a hydraulic line and/or airplane structure.

About 13 minutes after takeoff for a medical transport flight, while climbing through about 14,900 ft mean sea level (msl), the pilot reported to air traffic control (ATC) that he was smelling smoke in the cockpit and would be returning to the originating airport. The flight was cleared to return with a descent at pilot's discretion to 9,000 ft msl. The pilot replied, "okay," and said that it looked like he was going to lose some power shortly. The pilot then stated that he had smoke in the cockpit, declared an emergency, and requested that ATC contact the fire department. About 1 minute 15 seconds after the initial report of smelling smoke, the pilot made the last radio transmission of the flight stating that he had three people on board. The wreckage was located about 9 hours later in an area of brush and heavily forested terrain. Portions of the burned and fragmented wreckage were scattered along a debris path that measured about 2,400 ft in length, which is consistent with an inflight breakup. The center fuselage and cockpit areas were largely intact and displayed no evidence of fire; however, there was an area of thermal damage to the forward fuselage consistent with an inflight fire. The thermal damage was primarily limited to the floor area between the two forward seats near the main bus tie circuit breaker panel and extended to the forward edge of the wing spar. All exposed surfaces were heavily sooted. Some localized melting and thermal-related tearing of the aluminum structure was present. The primer paint on the floor panels under the right aft corner of the pilot seat and the left aft corner of the co-pilot seat was discolored dark brown. An aluminum stringer in this location exhibited broomstrawing indicating that the stringer material was heated to near its melting point prior to impact. A single wire located in the area exhibited notching consistent with mechanical rubbing. The main bus tie circuit breakers were partially missing. The remaining breakers were heavily sooted on their aft ends, and one breaker was thermally discolored. Areas of charring were on the backside of the panel. Examination of the wiring in this area showed evidence of electrical arcing damage. Four hydraulic lines servicing the landing gear system were located in this area, and all the lines exhibited signs of thermal exposure with melting and missing sections of material. Six exemplar airplanes of the same make and model as the accident airplane were examined, and instances of unsafe conditions in which electrical lines and hydraulic lines in the area of the main bus tie circuit breaker panel were in direct contact were found on all six airplanes. Some of the wires in the exemplar airplanes showed chafing between hydraulic lines and the electrical wires, which, if left uncorrected, could have led to electrical arcing and subsequent fire. Based on the unsafe conditions found during examination of the exemplar airplanes and the thermal damage to the area near the main bus tie circuit breaker panel on the accident airplane, including broomstrawing of the aluminum structure, electrical arcing damage to the wiring, and melting of the hydraulic lines, it is likely that an electrical wire near the tie bus circuit breakers chafed on a hydraulic line and/or airplane structure, which resulted in arcing and a subsequent in-flight fire that was fed by the hydraulic fluid.

Cal-Ore Life Flight is a 14 CFR Part 135 air carrier that holds on-demand operations specifications and is authorized to conduct business exclusively under the business name Cal-Ore Life Flight. The company headquarters are located at KCEC. The Director of Operations, Chief Pilot, and Director of Maintenance at the time of the accident were all based in Crescent City. At the time of the accident, Cal-Ore Life Flight operated 7 PA-31T airplanes and employed about 12 pilots. The company had 3 bases located in northern California. Wire Examination Multiple electrical wires that were connected to the main bus tie circuit breaker panel were removed from the wreckage and sent to the NTSB's Materials Laboratory for further examination. The wire identified as P3H contained both terminal ends. The entire wire section was missing insulation. A large area of welded conductors/arcing was present about 2 inches and 4.5 inches from the terminal ends. (See figure 1 and figure 2.) Figure 1 - Photograph of welded area on P3H Figure 2 - Micrograph of welded area on P3H The wire identified as L Main 2 was trapped inside a section of aircraft structure. A portion of the circuit breaker remained attached to the wire. About 11 inches of wire insulation were missing from the fractured end of the wire. The remaining insulation exhibited thermal discoloration and shrinking. Signs of thermal damage were present and decreased in severity moving away from the main bus tie circuit breaker end. The fractured end was frayed, and the exposed conductor exhibited notching. The fractured end was examined using a scanning electron microscope. No obvious signs of arcing on exposed sections of conductor were found. Several areas of the cross diameter on several conductors were flattened and appeared to have missing material. In addition, the fractured end was examined using electron dispersive spectroscopy to look for the presence of foreign materials on the wire conductors. No foreign material was found. (See figure 3 and figure 4.) Figure 3 - Photograph of fractured end of L Main 2 Figure 4 - Micrograph of notching on conductor surface of L Main 2 Hydraulic Line Examination Sections of the gear up, gear down, door open, and door closed hydraulic lines were removed from the airplane and sent to the NTSB's Materials Laboratory for further examination. The hydraulic lines were constructed of 5050-0 aluminum tubing. All the lines displayed visible signs of heat exposure, and several inches of material were missing from each of the lines. The fractured ends on all the lines were examined and found to exhibit patterns consistent with overstress fracture; some fractures exhibited features consistent with elevated temperature exposure as a contributing factor. On one of the fractured tubes, the gear up line, intergranular fracture features were found. A cross-section of fracture area showed severe grain separation along the grain boundaries, missing grains, and intergranular voids. There were no indications of microstructural features consistent with stress corrosion cracking or embrittlement. For additional information regarding the components examined for thermal damage and residue transfer, refer to the Materials Laboratory Group Chairman Factual Report, located in the public docket for this accident. Exemplar Airplane Examinations NTSB and FAA personnel examined the wiring in the area of the main electrical bus circuit breaker panel on six exemplar Piper PA-31T-series airplanes that were maintained by various individuals/operators. On all six exemplar airplanes, electrical lines and hydraulic lines were found in direct contact. Some of the wires in the exemplar airplanes showed chafing between hydraulic lines and the electrical wires. For additional information regarding the exemplar airplane examinations refer to the Systems-Exemplar Aircraft Factual Report located in the public docket for this accident. FAA Advisory Circular (AC) 43.13-1B, "Acceptable Methods, Techniques, and Practices-Aircraft Inspection and Repair," advises against wires and fluid lines being in contact due to the risk of chafing, which can lead to thermal stress and arcing in an area where flammable liquids are routed. This guidance material specifies a minimum 1/2-inch clearance between the wires and the fluid-carrying lines. On December 16, 2016, the FAA issued a Special Airworthiness Bulletin (SAIB), CE-17-05, concerning wiring on Piper aircraft including the PA-31T series. The SAIB provided information on wiring conditions in the area below the floor mounted circuit breaker panels that could lead to chafing, thermal stress, or arcing. The SAIB recommended best practices for securing high electrical current wires in the aircraft. On January 6, 2017, Piper Aircraft, Inc., issued Service Bulletin (SB) 1301. The SB described procedures for visually inspecting the area below the main circuit breaker panel and rerouting and replacing wires and/or parts as necessary. Based on the SAIB, the SB, and preliminary results from this investigation, on January 10, 2017 the NTSB issued Urgent Safety Recommendation A-17-001 concerning unsafe wiring conditions that may lead to arcing and cause fires on Piper PA-31T series airplanes to the FAA. The urgent recommendation requested that the FAA: Issue an emergency airworthiness directive (AD) that requires owners and operators of Piper PA-31T-series airplanes to take the actions recommended in Special Airworthiness Information Bulletin CE-17-05 immediately after the AD is issued. On February 7, 2017, the FAA issued AD 2017-02-06, requiring repetitive detailed visual inspection of the wiring below the floor mounted circuit breaker panels per the Piper SB 1301. The AD's effective date was February 22, 2017, and the AD required the initial inspection to be accomplished within 30 days after the effective date and then at repetitive intervals not to exceed 12 months. At 0107, KACV, located about 6 miles southwest of the accident site, was reporting, in part, wind 180° at 4 knots, visibility 1/2 statute mile in mist, runway 32 visual range 4,500 ft variable to greater than 6,000 ft, ceiling 200 ft overcast, temperature 55°F, dew point 54°F, and altimeter 29.85 inches of mercury. An autopsy was conducted under the authority of the Humboldt County Coroner, Eureka, California, on August 1, 2016. The examination revealed that the right leg of the pilot's flight suit was partially damaged from heat, and his right boot was melted over the top of its buckles. Both his right hand and right lower leg suffered thermal damage. The cause of death for the pilot was attributed to extensive blunt force trauma. The FAA's Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma, performed toxicology tests on specimens from the pilot, which were negative for carbon monoxide, alcohol and drugs. The pilot, age 54, held an airline transport pilot certificate with an airplane multi-engine land rating. Additionally, he held commercial pilot privileges for airplane single-engine land and rotorcraft-helicopter. He also held a type rating for A/BE-1900 airplanes and a certified flight instructor certificate with airplane single-engine land, multi-engine land, instrument airplane, and rotorcraft-helicopter ratings. His most recent second-class medical was issued on July 25, 2016, with the limitation that he must have available glasses for near vision. According to the operator's training records, the pilot completed training and was assigned for duty as a pilot-in-command in Piper PA-31T aircraft on May 12, 2016. He was hired with 7,300 hours total flight time, 1,378 hours of actual IFR, 179 hours of simulated IFR, 3,178 hours of night time, 5,174 hours of VFR cross country, and 3,100 hours of night cross country. In addition, at the time of the accident, he had accumulated about 125.5 hours in the accident airplane make and