EL MONTE, CA, USA
N5174D
Cessna 172N
A factory installed map light and it's switch were mounted midway down the left doorpost. About 1/2-mile from the airport, a passenger grabbed the left forward doorpost with her hand for support and a 'pop' noise was heard. Immediately, flames shot out from both sides of the plastic doorpost cover at the map light's level and black smoke filled the cockpit. The pilot tried to smother the fire, but each time the flames came back and seemed to grow more intense. An emergency downwind landing was made on runway 1 as the flames completely involved the cockpits left side. The cabin was destroyed in the ground fire. It was unclear if AD 80-04-08 was complied with following its issuance on February 16, 1980. The AD was due to instances of fuel leaks and/or in-flight fires resulting from contact between the map light switch terminals and an adjacent fuel line. The AD required a one-time visual inspection of the left doorpost area to ensure that the switch and the fuel line were at least 0.50-inch apart. It also required the installation of an insulator cover over the map switch electrical terminals. Due to a pilot squawk, a mechanic had repaired the map light the day before the accident. He found a loose wire and reconnected it to the switch. In questioning, the mechanic first said that there was nothing between the switch and the fuel line, then stated during a second interview that after review of the AD and parts manual diagrams, he definitely had reinstalled the insulator cover over the switch terminals. Review of the 172's design history revealed that models prior to the 172N had only one fuel supply line from each tank that were routed down the aft doorpost. Map lights installed prior to the 172N were located at the top of the doorpost in an area with more internal clearance. With the 172N model change, a second fuel feed line was added from each tank and routed down the forward doorpost in the channel with the wire bundles and the map light switch, with nylon spacers added to physically separate the wires from the fuel line. Also, the map light and switch were moved to their present locations. The only flammable components in the doorpost are the wire insulation, the nylon spacers, the plastic doorpost covering, and, the fuel in the fuel line (0.75 psi head pressure at the map light level). No evidence of electrical shorts in the doorpost circuits or flammable fluid leaks external to the doorpost, which would account for the ignition and severity of the doorpost fire, were found during the investigation. Three accidents were found in NTSB accident records where fires originated in the left forward doorpost; all three aircraft were Cessna 172N's. The SDR data base had three instances (only 172N's) where electrical arcs from the map light switch had punctured the fuel line in the left forward doorpost and caused an in-flight fuel leak. Other SDR's documented over 12 instances where electrical wires had chaffed into the fuel lines in either the left or right forward doorpost areas. Examination of two exemplar Cessna 172N's revealed that the internal configuration of the doorpost components were not in conformance to original drawing specifications due to interior replacements and repairs over the years.
HISTORY OF FLIGHT On February 19, 1999, at 1652 hours Pacific standard time, a Cessna 172N, N5174D, made an emergency landing at the El Monte, California, airport, after experiencing an in-flight cockpit fire. The aircraft was destroyed in the subsequent ground fire following the landing. The commercial pilot was not injured. The news reporter/passenger was treated for smoke inhalation at a local hospital and released the same night. An airport employee who responded to the burning aircraft and attempted to extinguish the fire was also treated and released for smoke inhalation. The flight was conducted under 14 CFR Part 91 by Jetcopters, Inc., as a traffic aerial observation flight for a local radio station and originated at the Van Nuys, California, airport, about 1600. Visual meteorological conditions prevailed and no flight plan was filed. The pilot was interviewed on the night of the accident by Safety Board investigators and the reporter was interviewed several days afterward. The pilot reported that they were orbiting over a traffic accident about 1/2-mile from the El Monte Airport. The pilot was in the right seat and the reporter was in the left seat. The reporter was reaching behind in the back seat area and grabbed the left doorpost with her hand for support. A "pop" noise was heard. Almost immediately, flames shot out from both sides of the Royalite molding covering the left doorpost at the level of the map light-mounting fixture. The pilot reported that black smoke filled the cockpit. He removed his shirt and attempted to suppress the fire, but was unsuccessful. According to the pilot and the reporter, each time the flames were smothered, they came right back and seemed to grow more intense. The reporter then moved to the back seat to escape the flames. The pilot declared an emergency with the El Monte Air Traffic Control Tower and set up for a downwind landing on runway 1. Both occupants said that during the time interval that the pilot maneuvered the aircraft into alignment with the runway and landed, the flames had progressed down the Royalite doorpost covering and spread to the left side cockpit sidewall upholstery and the left side of the cockpit glareshield/instrument panel. As the airplane was rolling to a stop, the reporter exited the right side door, followed immediately by the pilot. Review of video news footage taken by another station's helicopter, which was covering the same traffic accident, showed that about 1 minute elapsed between the airplane coming to a stop and the cockpit being fully involved in flames. Both the pilot and the reporter stated that they had not noticed any unusual fuel or electrical odors either during this flight or any of the flights in the days preceding the day of the accident. PERSONNEL INFORMATION Review of the Federal Aviation Administration (FAA) Airman record files disclosed that the pilot held a commercial pilot certificate with airplane ratings for single engine land, multiengine land and instruments. He also held a certified flight instructor certificate for airplanes. The most recent first-class medical certificate was issued without limitations on April 30, 1998. According to the pilot's logbooks, he had a total time of 1,050 hours. The pilot had been employed with Jetcopters, Inc., for about 1 year and principally flew traffic watch flights for the operator. The reporter was employed by Shadow Broadcasting Services as a broadcast traffic reporter, and had flown in single engine airplanes on a daily basis covering road traffic conditions for a local radio station. AIRCRAFT INFORMATION Review of Cessna Aircraft manufacturing records disclosed that the aircraft was manufactured in February 1979 as a Cessna 172N model, serial number 17272439. Jet Copters, Inc., purchased the airplane in June 1998 for principal use in traffic watch reporting flights. According to the aircraft maintenance records, it had accumulated a total time in service of 8,141 hours. The last annual inspection was performed on June 24, 1998, during the purchase process. The most recent 100-hour inspection was accomplished by Jet Copters, Inc., maintenance department on February 17, 1999, 47 hours prior to the accident. The airplane was equipped with a factory installed map light mounted about midway down the left doorpost with it's associated switch mounted about 6-inches lower on the post. Airworthiness Directive (AD) 80-04-08, became effective on February 16, 1980, and was applicable to the accident aircraft by serial number. The AD stated that compliance was to "preclude the possibility of a fuel leak or an in-flight fire due to contact between a map light switch and an adjacent fuel line . . ." and required a one-time inspection of the left doorpost area and accomplishment of Cessna Service Information Letter SE80-3. The pertinent provisions of the AD and Service letter directed a visual inspection of the metal fuel line and map light switch located in the left hand forward doorpost for chafing or arcing, and to provide at least 0.50-inch clearance between the map light switch and the fuel line in accordance with procedures in FAA Advisory Circular 43.13-1A. It also required the installation of a nomex cover (insulator), Cessna Part Number 0511080-1, over the electrical terminals on the rear of the map light switch. Cessna Service Information Letter SE80-3, dated January 21, 1980, indicated that a protective Nomex insulator on the rear of the doorpost map light switch would be installed on production aircraft from that date forward. It stated that the switch insulator would provide "improved coverage over the switch terminals." The AD and Service Information Letter are appended to this file. The accident aircraft's maintenance records were examined for evidence of compliance with the AD and the Service Information Letter. In the AD Compliance Section of the first airframe logbook, both AD 80-04-08 and SIL80-3 were noted as complied with, though no date of compliance was recorded. The maintenance entries in the airframe logbook were examined from January 1980 forward, and no entries were found which specifically noted either the AD compliance by name or the work which could be associated with compliance. The entry in the AD compliance section was observed to be in the correct date sequence when compared to the dates of other entries. A certified copy of the aircraft file was obtained from the FAA Aircraft Registry in Oklahoma City, Oklahoma, for review of FAA Form 337's. In addition, the FAA Form 337's in the aircraft maintenance records were examined to determine the modification history of the airplane as it related to the fuel or electrical system. No modifications or alterations to the airframe fuel system were found. Two FAA Form 337's were found, which detailed modifications to the electrical system in order to plug in and operate a portable AM broadcast radio for traffic reporting purposes. Review of the written installation descriptions and the electrical schematics attached to the Form 337's revealed no common connection between the installed circuits and the circuits routed in the left forward doorpost beyond the common connection to the main electrical power distribution bus. The operator reported that discrepancies and squawks are reported verbally by the pilots to the maintenance personnel, and no written records are kept. Interviews with company maintenance personnel disclosed that on February 16, 1999, one of the pilots informed the shop that the map light was inoperative in the accident airplane. Subsequent maintenance records reviews and interviews established that on February 17, 1999, one of the mechanics removed the Royalite doorpost covering and examined the map light area to try to determine why it was inoperative. The mechanic who performed the work reported that he found the washer for the map light mounting post on the cabin floor, so he reinstalled a washer and a nut on the map light post. He further stated that one of the wires between the map light and the switch was disconnected from the switch terminal and he determined that was the reason for the inoperative light. He reconnected the wire and secured the panel. The mechanic tested the light on the ground, found it to be working, and returned the airplane to service. The mechanic who performed the map light work was interviewed by Safety Board investigators on two separate occasions; by telephone on February 22 and in person on February 24. During the interviews, the investigators were aware of AD 80-04-08 and Cessna SIL 80-3, but asked non-leading general questions concerning what he observed behind the Royalite covering. The mechanic was asked if he recalled anything being behind the map light switch. He responded that he remembered nothing being behind the switch. In response to other questions about the general area condition he stated that he was focused on the map light post, switch, and their associated wiring, and did not specifically look at anything else; however, he did state that there was nothing unusual about the area which attracted his attention, to include fuel like odors. He did not recall whether he observed nylon "V" shaped spacers between the fuel line and the wiring circuits routed up the doorpost. During the in-person interview, the mechanic volunteered that he had reviewed the parts manual and AD 80-04-08 after the telephone interview. He stated that he definitely reinstalled the Nomex cover over the rear of the map light switch as depicted in the AD. At the request of Safety Board investigators, Cessna Aircraft provided proprietary drawings and production process specifications covering the design history and configuration details of doorpost area. Review of this data found that in models prior to the 172N (1976 and before), each wing fuel tank had only one supply line, which was routed down the aft doorpost to the cockpit fuel selector valve. In addition, map lights installed in models prior to the 172N were located at the top of the doorpost in an area with more internal clearance between the wires and the light mounting post. In 1977, with the model change to the 172N, a second fuel feed line was added from each tank and routed down the forward doorpost in the channel with the wire bundles. Cessna process specifications mandate a minimum 0.50-inch clearance be maintained between electrical conductors and any line containing flammable fluids. With the design change, nylon "V" shaped spacers were installed to physically separate to at least the minimum specified clearance the wires from the fuel line. With the 1980 production airplanes, the Royalite doorpost covering was modified with the addition of a stiffener to the edges. In 1978, the electrical system was changed to a 24-volt power system with a 60-amp alternator. Concurrent with this change, the map light and switch were moved into their current locations and the style of the map light was changed to the present one. Review of data provided by Cessna Aircraft identified the electrical circuits routed in the left forward doorpost. The following list includes the circuit name and the amperage draw for each in parenthesis: 1. Wing courtesy lights (1.7); 2. Pitot heat (2.2); 3. Overhead instrument flood lights (1.2); 4. Wing tip strobes (2.7); 5. Fuel sending units (0.5); and 6. Map light (1.7). The flammability of components within the doorpost was examined through design specification data provided by Cessna Aircraft. Fuel for the airplane is contained in two tanks, one in each inboard wing section, and is gravity fed to the engine. Lines from the forward and aft fuel tank pick-ups are routed in the aft and forward doorposts to the fuel selector valve on the cockpit floor. According to Cessna, at the point of the map light and switch, the fuel head pressure in the doorpost line is between 0.75 and 1.0 psi. The factory specified and installed plastic Royalite covering over the doorpost frame is flame retardant; however, Cessna noted that they are aware of companies which sell after market replacement interior plastic panels, the flame propagation properties of which are unknown. The only other components in the doorpost frame of known flammability are the wiring insulation and the nylon spacers. AIRPORT INFORMATION The airport is equipped with one 3,995-foot-long asphalt runway which is oriented on a magnetic heading of 010/190 degrees. A FAA Air Traffic Control Tower is located on the field and is attended from 0700 until 2130. According to the airport manager, the principal fire incident/rescue response is from the municipal fire department, which has a station within 1 mile of the airport. The airport does maintain a truck, which has 100 gallons of water and 450 pounds of "purple K." In addition, the truck carries a handheld 20-pound purple K extinguisher. All the chemicals were expended on the fire. The manager reported that the intent of the equipment is to provide time for occupant egress, not necessarily to extinguish a fire. The airport employees are not given formal fire fighting classes but rather initial indoctrination in how the equipment works. No recurrent training is required or given in fire fighting techniques. Review of the video footage taken by a television station's news helicopter showed that the airport truck was on scene within 1 minute of the occupant's egress. The truck stopped at a location downwind of the burning aircraft and the airport employee manning the truck approached the fire from that position. The fire suppression agents were then dispensed from about 30 feet away. The employee reported that a closer approach was prevented by his being engulfed in smoke from the fire. According to El Monte ATCT facility records, the municipal fire department was notified of the inbound aircraft with an onboard fire at 1650 via a communications radio direct link to the county 911 emergency operator. The municipal fire department responded and was on-scene 3 minutes after dispatch. The responding fire company reported that the aircraft cabin and inboard wing sections were fully involved as they rolled up. Conventional fire suppression techniques were utilized and the fire was extinguished within 4 minutes. WRECKAGE AND IMPACT The airplane came to rest between the right runway edge and the taxiway in a paved area about 1,100 feet from the approach end of runway 1. Extensive thermal destruction of the cabin and inboard wing sections was observed, with more damage noted to the left side. The cabin roof with wing carry through members was completely destroyed, with the left and right wings tips laying on the ground and the wings remaining attached to the fuselage by the lift struts. The right side of the cabin was thermally destroyed from the firewall aft to about the center of the baggage compartment. A heavily burned section of sidewall structure about 12 inches high from the firewall to the right side forward doorpost remained. The right forward doorpost from the floor to about the top of the instrument panel remained, although it was thermally damaged. The left side of the fuselage was thermally destroyed from the midpoint of the engine compartment back to the baggage compartment rear bulkhead. The sidewall structure was burned to within 1 or 2 inches of the cabin floor along the entire length of the cabin. A section of forward left doorpost 8-inches high remained in place, though it exhibited extensive thermal damage. The remaining burn damaged V-shaped patterns in the fuselage skin pointed to the remaining doorpost stub. No fuel staining was observed on the surviving exterior fuselage or wing skin panels in the areas where fuel lines are routed. The instrument panel was extensively burned, with much of it thermally destroyed. The radios, avionics, and flight instruments had fallen to the floor on the right side of the cockpit. The insulation on the cockpit side of the firewall was burned and blackened, with areas that were still intact. The firewall behind the insulation exhibited heat damage and discoloration; more so on the cockpit side than the engine side. The
An in-flight fuel fire in the left forward doorpost due to an electrical arc from an adjacent map light switch puncturing a main fuel feed line. The manufacturer's inadequate design in routing the fuel line in close proximity to electrical arc sources in the doorpost is causal. A factor in the accident is the probable failure of the doorpost internal component configuration to conform to original design specifications due to aircraft aging issues.
Source: NTSB Aviation Accident Database
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