JUNEAU, AK, USA
N96AK
Cessna 207A
The certificated commercial pilot, and four passengers, departed in a single-engine airplane on a scheduled commuter flight over terrain consisting of open water and wooded islands. Thirteen minutes later during a climb, the pilot noted smoke and heat coming from beneath the mid-cabin floor, and declared an emergency, reporting a fire in the airplane. The pilot ditched the airplane in shallow water along a shoreline of a small island. The fire, which was located under the right front seat between the cabin floor and the lower airplane belly skin, was self-extinguished during the ditching. Post-accident examination disclosed a fuel line, a bundled set of electrical wires, and a Loran antenna cable, had all been routed adjacent to each other under the cabin floor, contrary to FAA recommended methodology. The Loran antenna cable had been installed 10 years prior to the accident. Two small holes, consistent with arcing, were found in the fuel line. Evidence of arcing was found on one of the electrical wires. Plastic tie wraps attached the wire/antenna bundle to plastic clamps that were each secured to fuselage formers under the floor by bolts or screws. One clamp, adjacent to the holes found in the fuel line, was missing from its attach point. A screw and a small melted portion of a plastic tie wrap were found adjacent to each other in the fuselage belly, beneath the cabin floor. An annual inspection was accomplished six months before the accident. A 100-hour inspection was completed 11 days before the accident. Each company inspection checklist includes an inspection of the internal structure of the fuselage, and an inspection of antenna cables. The FAA recommended technique for attaching clamps to structure is the use of bolts with locknuts, or self-locking nuts.
HISTORY OF FLIGHT On June 9, 1998, about 1231 Alaska daylight time, a wheel equipped Cessna 207A airplane, N96AK, sustained substantial damage during an emergency landing and ditching, about 4 miles southwest of Juneau, Alaska. The airplane was being operated as a visual flight rules (VFR) scheduled passenger flight under Title 14 CFR Part 135, when the accident occurred. The airplane was operated by Wings of Alaska, Juneau, as Flight 17 to Hoonah, Alaska. The certificated commercial pilot, and the four passengers, were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Juneau International Airport, about 1213. The pilot reported that while climbing through 1,900 feet msl, approaching the coast of Barlow Cove (about 9 miles southwest of Juneau), she smelled smoke in the airplane. The passengers also indicated they smelled smoke. The pilot made a 180 degree turn to return to Juneau, and then saw smoke coming from the rear of the airplane, possibly from the baggage area. At 1226:50, the pilot called the Juneau Air Traffic Control Tower (ATCT) stating, "Juneau Tower, Wings 96, ah I'm declaring an emergency, I've got a fire." The airplane was cleared to land at Juneau. The passengers reported smoke and heat coming from the floor area, under the right front seat. The pilot told the passengers to use the airplane's Halon fire extinguisher to put out the fire. The airplane cabin filled with smoke, hindering the pilot's vision. Several passengers said they saw a red glow coming from the floor mounted vent, located between the front and second row seats. At 1228:48, in response to a position report request from the Juneau ATCT, the pilot reported, "ah I think I'm at the north side of Portland, and we're gonna try to put it down." The pilot selected a landing area near the northwest tip of Portland Island, located in the Favorite Channel of the Lynn Canal. The pilot said her first approach to a beach area in a southerly direction was too fast, and she decided to turn right toward another beach on the west side of the island. The pilot landed in the water, just short of the beach. The airplane received damage to the right wing, fuselage, and nose gear. After ditching, the fire was self-extinguished by ocean water, and the passengers waded ashore in ankle deep water. The accident occurred during the hours of daylight at latitude 58 degrees, 20.25 minutes north, and longitude 134 degrees, 44.25 minutes west. CREW INFORMATION The pilot holds a commercial pilot certificate with airplane single-engine land and sea, multiengine land, and instrument airplane ratings. In addition, the pilot holds a flight instructor certificate with airplane single-engine, and instrument airplane certificates. The most recent first-class medical certificate was issued to the pilot on June 27, 1997, and contained no limitations. According to the operator, the pilot's total aeronautical experience consisted of about 1,330 hours, of which 45 were accrued in the accident airplane make and model. The pilot was hired by the operator on April 20, 1998. She completed a 14 CFR Part 135.293/.299 check ride on May 2, 1998, in a Cessna 206. She completed a Part 135.293 check ride in a Cessna 207 on May 31, 1998. AIRCRAFT INFORMATION The airplane had accumulated a total time in service of 11,809.1 hours. Examination of the maintenance records revealed that a mechanical irregularity was noted on December 8, 1997; the navigation light circuit breaker had popped. The corrective action noted by a company mechanic was that a short was found, and corrected, in the left wing wiring. The most recent annual inspection was accomplished on December 30, 1997, 470.9 hours before the accident. In addition, a 100-hour inspection was completed on May 29, 1998, 45.8 hours before the accident. Each company inspection checklist includes an inspection of the internal structure of the fuselage, an inspection of the flight control cables, and an inspection of the antennas and antenna cables. The lower cabin/fuselage structure is comprised of laterally placed, vertically oriented, aluminum formers. In the area of the fire, from forward to aft in the cabin, the most forward lateral former is under the instrument panel, ahead of the fuel reservoir tanks. The second lateral former is located aft of the fuel reservoir tanks, adjacent to the forward cabin door post, and the wing lift strut attach points. The third former is located about midway between the forward and aft door post. The fourth former is located just forward of the main gear attach point, adjacent to the aft door post. Two longitudinal formers are attached near the outboard edges of the fourth lateral former, and angle inboard to the second lateral former. All of the formers have flanged openings (lightening holes) oriented along their respective spans. The cabin floor is riveted to the fuselage formers. Access holes in the floor, each with a removable cover, are provided for maintenance and inspection procedures. The lower belly skin of the airplane is riveted to the fuselage formers. Rear cabin floor ventilating air, both fresh and heated, is supplied by three ducts, including a duct extending under the center of the cabin floor. The floor duct outlet is a 90 degree plastic elbow that is flush mounted to the floor. The upper end of the floor duct has a screen covering the duct opening, and a removable, and rotatable airflow diverter. The lower portion of the duct, under the floor, has a flexible hose attached to the forward end of the duct. The hose is routed under the floor to the heat/ventilation system. Fuel for the engine is routed from the two wing fuel tanks. The lines from the tanks descend vertically down each side of the fuselage wall, under the interior floor of the cabin, and then forward horizontally to reservoir tanks, one for each fuel tank. The left and right horizontal fuel lines are formed in a modified "Z" shape. The right fuel line is routed from a union/fitting under the cabin floor, located just forward of the fourth lateral former, then outboard through the right longitudinal former and the third lateral former, then forward through the second lateral former to the right reservoir tank. The fuel lines are constructed of 5052 aluminum, and are 1/2 inch in diameter. Electrical wires for the tail position light, the tail mounted rotating beacon, and a Loran antenna, were routed horizontally under the floor, adjacent to the right fuel line. The operator installed molded plastic clamps to the vertical edges of the lighting holes. Each clamp is comprised of a plastic tab, through which a screw or bolt secured the clamp body to a former, and a small "U" shaped body, oriented 90 degrees to the tab. The wires and the antenna cable were attached to the body of the clamp by a plastic, self locking tie wrap. If the clamp is installed by the use of a screw, the screw is installed on one side of a former. The threads of the screw extend through the former, and slice new threads in the plastic body of the clamp tab on opposite side of the metal former. A review of major repair and alteration records (FAA Form 337), revealed the airplane's Loran receiver was installed on February 20, 1988. The cable was routed from the instrument panel, under the cabin floor, alongside the tail position light and rotating beacon wires, to about the landing gear attach point. The cable was then routed up to the top of the fuselage. Electrical circuit breakers are provided for each electrical circuit. The rotating beacon light circuit is provided with a 10 amp breaker. The position light circuit is provided with a 5 amp breaker. The circuit breakers are designed to open when their internal temperature is exceeded. COMMUNICATIONS A transcript of the air to ground communications between the airplane, and the Juneau ATCT, is included in this report. WRECKAGE AND IMPACT INFORMATION The airplane was removed from the accident scene before the arrival of the National Transportation Safety Board (NTSB) investigator-in-charge (IIC). Supplements A and B, of NTSB Form 6120.4, were not utilized for this investigation. The NTSB IIC examined the airplane wreckage at the operator's hangar in Juneau, on June 11, 1998. A Federal Aviation Administration airworthiness inspector, Juneau Flight Standards District Office, inspected the airplane prior to the arrival of the NTSB IIC. The inspector noted fluid in the fuselage, beneath the cabin floor, and he utilized an absorbent cloth to soak up the fluid. The fluid appeared to be water, mixed with an odor of aviation fuel. The nose gear assembly separated from the fuselage. The airplane's lower center keel area where the nose gear was attached, had upward crushing of the lower fuselage. The right wing had leading edge aft crushing and upward curling of the wing tip. The right aileron had similar aft crushing and tearing about 12 inches inboard from the tip. The trailing edge of the right wing and flap was wrinkled about mid-flap area. The floor of the interior cabin was covered by a plastic floor liner. It was charred, blackened, and melted in the area beneath the right front seat. A metal insulated mug, placed under the seat, was melted to the floor covering material. A floor mounted vent, located in the center of the floor between the first and second row of seats, was melted and charred. Examination of the fuselage area, beneath the cabin floor and lower skin of the airplane belly, was initially conducted by looking through the floor inspection access holes. Visible beneath the cabin floor were charred wires located longitudinally along the right side of the fuselage. The right fuel line from the right wing to the right reservoir tank was also visible in the same area. The cabin floor over the charred wires was unriveted, and removed by company maintenance personnel. The underside of the floor panel was blackened and charred. The fuselage formers under the floor were similarly blackened and charred. The charred wires were for the tail position light, the rotating beacon light, and the Loran antenna cable. The insulation from the wires, and the antenna cable, had a black and bubbled appearance. The wire/cable bundle was secured by a plastic clamp to the edge of a lightening hole located in the number one lateral former. The clamp was attached to the edge of the lightening hole by a small bolt and self locking nut. The clamp was not damaged, and the wire bundle was attached to the clamp by a plastic tie wrap. A second plastic clamp, and its attaching screw, was attached to a lightening hole edge through the second lateral former, about 3 inches inboard of the right fuel line. The clamp body and tie wrap were blackened and partially melted. The wire bundle was not retained by the tie wrap to the clamp. The clamp for the third lateral former, was missing from the lightening hole. A small portion of melted plastic was noted on the aft face of the former, at the lower, outboard edge of the lightening hole. The source of the melted plastic is unknown. A small portion of the locking end of a plastic tie wrap, and a Phillips head screw, were located lying in the belly area beneath the floor. The screw and the melted plastic tie wrap were located adjacent to each other, on the aft side of the third lateral former. No melted plastic, a screw or a bolt, were located on the forward side of the former. The portion of tie wrap had blackening and melting. The screw was blackened over its entire surface. Examination of the threads of the screw with a 10X magnifying glass revealed tiny areas of orange deposits. Also found, predominately near the base of the screw threads, were tiny areas of white colored material. A fourth clamp located at the lightening hole of the right longitudinal former, between the third and fourth lateral former, was still attached to the metal by a screw. The clamp was melted around the screw and deformed downward, but the clamp still held the wire/antenna bundle. The right fuel line was blackened and sooted, with numerous deposits of small beads of black material in the area of the fire. In the area between the third lateral former, and right longitudinal former, the fuel line's upper surface was cleaner and marginally sooted in comparison to adjacent areas. Air pressure was applied to the right fuel tank to check for leaks of the fuel system from the tank to the right reservoir tank. The examination revealed pinhead size holes in the upper surface of the right fuel line, in the area between the third lateral former, and right longitudinal former. This is the same area of the fuel line that was previously observed to have less sooting. The area of the holes was about 3 inches aft of the third lateral former, and about 2 1/2 inches inboard from the burned wire/antenna bundle. A plastic sleeve around the fuel line, about 4 inches in length, was shriveled and blackened. The forward edge of the sleeve was positioned about 3 3/8 inches aft of the area of the holes. The aft end of the sleeve was found positioned about 7 inches from the aft end of the fuel line. In comparison to a new fuel line, the aft edge of the plastic sleeve on a new fuel line is positioned about 12 3/4 inches from the aft end of the fuel line. On a new fuel line, the plastic sleeve covers the area of the fuel line holes, and is about 4 3/4 inches in length. The circuit breaker for the rotating beacon was tripped, and the rotating beacon switch was on. The pilot said that during the accident flight, the position lights on the airplane were not on. The circuit breaker for the position lights was not tripped. FIRE ASPECTS The fire was located under the floor of the cabin. The only apparent access to the flames for the fire extinguisher, from the passengers view, was through the floor vent; however, the vent's lower elbow was connected to the ventilation system, so no direct access to the flames was available. SURVIVAL ASPECTS The accident flight in a single-engine airplane, was conducted over terrain that varied from open water to wooded islands. The airplane was equipped with inflatable life vests. During the emergency, the pilot was busy flying the airplane, communicating with ATCT, and selecting a forced landing area. The passengers were very concerned about the presence of smoke and heat in the cabin. The airplane was ditched in shallow water, allowing the passengers to walk onto a beach. The passengers did not don their life vests as a precaution in the event the forced landing was in deep water, nor were they instructed to do so by the pilot. SEARCH AND RESCUE No formal search and rescue organizations were involved in the postaccident events. A local helicopter company from Juneau picked up the passengers. TESTS AND RESEARCH The airplane's right fuel line assembly, a three-wire bundle, and two circuit breakers, were sent to the National Transportation Safety Board's Materials Laboratory for examination. A Safety Board metallurgist reported the examination of the fuel line revealed two holes, aligned about 45 degrees to the axis of the line, with their centers about 0.143 inches apart. The holes were through the full thickness of the line, with partial thickness erosion around, and between the holes. The metal at the bottom of the holes was melted and resolidified, consistent with an electrical arc strike. The examination of the wire bundle revealed all three-wire elements were burned along the majority of their length. The insulation from each wire and cable was melted and bubbled. The antenna cable was 0.25 inches in diameter. The rotating beacon wire was 0.07 inches in diameter. The position bulb wire was 0.05 inches in diameter. The rotating beacon wire had two melted and resolidified electrical arc spots, located about 52.5 inches from the aft end of the wire bundle. The arc spots were about 0.075 inches apart. No other evidence of arcing was found on the wire bundle. The circuit breakers were tested by application of current across each breaker
The failure of company maintenance personnel to properly install a wire bundle clamp; chafing, electrical arcing, and subsequent leaking of a fuel line, which resulted in an in-flight fire. A factor associated with the accident was company maintenance personnel's failure to discover a missing clamp during a 100 hour inspection.
Source: NTSB Aviation Accident Database
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