CEN11FA383

An inadequate repair of the fuel tank that allowed the fuel leak to continue, ultimately resulting in an in-flight fire.

The weekend before the accident, a fuel leak was identified. The fuel leak was subsequently repaired, and a final inspection the morning of the accident flight reportedly did not reveal any evidence of a continued fuel leak. Shortly after takeoff, the flight crew noticed a faint odor in the cockpit and a small amount of smoke near the radio room. The flight crew immediately initiated a turn with the intention of returning to the departure airport. About that time, they received a radio call from the pilot of the accompanying airplane advising that there was a fire visible on the left wing. The accident pilot subsequently executed an emergency landing to a corn field. Emergency crews were hampered by the muddy field conditions, and the fire ultimately consumed significant portions airframe. In-flight photographs showed the presence of fire on the aft lower portion of the left wing between the inboard and outboard engines. Located in the same area of the fire were fuel tanks feeding the left-side engines. After landing, heavy fire conditions were present on the left side of the airplane, and the fire spread to the fuselage. A postaccident examination noted that the C-channel installed as part of the No. 1 main fuel tank repair earlier in the week was partially separated. During the examination, the tank was filled with a small amount of water, which then leaked from the aft section of the repair area in the vicinity of the partially separated channel. Metallurgical examination of the repair area revealed a longitudinal fatigue crack along the weld seam. The fatigue nature of the crack was consistent with a progressive failure along the fuel tank seam that existed before the accident flight and was separate from the damage sustained in the emergency landing and postlanding fire. The repair earlier in the week attempted to seal the leak but did not address the existing crack itself. In fact, the length of the crack observed at the time of the repair was about one-half the length of the crack noted during the postaccident examination, suggesting that the crack progressed rapidly during the course of the accident flight. Because the repaired fuel tank was positioned within the open wing structure, a fuel leak of significant volume would have readily vaporized, producing a flammable fuel vapor/air mixture. Although the exact ignition source could not be determined due to the fire damage, it is likely that the fuel vapor and liquid fuel encountered hot surfaces from nearby engine components, which initiated the in-flight fire.

A postaccident examination of the no. 1 main fuel tank was conducted under the direct supervision of the NTSB investigator-in-charge. The upper, inboard end of the fuel tank was deformed and ruptured consistent with damage sustained after the landing. However, the area repair area itself located at the center portion of the lower, inboard seam of the tank appeared to exhibit minimal deformation. The aft portion of the repair C-channel was partially separated from the tank seam. The remainder of the C-channel appeared to be securely bonded to the tank. The bolts installed at the time of the repair were intact and appeared to be secure. The tank was filled with a small amount of water, which was subsequently observed to leak from the aft section of the repair area in the vicinity of the partially separated C-channel. Further examination of the fuel tank was conducted by the NTSB materials laboratory. A longitudinal crack, about 7.2 inches in length, was located along the center of the weld seam. The fracture surface features were consistent with fatigue, consistent with a progressive failure at the weld seam. The sealant in the vicinity of the aft two repair bolts was thin and the cured sealant did not conform to the inside shape of the C-channel. The sealant along the remainder of the repair had adhered to the fuel tank and provided full coverage over the weld seam. Additionally, the cured sealant along this portion of the repair conformed to the inside shape of the C-channel. FAA regulations (14 CFR Part 1) define a major alteration as one that is not listed in the aircraft, aircraft engine, or propeller specifications: (1) that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) that is not done according to accepted practices or cannot be done by elementary operations. The regulations (14 CFR Part 43) related to a major alteration specifically include "changes to the basic design of the fuel, oil, cooling, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust systems" as airframe alterations. The FAA Major Repair and Alteration Data Approval Job Aid provides guidance to Aviation Safety Inspectors in evaluating requests for field approvals. The document notes that a change to, or addition of, permanent fuel tanks or fuel system components, may be eligible for approval by means other than a Supplemental Type Certificate (STC), but require FAA approved data. This data may be obtained from a Designated Engineering Representative (DER), Organization Designation Authorization (ODA) approved engineering data or through ACO coordinated field approval. The airplane records on file with the FAA included approvals for modification of the fuel crossfeed system and removal of outboard fuel tanks, radio room seating, and a tail wheel modification, as well as lighting and avionics upgrades. However, the file did not include any application for, or approval of, the most recent modification to the fuel tanks, which included removal of the fuel bladders. At 0952, the ARR Automated Surface Observing System recorded weather conditions as: Wind from 060 degrees at 10 knots; 10 miles visibility; clear skies; temperature 18 degrees Celsius; dew point 9 degrees Celsius; altimeter 30.16 inches of mercury. The airplane came to rest on its landing gear in an agricultural field located about 8 miles southeast of ARR. Ground tracks indicated that the emergency landing was performed toward the east; approximate course 090 degrees. There did not appear to have been any damage to the airplane as a direct result of off-airport landing. However, the in-flight and ground fire substantially damaged the airplane. In-flight photographs of the accident airplane showed the presence of fire on the aft lower portion of the left wing between the no. 1 (outboard) and no. 2 (inboard) engines. Also located in the same area of the fire were fuel tanks feeding the left-side engines. In photographs taken shortly after the accident aircraft landed, heavy fire conditions were present on the left side of the aircraft with the fire spreading to the fuselage. By the time the fire was extinguished, the inboard portion of both the left and right wings had been destroyed by fire as well as most of the fuselage. The engines, empennage, fuselage nose, and the outboard portions of both wings remained intact. The pilot held an Airline Transport Pilot certificate with single and multi-engine land airplane ratings. His certificate included type ratings for B-17, B-737, B-757, and B-767 airplanes. He was issued a first class airman medical certificate without limitations on May 17, 2011. He reported a total flight time of 14,178 hours, with 438 hours in B-17 airplanes. His most recent regulatory checkride was completed on April 17, 2011. The co-pilot held an Airline Transport Pilot certificate with single and multi-engine land airplane ratings. His certificate included type ratings for DC-9, B-757, B-767, and A320 airplanes. He was issued a first class airman medical certificate with a limitation for corrective lenses on March 4, 2011. He reported a total flight time of 15,000 hours, with 6 hours in B-17 airplanes. His most recent regulatory checkride was completed on August 26, 2010. The accident airplane was a Boeing B-17G "Flying Fortress," serial number 44-85734. It was mid-wing monoplane design, configured with a retractable, tail wheel landing gear. The cockpit and cabin were accessible through a door located on the right side of the fuselage just forward of the horizontal stabilizer, or through a hatch located in the bottom of the fuselage below the cockpit. The airplane was powered by four 1,200 horsepower Wright model R-1820-97 nine-cylinder, radial engines. Available information indicated that the accident airplane was delivered to the United States Army Air Force in 1945. In 1947, the airplane was included in a lot of aircraft sold for scrap. However, the accident airplane was not scrapped and was subsequently sold to the United Aircraft Corporation/Pratt & Whitney Aircraft Division for use as an engine development test bed. In 1967, the airplane was donated to the Connecticut Aviation Historical Association. Unfortunately, in 1979, the airplane was severely damaged in a tornado. In 1999, a private individual purchased the airplane with the intention of restoring it. During the restoration process, the airplane was sold to the Liberty Foundation. The restoration was completed in 2004 and the airplane was returned to an airworthy condition. The FAA issued a limited special airworthiness certificate in May 2005. At the time of the accident, the airplane was being operated as a historical demonstration/exhibition aircraft by the Liberty Foundation. The airplane was maintained under a progressive inspection program. The program was comprised of four incremental inspection procedures designated "A", "B", "C", and "D", which were to be conducted at 25-hour intervals, and periodic/non-routine inspection procedures as applicable. The most recent incremental inspections were completed on: March 25, 2011, at 2,501.9 hours total time airframe (TTAF); April 21, 2011, at 2,529.6 hours TTAF; May 14, 2011, at 2,552.7 hours TTAF; and June 10, 2011, at 2,579.0 hours TTAF. Documentation on file with the Federal Aviation Administration (FAA) revealed that a modification of the fuel system was completed during restoration of the airplane. This modification involved removal of the outboard fuel cells, also known as "Tokyo" tanks, and related fuel lines. In addition, the fuel tank-to-fuel tank transfer system was replaced with a fuel tank-to-engine cross-feed system. A corresponding FAA form 337, Major Repair and Alteration, dated April 26, 2005, was on file with the airplane records. An airplane maintenance logbook entry, dated February 24, 2011, at