CEN21LA274

The pilot heard a loud bang while starting the right engine and noticed flames exiting the right wing. An airport surveillance camera captured a wing explosion that lasted about 1 second, which substantially damaged the right wing. Examination revealed that the right wing outboard aft fuel bladder was brittle, with several holes due to age-related oxidation, and electrical wiring to the navigation lights was chafed in two locations, with evidence of electrical arcing. Review of maintenance records indicated that the airplane was inspected about 50 hours before the accident; the inspection included checking the fuel tanks for leakage, and electrical wiring for chafing. Based on the condition of the fuel bladder and wiring, it is likely that the most recent inspection was inadequate. About 2 years before the accident, the airplane’s left wing exploded due to ignition of fuel vapor. Following the most recent wing explosion accident, the operator instituted several maintenance protocols to aid in detection of a fuel leak, including top-off of the fuel system in conjunction with the wing inspection procedure checklist or unscheduled fuel system maintenance.

On June 16, 2021, at 1905 mountain daylight time, a Beech U-8F airplane, N62069, was substantially damaged when it was involved in an accident near Goodland, Kansas. The pilot sustained minor injuries. The airplane was operated by Bemidji Aviation Services Inc. as a Title 14 Code of Federal Regulations Part 135 cargo flight. The pilot stated that, while starting the right engine, he heard a loud bang and noticed flames coming out of the right wing. An airport surveillance video captured flames from the right wing for about 1 second. The pilot shut down the right engine with the mixture control and egressed the airplane. Examination revealed that the outboard 12 ft of the right wing was heavily damaged, with the forward spar deflected significantly aft and the right-wing leading edge curled aft. The outboard aft fuel bladder for the right wing was removed and observed to be in a degraded condition, with indications of fire damage. Electrical wiring to the navigation light was chafed and conductor was exposed in two locations. The outboard aft fuel bladder and wiring bundle from the right wing were both submitted to the National Transportation Safety Board Materials Laboratory for examination. The fuel bladder was constructed of nitrile butadiene rubber and was brittle in several areas, with multiple holes. A sample from the edge of one of the holes examined using a Fourier Transform Infrared spectrometer indicated degradation due to oxidation, with the embrittlement consistent with physical aging of the bladder material. The right wing wire bundle contained several areas where the insulation for the wires was chafed. One chafed area of the wiring for the navigation lights showed evidence of electrical arcing and welding. Electrical damage to the conductor surface indicated that the wire was energized at the time the damage occurred. The operator maintained the airplane with a maintenance manual accepted by the Federal Aviation Administration. The manual contained a continuous care inspection guide that required a routine inspection of the entire airplane every 60 flight hours, as well as eight more detailed inspections of specific portions of the airplane every 480 flight hours. On May 22, 2021, about 50 flight hours before the accident, a detailed inspection occurred that required following areas be checked: 1) Fuel Tanks, Vents, and Pumps – Check cells for leaks, plugged vents, pumps for leakage. 2) Fuel Caps and O-Rings – Check fuel caps and O-rings for fit and seal. 3) Fuel Quantity Transmitters – Check for leakage or visible damage. 4) Plumbing – Check for chaffing, leakage and damage, and proper attachment. 5) Electrical Wiring and Equipment – Check for security, chaffing damage and attachment. On July 9, 2019, the airplane was involved in a similar accident that the NTSB investigation attributed to an explosion of the outboard left wing due to electrical arcing and ignition of fuel vapors. Following the most recent wing explosion accident,the operator instituted several maintenance protocols to aid in detection of a fuel leak, including top-off of the fuel system in conjunction with the wing inspection procedure checklist or unscheduled fuel system maintenance.