Boeing 787 Dreamliner - Mark Wagner [77]
Boeing debated long and hard over the best ways to counter the lightning threat. In early 2006 the Seattle Times published a leaked internal review showing that concerns over wing-lightning protection existed as late as the previous November. In particular, a safety team argued that with the existing design, sparks still could occur in the fuel tank, potentially blowing up the aircraft. Despite a startling one hundred lightning strikes per second on average around the world, safety features on modern airliners had ensured no lightning-induced commercial disasters since 1963, when a Pan Am 707 had exploded in flight over Maryland. Although an Iranian Air Force 747 was also brought down by a suspected lightning strike in 1976, the safety record was otherwise startlingly good.
Boeing’s first bump in the road to the 787 was a structural issue with the ninth one-piece test barrel that failed inspection following porosity problems in April 2006. This was caused by attempts to use a defective mandrel. The rejected barrel, thought to be this section sitting outside Boeing’s Developmental Center by Boeing Field in May that year, was part of the early certification process for the 787 and taught valuable lessons before final manufacturing began for the real aircraft. Guy Norris
Boeing ultimately overcame the safety concerns by taking a multilayered approach to countering the lightning threat. The first layer of defense was an embedded bronze-copper alloy mesh in the outer ply of the composite wing skin where fasteners joined it to the underlying ribs, stringers, and spars. This conducted the electrical charge around the fuselage, creating the same sort of “Faraday cage” effect as found on aluminum aircraft. Fasteners that were exposed to the fuel tanks were also sealed, brackets and fuel tubes were insulated, and the connections between the tanks and structure were electrically bonded. Nonconductive sealant also was applied to prevent sparks and arcs from happening inside the edges of the fuel tanks, where skin fasteners and joints could have direct lightning attachment and ignite vapors.
Messier-Bugatti used a large dynamometer to test and certificate its 787 electric brake system. Along with the alternative option offered by Goodrich, these were the first-ever commercial applications of electric braking in place of the more conventional hydraulically actuated brakes. The aircraft’s eight main wheels were fitted with electromechanically actuated carbon brakes driving digitally through four controller units. Tests on Messier-Bugatti’s brakes were run in France to obtain technical standard order certification, with work being performed at Villeurbanne, near Lyon. This was also responsible for research and development as well as carbon disc production for Formula 1 racing cars. Messier-Bugatti
Last, a nitrogen-generating system (NGS) was added as standard to fill the space above the fuel in the tank with inert gas. NGS (see chapter 5) was adopted as an added safety measure throughout the industry following the 1996 loss of a TWA 747-200 due to an explosion caused by a suspected short circuit in the center fuel tank.
Similar protection also was given to the fuselage structure to prevent lightning from gouging holes in the composite and to protect the myriad electrical and avionics systems inside. Dedicated metal conductors and bonding straps were added to safely conduct electricity through the additional metallic conduits. All the electrical systems within the fuselage were similarly “earthed” to prevent short-circuiting by being