Boeing 787 Dreamliner - Mark Wagner [78]
SPECIAL CONDITIONS
The first of the FAA’s long-awaited special conditions were issued in March 2007 and focused on the aeroelastic interaction between the airframe structure and the fly-by-wire flight control system (FCS). The condition reflected the expansion of the 787 FCS into an all-axis system to handle lateral maneuvers and gust load alleviation. Another special condition stipulated that “suitable annunciation be provided to the flight crew when a flight condition exists in which nearly full control surface deflection occurs.” The FAA said this was required because, under certain conditions, the crew might not be aware of “excessive deflection or impending control surface deflection limiting,” with a resulting danger of loss of control.
The following month saw special crashworthiness conditions issued that required the composite wing and fuel-tank structure to withstand a post-crash fire long enough for passengers to evacuate safely. This was prompted by the dramatic difference in thermal conductivity between composites and aluminum, the latter being highly conductive and able to readily transmit the heat of a ground fire to fuel still in the tank. Far from being as bad as it sounds, this spread the heat over the wing surface and prevented localized hot spots, delaying structural collapse or burn-through beyond the time needed for evacuation. As carbon fiber had low thermal conductivity, the FAA called for additional tests and analyses to show that the 787 fuel tanks could resist a postcrash fire for at least five minutes.
Testing on the first experimental composite fuselage sections began in 2004 and paved the way for later development testing and assembly process work conducted by the structural partners. “They have not only been involved in the design, manufacturing, and assembly—but also in the structural tests,” said Randy Harley. The leading-edge slat structure was tested by Spirit in Kansas, and the trailing edge by Hawker de Havilland in Australia. The horizontal stabilizer was designed, built, and tested by Alenia in Italy, while the U.K.-designed landing gear was tested in Toronto, Canada.
By mid-2005 fuselage test work was ramping up, with three test barrels completed, including two versions of the original tail and aft fuselage Section 47, plus a constant-diameter barrel representing equally either a Section 43 or 46. “Now we’re working on a Section 41, and we will be doing around seven such test barrels in total,” said Walt Gillette, adding that by then the work of proving the material was essentially over. “From now on, we’re working on production efficiency, basically. At the end of this we will build a big piece of barrel and an additional half a piece of barrel for certification of the mechanical join of the major sections.”
It was not all plain sailing, however, and in May 2006 Boeing hit an unexpected bump in the development road when what was intended to be the ninth and final test specimen failed quality inspections. Porosity, or air spaces in the composite layers, was revealed by nondestructive inspection (NDI) of the thirty-three-foot-long constant-diameter test section. Built to assess a different mandrel tool and production process, the ninth test section had been added to check “improvements,” said Boeing.
Two extra barrels were added to conduct separate testing concurrently and to try to stay on schedule. Analysis revealed that the revised mandrel caused trapped gases to bubble when the lengthened section was being baked in the autoclave at the company’s developmental center site at Boeing Field. “While this was a pop-up for sure, this is exactly why we are doing it. It’s all about proving the technology at the development stage. If this was the first production barrel, then we’d be very upset,” said Boeing.
Tests also were under way on a first full-size structural wingbox involving a sixty-foot-long representative Mitsubishi-built