Carbon fiber potential use in next-generation narrow-body aircraft

Jens Hinirchsen, president of Aerospace Advisory Group LLC, evaluated factors likely to influence materials selection for anticipated replacements of the Airbus A320 and Boeing 737.

Carbon Fiber 2009 kicked off officially on Dec. 10 with a keynote address by Jens Hinirchsen, president of Aerospace Advisory Group LLC (New Alexandria, Pa.). Titled, "Will Composites Materials Be Attractive for the Next Generation Narrow-body Aircraft?," Hinirchsen's presentation evaluated factors likely to influence materials selection for anticipated replacements of the Airbus A320 and Boeing 737, the best-selling and most used planes in commercial aerospace.

He started by establishing structures that he believes are very or somewhat likely to be made from composites: empennage, floor beams, pressure dome, center wing box, pressure dome frame and unpressurized fuselage. The structures that he consider less likely to be made from composites, and the subject of his talk, include the pressurzized fuselage skins and stringers, standard frames, load-introduction frames and door cut-out doublers.

Regarding fuselage skins and the potential use of composites, Hinirchsen says the design driver will be damage tolerance, not strength, and reported that weight savings at the cost of increased maintenance will not be acceptable to Boeing or Airbus — or their customers. Every plane, he noted, is sold with a guaranteed maintenance cost for each part/structure, thus a composite fuselage skin must meet that maintenance cost requirement. Additionally, he also noted that the threat of barely visible impact damage will require carefully defined minimum thickness for skins. In particular, he said that airlines are most worried about damage from runway debris, which causes $4 billion annually in aircraft damage.

Exploring composite skin damage issues more deeply, Hinirchsen said airlines have two challenges: Determine the severity of composites damage in a skin structures and develop a skin structure that allows the plane to fly safely if inspection overlooks a barely visible damage. At Boeing, the damage threshold is 15 Joules: Impact of the skin by up to 15 Joules should produce no delamination and no surface fracture; a skin damaged by an impact of more than 15 Joules should resist growth of the damage.

Other factors that may determine composites use include automated tape laying and automated fiber placement speeds and tool occupation time — meaning that tooling used to produce composite structures must be amoritized quickly and thus will require fast material placement rates.

Hinirchsen ended with a fuselage skin thickness proposal for narrow-body aircraft, if composites are used: 3 mm for the upper fuselage, 3.5 mm for the side fuselage, and 4 mm for the bottom fuselage. "Potential of weight savings via composites in the fuselage is significantly reduced for narrow-body aircraft as strength/stiffness requirements result in skin thickness well below the threshold for structural thickness," he concluded.