Figure 1-9.  The De Havilland Mosquito used a laminated wood  construction with a balsa wood core in the fuselage. Figure 1-10.  The nearly all composite Cessna Citation Mustang  very light jet (VLJ). performance surfaces. Variable contour wings became easier  to construct. Increases in flight speed accompanying jet travel  brought about the need for thinner wings. Wing loading also  increased greatly. Multispar and box beam wing designs were  developed in response. In the 1960s, ever larger aircraft were developed to carry  passengers. As engine technology improved, the jumbo jet  was engineered and built. Still primarily aluminum with a  semimonocoque fuselage, the sheer size of the airliners of  the day initiated a search for lighter and stronger materials  from which to build them. The use of honeycomb constructed  panels in Boeing’s airline series saved weight while not  compromising strength. Initially, aluminum core with  aluminum or fiberglass skin sandwich panels were used on  wing panels, flight control surfaces, cabin floor boards, and  other applications.  A steady increase in the use of honeycomb and foam core  sandwich components and a wide variety of composite  materials characterizes the state of aviation structures from  the 1970s to the present. Advanced techniques and material  combinations have resulted in a gradual shift from aluminum  to carbon fiber and other strong, lightweight materials. These  new materials are engineered to meet specific performance  requirements for various components on the aircraft. Many  airframe structures are made of more than 50 percent  advanced composites, with some airframes approaching  100 percent. The term “very light jet” (VLJ) has come to  describe a new generation of jet aircraft made almost entirely  of advanced composite materials.  [Figure 1-10]  It is possible  that noncomposite aluminum aircraft structures will become  obsolete as did the methods and materials of construction  used by Cayley, Lilienthal, and the Wright Brothers.  The most common aircraft is the fixed-wing aircraft. As  the name implies, the wings on this type of flying machine  are attached to the fuselage and are not intended to move  independently in a fashion that results in the creation of lift.  One, two, or three sets of wings have all been successfully  utilized.  [Figure 1-12] Rotary-wing aircraft such as  helicopters are also widespread. This handbook discusses  features and maintenance aspects common to both fixed- wing and rotary-wing categories of aircraft. Also, in certain  cases, explanations focus on information specific to only  one or the other. Glider airframes are very similar to fixed- wing aircraft. Unless otherwise noted, maintenance practices  described for fixed-wing aircraft also apply to gliders. The  same is true for lighter-than-air aircraft, although thorough  The concentration of this handbook is on the airframe of  aircraft; specifically, the fuselage, booms, nacelles, cowlings,  fairings, airfoil surfaces, and landing gear. Also included are  the various accessories and controls that accompany these  structures. Note that the rotors of a helicopter are considered  part of the airframe since they are actually rotating wings.  By contrast, propellers and rotating airfoils of an engine on  an airplane are not considered part of the airframe. An aircraft is a device that is used for, or is intended to be used  for, flight in the air. Major categories of aircraft are airplane,  rotorcraft, glider, and lighter-than-air vehicles.  [Figure 1-11] Each of these may be divided further by major distinguishing  features of the aircraft, such as airships and balloons. Both  are lighter-than-air aircraft but have differentiating features  and are operated differently. General 1-5