BL 21.50 BL 47.50 BL 21.50 BL 47.50 BL 96.50 BL 96.50 BL 16.00 BL 23.25 BL 34.5 BL 47.27 BL 61.50 BL 76.50 BL 86.56 BL 96.62 BL 34.5 BL 47.27 BL 61.50 BL 76.50 BL 86.56 BL 96.62 WL 123.483 WL 73.5 WL 79.5 WL 97.5 WL 7.55 WL 9.55 Ground line 1-40 Figure 1-90.  Water line diagram.  Figure 1-89.  Butt line diagram of a horizontal stabilizer.  the wing. The upper surface of the wings typically have  fewer access panels because a smooth surface promotes  better laminar airflow, which causes lift. On large aircraft,  walkways are sometimes designated on the wing upper  surface to permit safe navigation by mechanics and inspectors  to critical structures and components located along the  wing’s leading and trailing edges. Wheel wells and special  component bays are places where numerous components and  accessories are grouped together for easy maintenance access.  Panels and doors on aircraft are numbered for positive  identification. On large aircraft, panels are usually numbered  sequentially containing zone and subzone information in the  panel number. Designation for a left or right side location on  the aircraft is often indicated in the panel number. This could  be with an “L” or “R,” or panels on one side of the aircraft  could be odd numbered and the other side even numbered.  The manufacturer’s maintenance manual explains the panel  numbering system and often has numerous diagrams and  tables showing the location of various components and under  which panel they may be found. Each manufacturer is entitled  to develop its own panel numbering system. are further divided into sequentially numbered zones and  subzones. The digits of the zone number are reserved and  indexed to indicate the location and type of system of which  the component is a part.  Figure 1-92  illustrates these zones  and subzones on a transport category aircraft.  Knowing where a particular structure or component is located  on an aircraft needs to be combined with gaining access to  that area to perform the required inspections or maintenance.  To facilitate this, access and inspection panels are located on  most surfaces of the aircraft. Small panels that are hinged or  removable allow inspection and servicing. Large panels and  doors allow components to be removed and installed, as well  as human entry for maintenance purposes.  The underside of a wing, for example, sometimes contains  dozens of small panels through which control cable  components can be monitored and fittings greased. Various  drains and jack points may also be on the underside of  The airframe, or fundamental structure, of a helicopter can be  made of either metal or wood composite materials, or some  The structures of the helicopter are designed to give the  helicopter its unique flight characteristics. A simplified  explanation of how a helicopter flies is that the rotors are  rotating airfoils that provide lift similar to the way wings  provide lift on a fixed-wing aircraft. Air flows faster over the  curved upper surface of the rotors, causing a negative pressure  and thus, lifting the aircraft. Changing the angle of attack of  the rotating blades increases or decreases lift, respectively  raising or lowering the helicopter. Tilting the rotor plane of  rotation causes the aircraft to move horizontally.  Figure 1-93 shows the major components of a typical helicopter. Helicopter Structures Access and Inspection Panels Airframe