have a separate engine logbook for any work performed on  the engine(s). Other aircraft have separate propeller logbooks. Large aircraft require volumes of maintenance documentation  comprised of thousands of procedures performed by hundreds  of technicians. Electronic dispatch and recordkeeping of  maintenance performed on large aircraft such as airliners  is common. The importance of correct maintenance  recordkeeping should not be overlooked. reference datum.  [Figure 1-88]  The reference datum  is an imaginary vertical plane at or near the nose of  the aircraft from which all fore and aft distances are  measured. The distance to a given point is measured  in inches parallel to a center line extending through  the aircraft from the nose through the center of the  tail cone. Some manufacturers may call the fuselage  station a body station, abbreviated BS.  Even on small, light aircraft, a method of precisely locating  each structural component is required. Various numbering  systems are used to facilitate the location of specific wing  frames, fuselage bulkheads, or any other structural members  on an aircraft. Most manufacturers use some system of  station marking. For example, the nose of the aircraft may be  designated “zero station,” and all other stations are located at  measured distances in inches behind the zero station. Thus,  when a blueprint reads “fuselage frame station 137,” that  particular frame station can be located 137 inches behind  the nose of the aircraft. To locate structures to the right or left of the center line of an  aircraft, a similar method is employed. Many manufacturers  consider the center line of the aircraft to be a zero station  from which measurements can be taken to the right or left  to locate an airframe member. This is often used on the  horizontal stabilizer and wings.  The applicable manufacturer’s numbering system and  abbreviated designations or symbols should always be  reviewed before attempting to locate a structural member.  They are not always the same. The following list includes  location designations typical of those used by many  manufacturers. In addition to the location stations listed above, other  measurements are used, especially on large aircraft. Thus,  there may be horizontal stabilizer stations (HSS), vertical  stabilizer stations (VSS) or powerplant stations (PPS).  [Figure 1-91]  In every case, the manufacturer’s terminology  and station location system should be consulted before  locating a point on a particular aircraft. Another method is used to facilitate the location of aircraft  components on air transport aircraft. This involves dividing  the aircraft into zones. These large areas or major zones  1-39 Figure 1-88.  The various fuselage stations relative to a single point of origin illustrated in inches or some other measurement (if of  foreign development). FS −97.0 FS −85.20 FS −80.00 FS −59.06 FS −48.50 FS −31.00 FS −16.25 FS 0.00 FS 20.20 FS 37.50 FS 58.75 FS 69.203 FS 89.25 FS 109.375 FS 132.00 FS 154.75 FS 177.50 FS 189.10 FS 200.70 FS 214.00 FS 224.00 FS 234.00 FS 273.52 WL 0.00 •  Fuselage stations (Fus. Sta. or FS) are numbered in  inches from a reference or zero point known as the  •  Nacelle station (NC or Nac. Sta.) is measured either  forward of or behind the front spar of the wing and  perpendicular to a designated water line.  •  Flap station (KS) is measured perpendicular to the rear  beam of the wing and parallel to, and outboard from,  the inboard edge of the flap.   •  Aileron station (AS) is measured outboard from,  and parallel to, the inboard edge of the aileron,  perpendicular to the rear beam of the wing.  •  Water line (WL) is the measurement of height in  inches perpendicular from a horizontal plane usually  located at the ground, cabin floor, or some other easily  referenced location.  [Figure 1-90] •  Buttock line or butt line (BL) is a vertical reference  plane down the center of the aircraft from which  measurements left or right can be made.  [Figure 1-89] Location Numbering Systems