manufacturer’s naming conventions.  For detailed operational guidance, refer to Advisory Circular  (AC) 90-100, U.S. Terminal and En Route Area Navigation  (RNAV) Operations; AC 90-101, Approval Guidance for  Required Navigation Performance (RNP) Procedures with  Authorization Required (AR); AC 90-105, Approval guidance  for RNP Operations and Barometic Vertical Navigation  in the U.S. National Airspace System and in Oceanic and  Remote Continental Airspace; and AC 90-107, Guidance  for Localizer Performance with Vertical Guidance and  Localizer Performance without Vertical Guidance Approach  Operations in the U.S. National Airspace System.  Magnetic variations for locations coded into airborne  navigation databases can be acquired in several ways. In  many cases they are supplied by government agencies in  the epoch year variation format. Theoretically, this value  is determined by government sources and published for  public use every five years. Providers of airborne navigation  databases do not use annual drift values; instead the  database uses the epoch year variation until it is updated by  the appropriate source provider. In the United States, this  is the National Oceanic and Atmospheric Administration  (NOAA). In some cases the variation for a given location is  a value that has been calculated by the avionics system.  These dynamic magnetic variation values can be different  than those used for locations during aeronautical charting  and must not be used for conventional NAVAIDs or airports.  Discrepancies can occur for many reasons. Even when the  variation values from the database are used, the resulting  calculated course might be different from the course  depicted on the charts. Using the magnetic variation for the  region instead of the actual station declination can result  in differences between charted and calculated courses and  XYZ may occur several times in a provider’s database, so  the avionics manufacturer must design a way to identify  these fixes by a more specific means than the three-letter  identifier. Selection of geographic region is used in most  instances to narrow the pilot’s selection of NAVAIDs with  like identifiers.  Non-directional beacons (NDBs) and locator outer markers  (LOMs) can be displayed differently than they are charted.  When the first airborne navigation databases were being  implemented, NDBs were included in the database as  waypoints instead of NAVAIDs. This necessitated the use  of five character identifiers for NDBs. Eventually, the NDBs  were coded into the database as NAVAIDs, but many of  the RNAV systems in use today continue to use the five- character identifier. These systems display the characters  “NB” after the charted NDB identifier. Therefore, NDB ABC  would be displayed as “ABCNB.”  Other systems refer to NDB NAVAIDs using either the NDB’s  charted name if it is five or fewer letters, or the one to three  character identifier. PENDY NDB located in North Carolina,  for instance, is displayed on some systems as“PENDY,”while  other systems might only display the NDBs identifier “ACZ.”  [Figure 6-31]  Using the VOR/DME Runway 34 approach at Eugene  Mahlon Sweet Airport (KEUG) in Eugene, Oregon, as  another example, which is named V34, may be displayed  differently by another avionics platform. For example, a  GPS produced by one manufacturer might display the  approach as VOR 34, whereas another might refer to the  approach as VOR/DME 34, and an FMS produced by another  manufacturer may refer to it as VOR34. These differences  can cause visual inconsistencies between chart and GPS  displays, as well as confusion with approach clearances  and other ATC instructions for pilots unfamiliar with specific  Issues Related To Magnetic Variation  Figure 6-31. Manufacturer’s naming conventions.  6-16 