Figure 2-44. Direct route navigation.  specified dimensions called class limits or categories.  for use in a particular structure (i.e., in the low or high  structures) may be used to define the en route phase of a  direct flight within that altitude structure.  An operational service volume has been established  for each class in which adequate signal coverage and  frequency protection can be assured. To facilitate use  of VOR, VORTAC, or TACAN aids, consistent with their  operational service volume limits, pilot use of such aids  for defining a direct route of flight in controlled airspace  should not exceed the following:  Figure 2-44 shows a straight line on a magnetic course  from SCRAN intersection of 270° direct to the Fort Smith  Regional Airport in Arkansas that passes just north of  restricted areas R-2401A and B and R-2402. Since the airport  and the restricted areas are precisely plotted, there is an  assurance that you will stay north of the restricted areas.  From a practical standpoint, it might be better to fly direct  to the Wizer NDB. This route goes even further north of the  restricted areas and places you over the final approach fix  to Runway 25 at Fort Smith.  The azimuth feature of VOR aids and the azimuth and  distance (DME) features of VORTAC and TACAN aids are  assigned certain frequency protected areas of airspace that  are intended for application to established airway and route  use and to provide guidance for planning flights outside  of established airways or routes. These areas of airspace  are expressed in terms of cylindrical service volumes of  2-22  4. Operation off established airways between 14,500  3. Operation off established airways below 18,000 feet  MSL—use NAVAIDs not more than 80 NM apart.  These aids are depicted on en route low altitude charts.  2. Operation off established routes from 18,000 feet  MSL to FL 450—use NAVAIDs not more than 260  NM apart. These aids are depicted on en route high  altitude charts.  1. Operations above FL 450—use NAVAIDs not more  than 200 NM apart. These aids are depicted on en  route high altitude charts.  NO T FOR NA VIGA TION FOR NA