Primary en route obstacle clearance area  1,000 feet above highest obstacle  6.7°  6.7°  6.7°  6.7°  51  51  4 NM  4 NM  2 NM  Secondary obstacle clearance area  V214  2 NM  Figure 2-41. Primary and secondar obstacle clearance area.  Figure 2-40. Secondary obstacle clearance area.  Figure 2-39. Non-mountainous obstacle clearance in the primary  area.  ±6.7° angle from the NAVAID. These  6.7° lines intersect the  outer boundaries of the secondary ar eas at the same point  as primary lines, 51 NM from the NAVAID. If the distance  from the NAVAID to the COP is  more than 51 NM, the  secondary area extends along the 6.7° line when the COP  is at mid-point. [Figure 2-40] In all areas, mountainous and  non-mountainous, obstacles that are located in secondary  areas are considered as obstacles to air navigation if they  extend above the secondary obstacle clearance plane.  This plane begins at a point 500 feet above the obstacles  (natural or man-made) upon which the primary obstacle  clearance area is based, and slants upward at an angle that  causes it to intersect the outer edge of the secondary area  at a point 500 feet higher. [Figure 2-41]  When flying airways, pilots normally change frequencies  midway between NAVAIDs, although there are times  when this is not practical. If the navigation signals cannot  be received from the second VOR at the midpoint of the  route, a COP is depicted and shows the distance in NM to  each NAVAID. [Figure 2-42] COPs indicate the point where a  frequency change is necessary to receive course guidance  from the facility ahead of the aircraft instead of the one  If the distance from the NAVAID to the change-over point  (COP) is more than 51 NM, the outer boundary of the  primary area extends beyond the 4 NM width along the  4.5° line when the COP is at midpoint. This means the  primary area, along with its obstacle clearance criteria, is  extended out into what would have been the secondary  area. Additional differences in the obstacle clearance area  result in the case of the effect of an offset COP or dogleg  segment. For protected en route areas, the minimum  obstacle clearance in the primary area, not designated as  mountainous under 14 CFR Part 95—IFR altitude, is 1,000  feet over the highest obstacle. [Figure 2-39] The secondary  obstacle clearance area extends along a line 2 NM on each  side of the primary area. Navigation system accuracy in  the secondary area has widths of route protection of a  Changeover Points  2-20 