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 6.7 angle from the NAVAID. These 6.7 lines intersect the outer boundaries of the secondary areas 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 midpoint. In all areas, mountainous and nonmountainous, 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 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 3-10]
The obstacle clearance areas for LF airways and routes are different than VHF, with the primary and secondary area route widths both being 4.34 NM. The accuracy lines are 5.0 in the primary obstacle clearance area and 7.5 in the secondary area. Obstacle clearance in the primary area of LF airways and routes is the same as that required for VHF, although the secondary area obstacle clearance requirements are based upon distance from the facility and location of the obstacle relative to the inside boundary of the secondary area.

When a VHF airway or route terminates at a NAVAID or fix, the primary area extends beyond that termination point. Figure 3-11 and its inset show the construction of the primary and secondary areas at the termination point. When a change of course on VHF airways and routes is necessary, the en route obstacle clearance turning area extends the primary and secondary obstacle clearance areas to accommodate the turn radius of the aircraft. Since turns at or after fix passage may exceed airway and route boundaries, pilots are expected to adhere to airway and route protected airspace by leading turns early before a fix. The turn area provides obstacle clearance for both turn anticipation (turning prior to the fix) and flyover protection (turning after crossing the fix). This does not violate the requirement to fly the centerline of the airway. Many factors enter into the construction and application of the turning area to provide pilots with adequate obstacle clearance protection. These may include aircraft speed, the amount of turn versus NAVAID distance, flight track, curve radii, MEAs, and minimum turning altitude (MTA). A typical protected airspace is shown in Figure 3-11. Turning area system accuracy factors must be applied to the most adverse displacement of the NAVAID or fix and the airway or route boundaries at which the turn is made. If applying nonmountainous en route turning area criteria graphically, depicting the vertical obstruction clearance in a typical application, the template might appear as in Figure 3-12 .

Turns that begin at or after fix passage may exceed the protected en route turning area obstruction clearance. Figure 3-13 on page 3-10 contains an example of a flight track depicting a turn at or after fix passage, together with an example of an early turn. Without leading a turn, an aircraft operating in excess of 290 knots true airspeed (TAS) can exceed the normal airway or route boundaries depending on the amount of course change required, wind direction and velocity, the character of the turn fix (DME, overhead navigation aid, or intersection), and pilot technique in making a course change. For example, a flight operating at 17,000 feet MSL with a TAS of 400 knots, a 25 bank, and a course change of more than 40 would exceed the width of the airway or route; i.e., 4 NM each side of centerline. Due to the high airspeeds used at 18,000 feet MSL and above, additional IFR separation protection for course changes is provided.