An instrument approach may be divided into as many as four approach segments: initial, intermediate, final, and missed approach.
The initial approach segment begins at an initial approach fix (IAF) and usually ends where it joins the intermediate approach segment.
Purpose: To provide a method for aligning your aircraft with the approach course by using an arc procedure, a course reversal, or by following a route that intersects the final approach course.
This segment begins at the intermediate fix (IF) which is usually aligned within 30° of the final approach course. If no fix is shown for this segment then it begins at a point where you are proceeding inbound to the final approach fix and are properly aligned with the final approach course.
Purpose: This is designed primarily to position your aircraft for the final descent to the airport.
For a nonprecision approach, the final approach segment begins either at a designated final approach fix (FAF) or at a point where you are established on the final approach course. When an FAF is not designate (on-airport VOR or NDB) this point is typically where the procedure turn intersects the final approach course inbound and is referred to as the final approach point (FAP).
For a precision approach the final approach segment begins where the glide slope is intercepted at the minimum glide slope intercept altitude.
Purpose: Allows you to navigate safely to a point at which, if the required visual references are available, you can continue the approach to a landing. If you cannot see the required cues at the missed approach point, you must execute the missed approach procedure.
The missed approach segment begins at the missed approach point (MAP) and ends at a designated point, such as an initial approach or enroute fix. The MAP depends on the type of approach you are flying. If it’s a precision approach then the MAP occurs when you reach a designated altitude on the glide slope called the decision height (DH). For a nonprecision approach it’s when you hit either a fix defined by a navaid or after a specified period of time has elapsed since crossing the FAF.