text: Air Intelligence Preparation of the Battlefield
  C-15. Key Terrain. Key terrain is any locality or area in which the seizure, retention, or control of it will
  afford a marked advantage to either combatant. In the aerial dimension, key terrain consists of terrain features
  which canalize or constrain air threat systems, and terrain with an elevation higher than the maximum ceiling
  of these systems. Additionally, areas that can be used for airfields, missile and UAS launch sites, landing and
  drop zones, or forward arming and refueling points also need to be considered as key terrain, since these
  areas could be used to support friendly or threat air operations. Terrain can be used as an aid to navigation.
  Man-made features are also used as cues to navigate to targets.
  C-16. Air avenues of approach. Air avenues of approach are evaluated using the same criteria as for the
  ground. A good air avenue of approach will permit maneuver while providing terrain masking from surface-
  to-air weapon systems. Some common air avenues of approach are valleys, direct lines from the enemy point
  of origin, and river beds. See figure C-1. Consider the following in determining air avenues of approach, both
  ingress and egress:
  * Type of air threat, attack profile, and ordnance. UASs are small and elusive. They usually fly low.
  Altitude can vary. Once in the target area, they may fly an orbit attempting to stay out of
  engagement range of ADA. Most surface-launched cruise missiles are terrain following and they
  use terrain masking. Cruise missiles can fly indirect routes at low altitudes and can attack or surveil
  from any direction. Tactical air-to-surface missiles usually fly direct routes from launch platform
  to the target. Rotary-wing aircraft primarily conduct contour flights. They follow ridge lines and
  military crests, using the terrain to mask their approach to the target area. Fixed-wing aircraft
  usually follow major terrain or man-made features. Depending on range, they may fly a straight
  line to the target. Ordnance or payload may affect range and altitude of the air system and thus
  influence the selection of avenues of approach. RAM fly ballistic trajectories from the launch point
  to the objective.
  Figure C-1. Example of air avenues of approach, based on terrain aspect