text: Chapter 4
  Night Approach
  4-17. Within the night approach and exit path, the maximum obstruction angle should not exceed 4
  degrees measured from the center of the landing point to a distance of 3,000 meters (9,843 feet,
  Figure 4-5). The maximum obstacle height at 3,000 meters is 210 meters (689 feet). The field-expedient
  formula is that for every meter of vertical obstacle, you must have 14 meters from the center of the landing
  point to the obstacle. That is, a landing point must be 280 meters from a 20-meter tree if the helicopter
  must approach or exit directly over the tree. Another night operation planning consideration is the
  helicopter approach and exit path area and the maximum obstacle height within that area. These criteria
  apply to both the approach path to the landing point as well as the exit path from the landing point. First,
  we must define the area that is the approach and exit path.
  Figure 4-5. Maximum angle of approach (night).
  APPROACH AND EXIT PATH
  4-18. The approach and exit path is a 16-degree (277 mils) sector or arc extending outward and is
  measured from the center of the landing point (Figure 4-6). The "V"-shaped approach and exit path is
  shown by the dashed and dotted line in the illustration. The 4-degree maximum obstruction angle applies to
  the entire area within the approach and exit path (both the dark and light shaded area) measured from the
  landing point center to a distance of 3,000 meters.
  Figure 4-6. Approach and exit path.