text: Helicopter Landing Zones
  APPROACH AND DEPARTURE DIRECTIONS
  4-14. Ideally, to land or take off, especially at night, the helicopter pilot generally chooses the approach
  or departure path facing into the wind, over the lowest obstacle, and along the long axis of the site. The
  departure heading must be within 45 degrees left or right of land heading.
  Prevailing Wind
  4-15. Always attempt to land a helicopter facing into the wind. Wind direction within 45 degrees left or
  right of land heading is considered a head wind. Depending on the helicopter's capabilities, if only one
  direction offers a good approach, or to make the most of available landing area, the pilot might be able to
  land with a crosswind of 0 to 9 knots or a tailwind of 0 to 5 knots. When wind speeds exceed 9 knots, the
  pilot must land into the wind. The same considerations apply to departures from landing sites. Except when
  the crosswind velocity exceeds 9 knots during a landing, the prevailing wind requires less attention than it
  does on the approach and departure routes. The wind affects smaller aircraft more than larger, more
  powerful ones.
  Approach and Departure Obstacle Ratio
  4-16. For HLZs that are bordered on the approach and departure ends by tall obstacles such as trees,
  power lines, or steep mountains, planners figure on an obstacle ratio of 10 to 1. That is, if a helicopter must
  approach or depart directly over a 10-foot tall tree, then the landing point must have 100 feet of horizontal
  clearance. If they have coordinated with the aviation unit commander, qualified pathfinders might have the
  authority to reduce the obstacle ratio to no less than 5 to 1. Reducing obstacle ratio is a last-resort means to
  make an HLZ suitable to accomplish the mission, second only to reduction of TDP size. Obstacle ratios are
  reduced in a deliberate manner after careful consideration of all factors, and only to the minimal reduction
  possible. The obstacle ratio should first be reduced over the route that the helicopters will be the lightest.
  For example, if the mission of the aviation unit is an insertion, they will be loaded on the approach and will
  require the most power to ingress and land, needing the longest glide path possible. After the unloading of
  troops and equipment, the aircraft will be lighter and will be able to use a shorter departure route. In this
  case, the pathfinder would reduce the obstacle ratio on the departure end and maintain a 10 to 1 or greater
  ratio on the approach end (Figure 4-4).
  Figure 4-4. Maximum angle of approach (daylight).