text: Helicopter Landing Zones
  Clear Around Obstacles
  4-12. Ground troops must do everything they can to improve landing point surfaces so aircraft can land.
  In general, an obstacle is a stump, rock, hole, or other object, 18 inches or larger, that might damage the
  aircraft or impede aircraft landing. No obstacles can be in a TDP in which an aircraft is going to land.
  Note, however, that even if pathfinders cannot clear ground obstructions, they can perform some helicopter
  operations without the helicopter landing. They must still clear and mark the area just as they would if the
  helicopter were going to land. Helicopters are given an advisory and hover above the ground obstructions
  that prevent them from landing.
  GROUND SLOPE
  4-13. Pathfinders choose landing sites with relatively level ground. For the helicopter to land safely, the
  slope should not exceed 7 degrees (Figure 4-3, page 4-6). Whenever possible, pilots should land upslope
  rather than downslope. All helicopters can land where ground slope measures 7 degrees or less and no
  advisory is required. When the slope exceeds 7 degrees, observation and utility helicopters that utilize
  skids for landing must terminate at a hover to load or off-load personnel or supplies. When the slope
  measures between 7 and 15 degrees, large utility and cargo helicopters that use wheels for landing are
  issued an advisory, and they land upslope. When the slope exceeds 15 degrees, all helicopters must be
  issued an advisory and terminate at a hover to load or off-load personnel or supplies.
  Note: To determine slope in percentage or degrees, express all measurements in either feet or
  meters, but not both. If the map sheet expresses elevation in meters, multiply by three to convert
  into feet. If the map sheet expresses elevation in feet, divide by three to convert to meters.
  CAUTION
  Never land an aircraft facing downslope, if possible.