text: FM 7-100.1________________________________________________________________________________
  enemy fighters and ground fires. Attack helicopters can escort lift helicopters
  during a heliborne operation to protect them from ground fires.
  13-23. Passive defense measures taken during the air movement phase in-
  clude conducting movement during hours of darkness, using more than one
  flight route, maintaining radio silence, and flying at low altitudes. The
  OPFOR can use electronic warfare measures during air movement, including
  escort jammers, which suppress enemy air defense and surveillance systems.
  Air Drop or Heliborne Landing
  13-24. Airborne forces normally conduct combat air drops at an altitude of
  from 150 to 300 m. They emphasize the necessity of dropping at low altitude
  to minimize the amount of time individuals are in the air. Low-altitude drops
  also increase the likelihood that a unit’s personnel and equipment would land
  close together.
  13-25. Forces inserted by helicopter have the advantage of arriving on the LZ
  as organized units. To minimize their vulnerability to ground fires, helicopters
  remain on the ground in the LZ only long enough to disembark troops. If the
  LZ is under effective enemy fire, the landing force commander, after consult-
  ing the aviation commander, may divert the force to an alternate LZ.
  Drop Zone or Landing Zone Procedures
  13-26. The air drop or landing and reorganization phase is the second most
  vulnerable period in an operation, following the air movement phase. The
  airborne or heliborne force must clear the DZ or LZ quickly, before the enemy
  arrives to counter it.
  13-27. If the airborne force is dropped or landed during daylight hours,
  personnel either move directly to their predesignated attack positions or, if
  the DZ or LZ is not on the objective, first assemble in battalion assembly
  areas. If the drop or landing occurs at night, personnel may first assemble as
  companies and then move to battalion assembly areas, before occupying pre-
  designated attack positions.
  13-28. If the DZ is under strong enemy attack, personnel assemble and
  move immediately to the perimeter to establish defensive battle positions.
  Personnel use any available light armored vehicles to reinforce battle po-
  sitions, and do not sort out the vehicles until after repelling the enemy attack.
  13-29. If the DZ is not on the objective and units assemble first, they try to
  avoid combat with enemy ground force units and hide from an air threat. If
  required to actively defend against an air attack, at least one entire platoon
  per company or one company per battalion is responsible for the mission. For
  a planned follow-on air landing, the initial landing force leaves a rear detach-
  ment at the DZ. This detachment provides security on the DZ for the landing
  of the follow-on force.
  13-30. The heliborne force lands on its objective if possible. If it is not on the
  objective, the LZ should be as close as possible but outside of the direct fire
  range of enemy forces at the objective. Once on the ground, the heliborne land-
  ing force organizes rapidly in an assembly area.