LZ stands for landing zones. During Vietnam war, and more than any other conflict, the helicopter was the primary method of insertion for the LRRPs behind the enemy lines. The speed and the convenience of the Huey helicopter was unmatched by any other vehicles. In a flash, a commander could insert a small team of Rangers exactly where he wanted and this with a minimal amount of assets deployed.
In order to land safely, helicopters required a minimum of clearance on the ground. Usually river beds, rice paddies or jungle clearings were used due to their tendencies to be deprived of trees and high foliage. If obstacles were to hit the rotors while the helicopter tried to land, catastrophic events could happen ranging from the chopper drifting, not having enough lifting force to stay above ground or even damaging the rotors leading to all sorts of bad stuff including, but not limited to, helicopter crash.
Most of the time the main LZ is determined by you and your command before you deploy. Unless things starts to go really bad, the main LZ is to be used. The main reason behind this is that everybody knows where it is and everyone knows it has enough clearance. Above all, the LZ is usually the center of your ops box, or most commonly called, AO. In a terrain like found in Vietnam where rolling hills, dense jungles and thick vegetation is predominant, expect the enemy to be quite aware of where and what constitutes as a potential LZ. Depending on your heat score, the enemy might be waiting for you at these locations or even worse, have them booby trapped.
Types of Landing Zones
The primary zone for insertion and exfiltration. Located in the middle of the AO and is large enough to allow un-impeded landing for the Huey. Marked red on the map. If the situation allows, it is best to aim to exfiltrate through this LZ. However, the enemy often investigates the main LZ after infiltrating and as a result, remain in the vicinity patrolling. This can lead to very dangerous situation if not handled properly. This is the only available form of exfiltration before reaching the rank of Captain.
Locations spotted and identified by Rangers during the reccee. These are clearings large enough to allow Huey to land when marked by smoke. The smoke grenade has to be deployed exactly in the middle of the LZ, if the marker is off the target by too large amount, the pilot refuses to land. Marked blue on the map. Useful if the main LZ is too hot or is un-accessible due to hostile presence.
Locations in the AO the commander considers as large enough to allow the Huey to land in. These also require to be marked by smoke, but are far more demanding than secondary LZ's. If the clearing is not large enough or not marked precisely enough, the pilot will not land. Not marked on the map and is suggested to avoid using unless the situation demands the use of such location for extract.