The helicopter scenes from "Apocalypse Now" have a great deal of artistic license and bear no resemblance to what a real Vietnam CA (combat assault) looked like. The goat rope of different aircraft, altitudes and equipment shown in the movie makes most Vietnam vets laugh. The real thing was a sight to behold, and a testament to the training and discipline of hundreds of aircrew who did it like it was easy.
I have personally flown several hundred CA's, and this is my impression of what it looked like:
A battalion-sized lift would use assets from a Cav Troop, a lift company, USAF FACs and fast movers, and one or two artillery batteries in a combined arms operation.
The Cav would usually have found the objective a few hours or perhaps a day before. OH-6 and Cobras would scout areas to look for sins of bad guy activity. Commo wire in the trees, signs of truck activity, an established basecamp are all signs that a likely objective is identified. The spot reports that they sent back to the local TOC (Tactical Operations Center) would plant the seed for the CA.
The assault would be planned by the local infantry battalion commander and his staff, usually the CO of the lift company would be involved, too. Planning would usually take a few hours, but on-call assets could start a CA in minutes, if necessary.
The lift company would use up to 18 UH-1H machines each carrying about 7 to 9 troops, depending on the season (temperature) and altitude of the LZ. Each lift would therefore bring in a full infantry company of 100 to 140 troops. Three companies was a battalion, so the lift would be done in three insertions, usually by using a pickup zone that was no more than 10 minutes (20 miles) away, so the entire battalion would be committed in less than an hour . If the objective was really big, two lift companies would be used, but these were rare. I flew gun cover on one 50 ship lift, it was a big to-do.
The scouts would mark the LZ, and arty prep would start prior to the lift. Often, the prep would soften up several possible LZ's so the actual one would not be announced. Just prior to the insertion, fast movers (F-4's or A-6's) might drop big HE in the LZ perimeters if the job were a really hot one. Often, just arty prep would be used. The last white of the arty prep showed the last round clearly for us, using an airburst Willie Pete round. That was usually 20 to 30 seconds from the LZ time, so the insertion crews could see the last round at about 1 mile final approach.
The lift companies each had gun platoons with UH-1C gunships to provide organic gun cover for their slicks. These guys would ride heard on the slick formation, usually two paired teams, one pair at each side of the formation. They would suppress the LZ and engage active enemy that popped up to oppose the landing. Slicks usually broke into 4 or so diamonds for a square or circular LZ, so each wave was actually a series of smaller insertions, spaced at 30 seconds to 1 minute intervals (to allow the gun cover to escort each section in and out and then pick the next section up.) Each full wave took less than 2 minutes, and usually went off silently, with a few simple calls, like "Trail's up" when the last Huey in the wave could see that all aircraft were loaded at the PZ. Often, the guns would call their breaks to let the wingmen know when to roll in to maintain continuous cover. The last 30 seconds of an insertion were tense and strange because of the silence.
The entire team would head back to the PZ for the next wave, usually leaving a gun team behind to help the troops in the LZ with supporting fire.
A glance back across the sky would show the entire lift in lose formation heading back to the PZ, then lifting in perfect formation heading back inbound. The formation might shift depending on how fancy everyone felt, but usually they used a set of 4 ship diamonds for small LZ's and a long staggered trail if the LZ was large or linear.
The formation would tighten up to about 1 rotor disk separation on short final, for best cohesion, and also to keep the troops close to each other when they get out. The door guns on the slicks were very effective in suppressing individual targets on short final and while in the LZ. Everybody stayed cold on the guns unless fire was taken, then all hell could break loose.
A more reasonable look at the concept of Air Assault is seen in "We Were Soldiers". The guy who helped invent these tactics was Gen Harry Kinnard, who is a character in "Soldiers" seen walking down the hall with Hal Moore (Mel Gibson) when Moore first comes to Ft. Benning.
Kinnard formed the 11th Air Assault, which became the 1st Air Cav Division. Kinnard was with Gen MacCauliff in the 101st Airborne at Bastogne in 1944. It was Kinnard that suggested using "Nuts" as the response to the German request for surrender, since MacCauliff's first response was "F**K EM!" and thus less likely to be recorded in the history. I met Harry several years ago, he lives in Virginia now, when I last heard.