TRY TO IMAGINE the Vietnam war without helicopters. It can't be done. Wherever there were troops, choppers picked them up and put them down, fed them their supplies, ferried in the visiting brass (and press), carried out the dead and wounded.

As they beat their way through Vietnam's hot air--over the triple canopy jungle or the center of Saigon--choppers were the rhythm section of the war.

From a Huey, the common name for the wonderfully versatile Bell HU-1 Iroquois, you see the world a lot better than in a Chevrolet. The distance, plus the rotor noise, blocks out sounds. The landscape floats below (if you don't like it, wait a minute and it changes), the open doors are huge picture windows. Fly over Saigon and hear no traffic. Over a battle, no gunfire. Even the bullets aimed at your Huey make the seemliest of noises, hardly ruffling the flying dream. Snap the lid of a Zippo lighter closed. That was the sound. "Tick," is the word Robert Mason uses. He heard a lot of ticks.

The sounds and sensations of flying helicopters dominate the best parts of this memoir of a year and more than 1,000 missions in Vietnam. Mason was there early in the U.S. mainforce war. His 12 months began in September 1965 when he landed with the First Cavalry Division (Airmobile).

It is hard in 1983 to remembr how the First Cav was going to change the war. In December 1965, Defense Secretary Robert S. McNamara visited the Cav at An Khe and emerged "brimming over with praise," in Newsweek's phrase. Any doubts that a division built around the mobility of its helicopters would be effective were routed. McNamara said: "There is no other division in the world like it." Airmobile! On the rotors of its Hueys, the Cav was going to sweep Charlie from the battlefield. The Cav's pilots, former Warrant Officer Mason writes, lost their optimism early.

Mason's book could have had an enormous impact had it been written--and published-- soon after his tour in Vietnam. Here is a combat veteran saying that the U.S. effort wasn't working. If you were wondering who to believe in 1967 or 1968, Chickenhawk would have been persuasive evidence.

The American bodies piling up. The bits of flesh stuck under the seat of Mason's chopper. The Vietnamese prisoners shot in the head in retaliation for atrocities against Americans. A pilot's decision to pull a pistol on his despised commander nicknamed Major Disaster. It was, Mason writes, a screwed-up war.

But that is not news now. Nor, for all the power of some of the incidents Mason describes, is it news that U.S. firepower chewed up the countryside nor that some U.S. officers were incompetent, some missions were stupid and a lot of LZs (landing zones) were hot.

Mason's account of his life on the ground in Vietnam seems in part an aimless piling up of anecdotes, some of them so pat as to seem right out of a script. When he flies, so does his book.

"My first impression of the machine was that it was pure silk," Mason writes of his maiden in a Huey. "The machine left the ground like it was falling up."

A helicopter is controlled by foot pedals and two sticks called the collective and the cyclic. When Mason has his feet on the pedals and his hands on those sticks, his book takes off. He flies into landing zones too small for his Huey's 48-foot rotor diameter by tilting the chopper over sideways as he approaches. He gets out by deliberately chopping tree branches with the rotor blades as he lifts off. Mason wiggles his Huey into the tightest places sometimes carrying loads well over the safe limit, and following the advice of his pilot guru Leese, he keeps the faith: No backing away, no LZ too small. No load too heavy.

But fear is always with him. In the end it takes over his dreams and, for a time, his life. "Chickenhawk," given the word's place in the homosexual lexicon, is an unfortunate title. For Mason, as he explains, the word marries his (and other pilots') alternating moments of fear and bravery.

Mason transfers out of the Cav to find a quieter place to spend his last months in Nam. The combat assaults into LZs with names like X-Ray, Dog and Quebec to drop off "another load of wild-eyed boys" wore him down. The war, he thought, was going badly. The landing zones were too hot. The Viet Cong knew too much. He counted too many bullet holes in his Hueys and even though he wasn't wounded, Mason became a Vietnam casualty.

The downside of being back in the United States hit him almost unbelievably soon. At the Honolulu airport en route home, a young woman asked if he was a returning vet. Then: "She glared at me and said, 'Murderer.' "

Only at the end of Chickenhawk does the reader understand why Mason didn't write his memoir earlier. The pilot who never cracked up under fire, fell from the sky when he got back home.

Nightmares. Babies on pitchforks. Half a bottle a day. Veterans Administration psychiatrists.

He taught at a helicopter school for a while, but the bad dreams overcame him and the Army grounded him. The nightmares and the alcohol and the pills got so bad Mason didn't want to fly any longer. His love affair with the choppers was over and he was discharged in 1968.

For Mason and his wife, Patience, it got worse. Chickenhawk ends with a barebones 10- page account of Mason's life post-Vietnam.

The last paragraphs catch you up short.

"The car broke down and the bills began to pile up. For the time I had spent writing (Chickenhawk), I got four rejections.

"What did the desperate man do?"

He did what a lot of desperate men have done and he got caught. It's a sad story.