On a spring night seven months ago, an ancient DC4 aircraft flew over south Texas, headed for an isolated ranch with a primitive airstrip.

But before it could reach its destination at Orange Grove, the plane was spotted by air traffic controllers. Three U.S. Customs planes scrambled to intercept the DC4 and rode it down to the airfield at Cotulla, Tex. For the last 86 miles of its flight, the two pilots of the DC4 dumped their cargo of Colombian marijuana. Five tons were later recovered.

If there were any drug users on the ground during that 86-mile flight, it must have seemed like a doper's dream: marijuana falling out of the sky in burlap bales.

On the ground at Cotulla, the Customs agents seized the pilots, their aircraft and a small arsenal of weapons -- .22 caliber pistols with silencers and .38 caliber pistols with bullets tipped with neoprene impregnated with cyanide.

At the ranch where the plane was headed, the agents found trucks, a tanker loaded with aircraft fuel and seven ground crewmen waiting to refuel the DC4 for a flight to Indianapolis with its cargo of pot.

This episode was one of many in the new war in the air between cops and crooks, a war in which the prize is drugs.

Increasingly, smugglers are using large cargo planes to make fast, long haul drug flights -- usually undetected -- from Central and South America into the Gulf Coast and now the Southwest.

As recently as 1977, drug smuggling was characterized by the single engine plane carrying a relatively small amount of Mexican marijuana into a border town in the desert Southwest. Today it is becoming the province of twin-enginge and four-enginge aircraft, often planes that were sold as surplus by the airlines.

They are bringing in not only ton-loads of Colombian marijuana but also second cargoes such as cocaine and methaqualone (Quaaludes).

Analysts here at the U.S. Drug Enforcement Administration's El Paso Intelligence Center (EPIC) calculate that planes now account for at least one third of the marijuana smuggled into the United States from Colombia, nearly double their share of a few years ago.

While most DEA attention has focused on merchant ships that bring huge cargoes of marijuana to the United States in one trip, a speedy cargo plane can bring in as much pot by running a number of flights in the time it takes a ship to make a single trip. And the plane is exposed to possible apprehension for far less time.

Analysts estimate that more Quaaludes come into the country by airplane than by any other means.

"The airplane," says EPIC aviation analyst Bill Small, "is the only vehicle that can smuggle from anywhere in the world to anywhere in the country." Risk as high as pay

And so a town like Tucson, once a distribution center for Mexican marijuana in the early 1970s, has lost sway to a number of other touchdown towns for drug pilots. Places like Kim, Colo.

In that southeast Colorado town, a four-engine DC7, a model that was once the pride of commercial airlines, landed early last April on a two-mile-long mesa and unloaded three truckloads of marijuana.

The plane, inexplicably, was abandoned there. Chemical analysis of marijuana debris on the craft linked Kim to Colombia.

Such illicit importation has created its own little but profitable world, one where pilots must often crawl over their cargoes to get to the cockpit. For such inconveniences, petty and otherwise, pilot pay is calculated in thousands, but the risk is as high as the pay.

In the three years from 1975 through 1977, 41 crewmen are known to have died in air crashes involving dope smuggling. Last year alone, 59 died and in just the first six months of this year, at least 39 crewmen aboard drug flights were killed in crashes.

All told, there were 139 known crashes involving drug transport planes last year compared to 102 in the first half of this year.

The trafficking, furthermore, utilizes more than half of the planes reported as stolen in the United States. Sometimes, it is custom-order theft. The word was out from Juarez at one time that $15,000 would be paid for a Cessna 206; about that time, the University of New Mexico's Cessna 206 was stolen.

Into this new air war the federal government has thrown a tiny air force of men and planes, planes that in many cases were seized from the smugglers themselves.

The Customs Service operates eight facilities along the border from California to Florida, including a new one in Houston to combat Gulf Coast trafficking, from which pilots and enforcement officers are launched in pursuit of smugglers.

They are skillful and daring pilots, skimming the desert at 150 miles an hour or more, pulling up quickly to clear power lines, or bringing helicopters down on mountaintops so windy the pilots toss rolls of toilet paper to read the air currents. They provide surveillance on an endless expanse of opportunities for drug smugglers to fly into the country.

These pilots pack stainless steel .357 magnums and machine guns with collapsible stocks. They tuck them in their attache cases and walk off to war with the dopers, or crooks, as they call them. Satellite communication

An element in this war is the secret and sophisticated DEA intelligence center, EPIC, with satellite ties to agents in such distant countries as Colombia. From a car in Guajira, the drug rich northern peninsula of Colombia, an agent can tap by satellite communication into EPIC's computerized intelligence files containing names and numbers of thousands of airplanes and ships.

Customs agents sometimes camp in the desert for days with mobile units used to fill in the gaps in radar coverage along the border, while other staff on a part-time basis Federal Aviation Administration air traffic control centers to monitor any errant blips coming from over Mexico.

Court-ordered wiretaps put bugging transponders on private aircraft, and when these wired planes -- eight, currently -- take off; they become, on radar scopes, glowing suspects.

But officials openly acknowledge that, outside of an occasional hit, the smugglers are so far winning.

"It's hard catching an aircraft," says Arthur Fluhr, a former "French Connection" agent who is now director of EPIC. "It's just a difficult thing to do. And with all the seizures, we still haven't seen a change in supply or price.

"In the 18 months beginning in January '78, one and a half tons of cocaine and a ton of Quaaludes were seized from [private] aircraft. When you think of what we're getting -- my God, how much got away?"

Joe Beaver, director of the Customs Services' El Paso air force, adds: "We're always a little late, so they beat us for a long time before we ever catch." In short, the dopers are usually a step ahead in a business where innovation and invention mean freedom.

Fluhr and others at EPIC are hoping that a three-month-old classified crackdown called "Operation Tigre-Falcon" will help turn the tide in the air war. It is a secret effort waged from a special command post in the third floor operations room of the intelligence center.

Fluhr would say only that it involves increased surveillance and improved communications between agents in the United States and those in Central and South America.

But another intelligence officer notes, "Everything they do, we react. And everything we do, they react."

This accounts in part for the increase in air smuggling along the Gulf Coast and into the Southwest. Increased enforcement and new anti-smuggling state laws in Florida, officials say, have forced some importers elsewhere.

The Mexican government's spraying of marijuana fields dried up supplies there. Customers then acquired a taste for the more potent Colombian marijuana. So now the big planes with much greater range are flying in from Colombia.

Generally they set out from Colombia or the Yucatan and fly to Florida or the Gulf Coast across the open water. Or they may fly over Mexico, dropping down low under the inadequate radar that marks places like Big Bend, Tex.

Crossing into the United States -- "right on the deck," as such low-level flying is called -- they seek out the north-south valleys between the mountain ranges, flying low and without running lights, so low they have to look up at the hills -- if there is enough light to see them. Suddenly they climb, popping onto radar as though they just took off, or just setting their planes down on a remote airstrip, ranch or road.

Intelligence information and radar spotting from the Albuquerque air traffic control center seem to work best here in detecting smugglers. On a recent day, officials at EPIC were waiting for one or two big planes to take off from Colombia. When they would cross the border and where was unknown. Customs patrol flight

I flew for nearly three hours that evening with a Custom patrol. We were in a blue and white twin engine plane over Marfa, Tex., when we got our first tip. It came from an Air Force plane. The radio conversation went like this:

6:43 p.m.: "John, we're tracking a target that's about 150 miles southwest of you over Mexico, 170 knots -- the profile looks good."

6:48 p.m.: "John, I've been tracking another target that looks like he's headed to Van Dorn. In addition, we've picked up another target up ahead of you."

6:56 p.m.: "I show him at 4,600 and you at 8,500 and seven miles in front."

We are still looking. "Target's three miles ahead. He must be right on the deck." We are at 6,700 feet.

"Target appears to be going to Van Dorn, four miles from you."

We are still looking.

Now we are coming up on the airport and we don't see anything.

"Stand by just a moment."

Then from the Air Force plane: "We no longer hold the target. If you check out the [Van Horn] airport we'll keep looking north."

We never see the plane and in fact we have been chasing a train picked up by the Air Force radar."John, you ever sieze a train before?"

Then at 7:12: "John, we got something that looks pretty good over by Columbus. Can you pour it on and make tracks for El Paso?"

So it went, a few transmissions more from the Customs officer on the Air Force plane, and a few others from the Customs officer at the Albuquerque air traffic control center. But not a single target that we could chase and bring down to see if someone was bringing drugs into the country.

So, I say, "No marijuana got into the United States tonight."

Duane Long, a Customs enforcement officer, smiles and laughs.