Four days before the Air Line Pilots Association struck United Airlines May 17, United Chairman Richard J. Ferris outlined an optimistic plan for breaking the strike.

Ferris assumed that 500 new pilots who recently completed training would go to work for United the minute the strike started, that many of United's experienced pilots would cross the picket lines, and that more replacement pilots could be hired and trained quickly to persuade those still striking that their cause was lost.

What will happen, Ferris was asked, if the union shows good discipline? If the pilots stay out?

"Impossible," he said emphatically. "Impossible. You know that."

If the strike against United continues for many weeks, a likely prospect after the collapse of negotiations early yesterday, Ferris may yet be proved right.

But until now, the pilots have crimped the first phase of United's strikebreaking plan by effectively organizing those 500 trained but not hired pilots. Only four of them went to work. And according to the ALPA, only 270 of its 4,900 active members working for United crossed the picket lines.

With so few pilots available to fly, United has been able to put only 209 flights into the air each day, out of its normal schedule of 1,500.

United now says it will never hire those 500, that they were offered jobs -- after the strike began -- and did not come to work. The pilots say they cannot turn their backs on the 500.

So a war of attrition has begun between the nation's largest airline and a pilot group that represents more than 17 percent of ALPA's national membership. A prolonged strike is almost a self-fulfilled prophecy, as both sides have prepared thoroughly.

United, which in 1979 took a 58-day strike from the International Association of Machinists and Aerospace Workers without trying to fly, made sure it would try this time. "We've run the numbers on all sort of cases and understand what we're up against," said John Cowan, United's senior vice president for administration and finance.

United's pilots, proud and highly experienced, have received what is generally regarded as the finest civil-aviation training in the world, not to mention some of the finest salaries, more than $80,000 annually on average.

Their way of life has been seriously threatened by the forces of airline deregulation, which have decreed that pilots are not worth what they were when an airline could automatically pass through to the customer the costs of salary increases. A perfectly competent non-union deck officer can be hired to fly a Boeing 727 at People Express or Continental for $40,000, not $80,000.

The pilots have spent more than $500,000 on two slickly produced nationwide satellite television conferences as part of their "family awareness program." The first floor of an office building across the street from Chicago's O'Hare International Airport houses the United Pilots' Strike Committee and is outfitted much like a political campaign center, with telephone banks and hordes of volunteer workers, mostly pilots' wives, wearing "We Are United" buttons.

They are there, day and night, to answer questions and give advice and mutual support to make sure the strike isn't lost at the kitchen table when concerns about the mortgage begin to impinge on feelings of union loyalty.

The ALPA has already lost at Continental, which declared bankruptcy before unilaterally cutting pilots' pay in half in September 1983. The ALPA struck and is still on strike, but Continental has more flights than it did before the strike and was filling them last week with rerouted United passengers.

"We were not prepared for a strike at Continental," said ALPA President Henry A. Duffy, "We were trying to negotiate a contract." The mistake has not been repeated.

The ALPA has been willing to make major salary concessions at financially troubled carriers such as Western, Republic, Eastern and Pan American, and even helped United by forgoing scheduled pay raises in 1981. "We've got a good track record of helping out, but not at airlines that are doing well," Duffy said. United had an operating profit of $564,000 last year and a net profit of $259,000 on revenues of $6.2 billion.

Pilots at other airlines understand the stakes. Marching together in the picket line at O'Hare recently were Mark McCoy, a United second officer, Mark Mathison, a Delta first officer, and John Mallery, a Trans World Airlines first officer. Both Delta and TWA are seeking new contracts with the ALPA, and the United result is expected to set the pattern.

"Is there any way to make the public understand about the problem in the cockpit when one guy is making thousands of dollars more than the other, the tension, the potential danger?" Mathison asked.

What the pilots want the public to grasp is that tension in the cockpit about salaries is a safety issue, one of ALPA's most predictable fallback positions. That is a tough sell.

The ALPA has a history of not striking very effectively and of not honoring other unions' pickets lines. ALPA's members make big salaries, vote Republican and talk to their stockbrokers between flights. When a bearded member of the ALPA contract negotiating committee showed up for duty on the picket line at O'Hare, he was not permitted to carry a sign. "No beards on the line," is the rule.

The issue that first separated ALPA and United was United's plan to pay newly hired pilots on a lower scale than existing pilots. That was solved in the last hours of negotiation. Then the 500 unhired pilots became the problem.

Both issues were sharply described by an elegantly dressed gentleman at O'Hare. "I don't see how you can go on strike to protect the rights of the unborn," he said as he poked his index finger into the picket sign of a striking United captain. "Four of my flights have been canceled and I won't tolerate it."

The pilot tried to explain, but the gentleman would have none of it. He wheeled on the heels of his expensive shoes and shouted, "I'm going over to American."